[Pg i]

INHERITANCE OF CHARACTERISTICS
IN DOMESTIC FOWL.

BY

CHARLES B. DAVENPORT,
Director of the Station for Experimental Evolution,
Carnegie Institution of Washington.

WASHINGTON, D. C.
Published by the Carnegie Institution of Washington.
1909

[ii]

Carnegie Institution of Washington Publication No. 121.

Papers of the Station for Experimental Evolution, No. 14.

PRESS OF J. B. LIPPINCOTT COMPANY
PHILADELPHIA

TABLE OF CONTENTS.

INHERITANCE OF CHARACTERISTICS
IN DOMESTIC FOWL.
BY
CHARLES B. DAVENPORT.

[2]

INTRODUCTION.

A series of studies is here presented bearing on the question of dominance and its varying potency. Of these studies, that on the Y comb presents a case where relative dominance varies from perfection to entire absence, and through all intermediate grades, the average condition being a 70 per cent dominance of the median element. When dominance is relatively weak or of only intermediate grade the second generation of hybrids contains extracted pure dominants in the expected proportions of 1:2:1; but as the potency of dominance increases in the parents the proportion of offspring with the dominant (single) comb increases from 25 per cent to 50 per cent. This leads to the conclusion that, on the one hand, dominance varies quantitatively and, on the other, that the degree of dominance is inheritable.

The studies on polydactylism reveal a similar variation of potency in dominance and show, in Houdans at least, an inheritance of potency ([table 11]), and moreover they suggest a criticism of Castle's conclusion of inheritance of the degree of polydactylism.

Syndactylism illustrates another step in the series of decreasing potency of the dominant. On not one of the F₁ generation was the dominant (syndactyl) condition observed; and when these hybrids were mated together the dominant character appeared not in 75 per cent but in from 10 per cent to 0 per cent of the offspring. The question may well be asked: What is then the criterion of dominance? The reply is elaborated to the effect that, since dominance is due to the presence of a character and recessiveness to its absence, dominance may fail to develop, but recessiveness never can do so. Consequently two extracted recessives mated inter se can not throw the dominant condition; but two imperfect dominants, even though indistinguishable from recessives, will throw dominants. On the other hand, owing to the very fact that the dominant condition often fails of development, two extracted "pure" dominants will, probably always, throw some apparent "recessives." Now, two syndactyls have not been found that fail (in large families) to throw normals, but extracted normals have been found which, bred inter se, throw only normals; hence, "normal-toe" is recessive. In this character, then, dominance almost always fails to show itself in the heterozygote and often fails in pure dominants.

The series of diminishing potency has now brought us to a point where we can interpret a case of great difficulty, namely, a case of rumplessness. Here a dominant condition was originally mistaken for a recessive condition, because it never fully showed itself in F₁ and F₂. Nevertheless, in related individuals, the condition is fully dominant. We thus get the notion that a factor that normally tends to the development of a character may, although present, fail to develop the character. Dominance is lacking through impotence.

The last term of the series is seen in the wingless cock which left no wingless offspring in the F₁ and F₂ generations. In comparison with the results gained with the rumpless cock, winglessness in this strain is probably dominant but impotent.

When a character, instead of being simply present or absent, is capable of infinite gradations, inheritance seems often to be blending and without segregation. Two cases of this sort—booting and nostril-height—are examined, and by the aid of the principle of imperfect dominance the apparent blending is shown to follow the principle of segregation. Booting is controlled by a dominant inhibiting factor that varies greatly in potency, and nostril-height is controlled by an inhibiting factor that stops the over-growth of the nasal flap which produces the narrow nostril.

The extracted dominants show great variability in their progeny, but the extracted recessives show practically none. This is because a positive character may fail to develop; but an absent character can not develop even a little way. The difference in variability of the offspring of two extracted recessives and two extracted dominants is the best criterion by which they may be distinguished, or by which the presence (as opposed to the absence) of a factor may be determined.

The crest of fowl receives especial attention as an example of a character previously regarded as simple but now known to comprise two and probably more factors—a factor for erectness, one for growth, and probably one or more that determine the restriction or extension of the crested area.

The direction of lop of the single comb is an interesting example of a character that seems to be undetermined by heredity. In this it agrees with numerous right and left handed characters. It is not improbable that the character is determined by a complex of causes, so that many independent factors are involved.

A series of studies is presented on the inheritance of plumage color. It is shown that each type of bird has a gametic formula that is constant for the type and which can be used with success to predict the outcome of particular combinations. New combinations of color and "reversions" receive an easy explanation by the use of these factors. The cases of blue, spangled, and barred fowl are shown also to contain mottling or spangling factors.

CHAPTER I.
THE SPLIT OR Y COMB.

A. INTERPRETATION OF THE Y COMB.

When a bird with a single comb, which may be conveniently symbolized as I, is crossed with a bird with a "V" comb such as is seen in the Polish race, and may be symbolized as oo, the product is a split or Y comb. This Y comb is a new form. As we do not expect new forms to appear in hybridization, the question arises, How is this Y comb to be interpreted? Three interpretations seem possible. According to one, the antagonistic characters (allelomorphs) are I comb and oo comb, and in the product neither is recessive, but both dominant. The result is a case of particulate inheritance—the single comb being inherited anteriorly and the oo comb posteriorly. On this interpretation the result is not at all Mendelian.

According to the second interpretation the hereditary units are not what appear on the surface, but each type of comb contains two factors, of which (in each case) one is positive and the other negative. In the case of the I comb the factors are presence of median element and absence of lateral or paired element; and in the case of the oo comb the factors are absence of median element and presence of lateral element. On this hypothesis the two positive factors are dominant and the two negative factors are recessive.

The third hypothesis is intermediate between the others. According to it the germ-cells of the single-combed bird contain a median unit character which is absent in the germ-cells of the Polish or Houdan fowl. This hypothesis supposes further that the absence of the median element is accompanied by a fluctuating quantity of lateral cere, the so-called V comb.

The split comb is obtained whenever the oo comb is crossed with a type containing the median element. Thus, the offspring of a oo comb and a pea comb is a split pea comb, and the offspring of a oo comb and a rose comb is a split rose. The three hypotheses may consequently be tested in three cases where a split comb is produced.

Table 1.

The first and third hypotheses will give the same statistical result, namely, the products of two Y-combed individuals of F₁ used as parents, will exhibit the following proportions: median element, 25 per cent; split comb, 50 per cent; and no median element, 25 per cent. These proportions will show themselves, whatever the generation to which the Y-combed parents belong, whether both are of generation F₁, or F₂, or F₃, or one parent of one generation and the other of another. Other combinations of parental characters should give the proportions in the progeny shown in table 1.

On the second hypothesis, on the other hand, the proportions of the different kinds occurring in the progeny will vary with the generation of the parents. This hypothesis assumes the existence in each germ-cell of the original parent of two comb allelomorphs, M and l in single-combed birds and m and L in the Polish fowl, the capital letter standing for the presence of a character (Median element or Lateral element) and the small letter for the absence of that character. Consequently, after mating, the zygote of F₁ contains all 4 factors, MmLl, and the soma has a Y comb; but in the germ-cells, which contain each only 2 unlike factors, these factors occur in the following 4 combinations, so that there are now 4 kinds of germ-cells instead of the 2 with which we started. These are ML, Ml, mL, and ml. Furthermore, since in promiscuous mating of birds these germ-cells unite in pairs in a wholly random fashion, 16 combinations are possible, giving 16 F₂ zygotes (not all different) as shown in table 2.

Table 2.

It is a consequence of this second hypothesis that, in F₂, of every 16 young 9 should have the Y comb; 3 the I comb; 3 the oo comb, and 1 no comb at all. It follows further that the progeny of two F₂ parents will differ in different families. Thus if a Y-combed bird of type a be mated with a bird of any type, all of the progeny will have the Y comb.

From Y-combed parents of various types taken at random 4 kinds of families will arise having the following percentage distribution of the different types of comb:

• 1. Y comb, 100 per cent.
• 2. Y comb, 75 per cent; I comb, 25 per cent.
• 3. Y comb, 75 per cent; oo comb, 25 per cent.
• 4. Y comb, 56.25 per cent; I comb, 18.75 per cent; oo comb, 18.75 per cent; absent, 6.25 per cent.

Again, mating two extracted I combs of F₂ should yield, in F₃, two types of families in equal frequency as follows:

• 1. I comb, 100 per cent.
• 2. I comb, 75 per cent; no comb, 25 per cent.

Again, mating two extracted oo combs of F₂ should yield, in F₃, two types of families in equal frequency, as follows:

• 1. oo comb, 100 per cent.
• 2. oo comb, 75 per cent; no comb, 25 per cent.

Single comb × Y comb should give families of the types:

• 1. Y comb, 100 per cent.
• 2. Y comb, 50 per cent; I comb, 50 per cent.
• 3. Y comb, 50 per cent; oo comb, 50 per cent.
• 4. Y comb, 25 per cent; I comb, 25 per cent; oo comb, 25 per cent; absent, 25 per cent.

Mating oo comb and Y comb should give the family types:

• 1. Y comb, 100 per cent.
• 2. Y comb, 50 per cent; oo comb, 50 per cent.
• 3. Y comb, 50 per cent; I comb, 50 per cent.
• 4. Y comb, 25 per cent; oo comb, 25 per cent; I comb, 25 per cent; no comb, 25 per cent.

Finally, I comb and oo comb should give the following types of families:

• 1. Y comb, 100 per cent.
• 2. I comb, 100 per cent.
• 3. Y comb, 50 per cent; oo comb, 50 per cent.
• 4. I comb, 50 per cent; no comb, 50 per cent.

Now, what do the facts say as to the relative value of these three hypotheses? Abundant statistics give a clear answer. In the first place, the progeny of two Y-combed F₁ parents is found to show the following distribution of comb types: Y comb 471, or 47.3 per cent; I comb 289, or 29.0 per cent; oo comb 226, or 22.7 per cent; and no comb 10, or 1 per cent. The presence of no comb in F₂ speaks for the second hypothesis, but instead of the 6.25 per cent combless expected on that hypothesis only 1 per cent appears. There is no close accord with expectation on the second hypothesis.

Coming now to the F₃ progeny of two Y-combed parents, we get the distribution of families shown in table 3.

Table 3.

An examination of these families shows not one composed exclusively of Y-combed individuals nor those (of significant size) containing Y-combed and I-combed or oo-combed individuals exclusively, much less in the precise proportion of 3:1, yet such should be the commonest families if the second hypothesis were true. Notwithstanding the marked deviation—to be discussed later—from the expected proportions of I, 25 per cent; Y, 50 per cent; oo, 25 per cent, the result accords better with the first or third hypothesis. Since on either of these hypotheses the same proportions of the various types of comb are to be expected in the progeny of Y-combed parents of whatever generation, it is worth recording that from such parents belonging to all generations except the first the results given in table 4 were obtained, and it will be noticed that these results approach expectation on the first or third hypothesis.

Table 4.

The progeny of two extracted single-combed parents of the F₂ generation give in 3 families the following totals: Of 95 F₃ offspring, 94 have single combs; one was recorded from an unhatched chick as having a slightly split comb, but this was probably a single comb with a slight side-spur, a form that is associated with purely I-combed germ-cells. This result is in perfect accord with the second and third hypotheses, but is irreconcilable with the first hypothesis.

The progeny of two extracted oo-combed parents is given in table 5.

Table 5.

The distribution of offspring in the 24 families of table 5 is in fair accord with any of the three hypotheses, but seems to favor the second, for that hypothesis calls for families with combless children, whereas such are not to be expected on the first hypothesis. Moreover, agreement with the second hypothesis is fairly close, for that calls for 3 families with combless children and there were actually 3 such families showing a total of 1.8 per cent combless, where expectation is 2.8 per cent. What is opposed to any hypothesis is the appearance of some Y-combed offspring; and to account for this the hypothesis is suggested that the germ-cells of some parents with oo comb contain traces of the I-comb determiner. The word "traces" is used because the median element in these Y-combed offspring is practically always very small. It is fair, consequently, to conclude that oo × oo gives oo-combed, and occasionally combless, offspring. This conclusion is further supported by the statistics derived from extracted oo comb of all generations bred inter se, which give: Y 11, oo 427, and no comb 8, where the 11 Y-combed birds are those just referred to as progeny of F₂ parents. The non-median comb, consequently, probably contains only non-median germ-cells.

Table 6.

The mating of extracted I comb and Y comb, both of the second (or later) hybrid generation, gives the following distribution of types in the offspring (table 6): Y comb 95 (49 per cent); I comb 95 (49 per cent); oo comb 4 (2 per cent). In detail the results given in table 6 accord badly with the second hypothesis, which demands some families with 100 per cent Y comb.

The mating of extracted oo comb×Y comb, where both parents are of the second hybrid generation, gave the distribution of comb types in the 6 families that are recorded in table 7.

Table 7.

The single comb recorded in the case of 7 birds is doubtless merely the limiting condition of a Y comb in which the median element is developed to its fullest extent. All but 2 of the 7 were recorded from early embryos when an incipient bifurcation would be more difficult to detect. This explanation applies generally, and accounts for the usual excess of I comb when compared with Y comb, as for instance in table 3, page 7. Returning to table 7, it is, consequently, probable that only the Y-combed and non-median-combed offspring are produced and that they are in the proportion of 99 to 115 or of 46 per cent to 54 per cent. If we add together all records of a oo×Y cross, disregarding the generation of the parents, we get a total I comb 5,[1] Y comb 177, oo comb 172, and absent 3, or 182 (51 per cent) with the median element and 175 (49 per cent) without. Thus the oo×Y cross gives the 1:1 proportion called for on the first and third hypotheses and not at all the variety required by the second hypothesis.

Table 8.

Finally, we must consider the result of mating a bird without papillæ (No. 1420, pen 704) with a median-combed hen (480). When this typical single-combed hen was used the 49 progeny were all of the Y type.[2] This proves that the combless type behaves only as an extreme of the non-median type.

When Y-combed hens were used with the combless cock the offspring had Y comb and non-median-comb in nearly equal numbers, 23:27 (table 8), but the latter included an unusually large proportion of combless fowl (15 in 27). When a combless hen (No. 4257) was used, 9 of the offspring had oo comb and 2 no comb; not a greater proportion of combless birds than in the no-comb×Y-combed cross. All of these facts indicate that "comblessness" is not entire absence of the comb factors, but a minimum case of the oo or paired comb. This result is opposed to the second hypothesis.

The statistics of all matings between I, Y, and no comb on the one side and no comb on the other thus speak unanimously for the conclusion that in these matings we are not dealing with 2 pairs of allelomorphs, but with a single comb and its absence (third hypothesis) or with a case of particulate inheritance (first hypothesis). Moreover, it must be said that the split comb is obtained also when the Polish-Houdan comb is crossed with a pea comb or a rose comb; and the pea and rose combs can not be said to have "lateral comb absent," as required by the second hypothesis. Consequently the second hypothesis is definitely excluded.

It now remains to decide between the two remaining hypotheses. First of all, it may be said that the perfection with which I and oo combs can be extracted from Y-combed birds indicates that we are here dealing with a case of Mendelian inheritance and, in so far, favors the third hypothesis. To accord with the theory of particulate inheritance, of which the first hypothesis is a special case, the two united characters should transmit the mosaic purely; but this they do not do. Hence the third hypothesis is to be preferred to the first.

Comblessness is a necessary consequence of the second hypothesis and is inexplicable on the first hypothesis. On the third hypothesis it may be accounted for as follows: Absence of single comb is allelomorphic to its presence. The lateral comb is a character common to fowl either with or without the median comb, but it is ordinarily repressed in the birds with single comb and gains a large size when the median element is absent. It is a very variable element. At one extreme it forms the cup comb; at the other there is an absence of any trace of comb. My own records show all grades between these extremes, including minute papillæ on both sides of the head or on one side only, low paired ridges, the butterfly comb, and cup comb shorter than normal. This variability of the lateral element is comparable to the fluctuation in size of the single comb itself, as illustrated by the Single-comb Minorca on the one hand and the Cochin on the other. It is comparable, also, to the fluctuation in the paired part of the Y comb, which we shall consider in the next section, and to the variability of the oo comb as met with in the pens of fanciers.

The foregoing considerations do not, at first sight, account for the Y comb as seen in F₁. Yet they provide us with all the data for an explanation. Median comb of the Minorca dominates over no median of the Polish, and so in F₁ we have the median element represented. But, on the well-known principle of imperfection of dominance in F₁, the median comb is usually incomplete and, probably for some ontogenetic reason, incomplete only behind. The incompleteness behind permits the development there of the elsewhere repressed lateral comb, and we therefore have the Y comb—evidence at the same time of a repressed lateral-comb Anlage in the single-combed birds and of imperfection of dominance of the single comb in the first hybrid generation.

B. VARIABILITY OF THE Y COMB AND INHERITANCE OF THE VARIATIONS.

As already stated, the proportions of the median and the lateral elements in the Y comb are very variable; the median element may, indeed, constitute anywhere from 100 per cent to 0 per cent of the entire comb. Even full brothers and sisters show this variability. Thus the offspring of No. 13 ♀ Single-comb Minorca and No. 3 ♂ Polish have the median element of the Y comb ranging from 0 per cent to 70 per cent of the whole comb. Notwithstanding this variability of the median element in any family there is a difference in the average and the range of variability in families where different races are employed. Thus the offspring of two Polish × Minorca crosses show an average of 46 per cent of the median element in the comb; the Houdan × Minorca cross gives combs with 60 per cent of the median element; and in the combs of the offspring of two Houdan × White Leghorn crosses there is, on the average, 71 per cent of the median element. The Houdan × Dark Brahma (pea comb) gives combs with an average of 87 per cent median element and the Polish × Rose-comb Minorca cross gives 89 per cent median. The rose-combed hens used in this last cross were heterozygous, having single comb recessive; consequently they produced also chicks with typical Y combs. Such had, on the average, only 59 per cent of the median element and were thus in striking contrast with the slightly split rose combs. In the case of the partially split rose combs the median element ranged from 60 per cent to 100 per cent of the whole length of the comb; but in the split single combs the range is from 0 to 100 per cent. Thus, in the two cases, the proportion of the median element and the range of its variability differ greatly.

Also, in generations subsequent to the first, the Y comb exhibits this same variability. We have already seen that the progeny of the Y-combed offspring of any generation may be compared with those of any other, and so we may mass together the progeny of all hybrid generations so long as they are derived from the same ancestral pure races.

Fig A.—The frequency of the different forms of Y comb, each form being based on the percentage of the median element of the Y comb to the entire length of comb.

In inquiring into the meaning of this variability we must first construct the polygon of frequency of the various grades of median element. This is plotted in fig. A, which is a composite whose elements are, however, quite like the total curve. There is one empirical mode at 70 per cent and another at 0 per cent. The smaller mode at 50 per cent is, I suspect, due to the tendency to estimate in round numbers, and may be, in this discussion, neglected. From this polygon we draw the conclusions, first, that the median element in the Y comb tends to dominate strongly over the absence of this element, as 7:3, and, second, that dominance is rarely complete. Yet there is an important number of cases, even in F₁, where the median element is almost or completely repressed (down to 10 to 0 per cent of the whole) and the comb consists of two high and long lateral elements—the "cup comb" of Darwin. There are, then, in the offspring of a median-combed and a non-median-combed parent, two types with few intergrades—the type of slightly incomplete dominance of the median element and the type of very incomplete dominance.

We have now to consider how these two types of comb and their fluctuations behave in heredity. When two parents having each combs of the 70 per cent or 80 per cent median type are mated, their offspring belong to the three categories of I, Y, and "no-median" comb, but the relative frequency of these three categories is not close to the ideal of 25 per cent, 50 per cent, and 25 per cent, respectively. For there is actually in 336 offspring a marked excess of the I comb, 36 per cent, 44 per cent, and 20 per cent, respectively, resulting. When, on the other hand, two parents having each combs of the 10 per cent and 0 per cent types are mated their offspring are of the same three categories and the proportions actually found in 241 offspring (28 per cent, 47 per cent, 25 per cent) closely approximate the ideal. It is clear, then, that even the cup comb, without visible median element, has such an element in its germ-cells and is totally different in its hereditary behavior from the Polish comb, in which the median element is absent, not only from the soma, but also from the germ-cells.

We have seen in the last paragraph that the Y comb with only 10 per cent to 0 per cent median element has germ-cells bearing median comb as truly as the Y comb containing 70 per cent to 80 per cent median element, but we have also seen that in the latter case there is an excess of single-combed progeny. We have now to inquire whether, in general, there is a close relation between the proportion of median element in the comb of the parents and the percentage of single-combed offspring. These relations are brought out in the lower half of table 9.

Table 9.—Frequency of the different proportions of single element in the combs of offspring of parents having the average proportion of median element given in the column at the left.

The proportion of single-combed offspring in the total filial population is 30.0 per cent, a departure of such magnitude from the expected 25 per cent as to arrest our attention. Further inspection of table 9 shows that the excess of single-combed offspring is found only in the lower half of the series. When the percentage of median element in the parents is under 50 the proportions of I, Y, and no-median combs are as 25.5 per cent, 49.8 per cent, 24.7 per cent, or close to expectation; but when the percentage is 50 or over the proportions are, on the average, 33.6 per cent, 45.2 per cent, and 21.2 per cent, a wide departure from expectation, 1108 individuals being involved. An examination of table 9 shows, moreover, that the proportion of offspring with single comb rises steadily as the proportion of the median element in the parentage increases from 50 per cent. The meaning of this fact is at present obscure, but the suspicion is awakened that, while the "cup comb" and the more deeply split combs are typical heterozygotes the slightly split combs are a complex of 2 or more units, one of which is "single comb." But that this is not the explanation follows for two reasons: first, that even in the F₁ generation slightly split combs are obtained, and, second, that the offspring of the cup combs are much more variable than those of slightly split combs (70 to 90 per cent median). What is strikingly true is that, from 50 per cent up, as the proportion of the median element in the parents increases the percentage of single-combed offspring rises.

The matter may be looked at in another light. Median comb is dominant over its absence. Typically, we should expect F₁ to show a single comb; the Y comb that we actually get is a heterozygous condition due to the failure of the median comb to dominate completely. Typically we should expect F₂ to reveal 75 per cent single combs, of which 1 in 3 is homozygous and 2 in 3 are heterozygous. Owing to the failure of single comb always to dominate completely in the heterozygotes, we expect to find some of the 75 per cent with the Y comb. When in the parents dominance has been very incomplete in the heterozygote (as is the case in the 0 per cent to 40 per cent median-combed parents) we find it so in the offspring also and all heterozygotes show a Y comb of some type. But when in the parents dominance has been strong in the heterozygote (50 per cent to 90 per cent) it is so in the offspring also and only a part of the heterozygotes show the Y comb; the others show the single comb and thus swell the numbers of the single-combed type. The only objection to this explanation is found in the reduction in the percentages of the no-median type. Thus, adding together the homozygous and heterozygous median-combed offspring and comparing with the non-median-combed, we find these ratios:

There is a great deviation from 25 per cent in the "non-median" offspring of the 90 per cent parents, but in this particular case the total number of offspring is not large, and the deviation has a greater chance of being accidental. Altogether this explanation of the varying per cents of single comb on the ground of inheritance of varying potency in dominance seems best to fit the facts of the case.

From the foregoing facts and considerations we may conclude that the Y comb represents imperfect dominance of median over no-median comb; that there is a fluctuation in the potency of the dominance, so that the proportion of the median element varies from 0 to over 90 per cent; that the more potent the dominance of median element is in any parents the more complete will be the dominance in the offspring and the smaller will be the percentage of imperfectly dominant, or Y-combed, offspring. Dominance varies quantitatively and the degree of dominance is inheritable.

The index of heredity may be readily obtained in the familiar biometric fashion from table 9. This I have calculated and found to be 0.301 ± 0.002. This agrees with Pearson's theoretical coefficient of correlation between offspring and parent. The index is larger than it would otherwise be because it is measured with an average of the parents and these parents assortatively mated. But this instance is, in any case, an interesting example of strong inheritance of a quantitative variation.

What, it may be asked, is the relation of these facts to the general principle that inheritance is through the gametes? Why, when a gamete with the median element unites with a gamete without that element, does the zygote develop a soma that in some cases shows a nine-tenths median and sometimes a one-tenth median element? We have seen that the Y comb is a heterozygous form due to imperfection of dominance of the median element; but why this variation in the perfection of the median element? This is probably a piece of the question, why any dominance at all. We find, in general, that the determiner of a well-developed organ dominates in the zygote over the determiner of a slightly developed condition of that organ or its obsolete condition. It is as though there were in the zygote an interaction between the strong and the weak form of the determiner, and the strong won; but sometimes the victory is imperfect. In the specific case of comb the interaction between median and no-median leads to a modification, weakening, or imperfection of the median element, and this weakening varies in degree. Sometimes the weakening is inappreciable—when the comb is essentially single; sometimes it is great, and the result is a comb in which the median element is reduced to one-half; sometimes, finally, the determiner of median comb is so completely weakened by its dilution with "no-median" as not to be able to develop, and we have the cup comb with only a trace of the median element. Nevertheless, such a cup comb is heterozygous and produces both single-combed and Polish-combed germ-cells. Thus the variation in the extent of the median comb seems to point to variations in relative potency of the median comb over its absence.

CHAPTER II.
POLYDACTYLISM.

The possession of extra toes is a character that crops out again and again among the higher, typically 5-toed vertebrates. Many cases have been cited in works on human and mammalian teratology (cf. Bateson, 1904, and Schwalbe, 1906), and it is recognized that this abnormality is very strongly inherited in man. Bateson and Saunders, and Punnett (1902 and 1905), Hurst (1905), and Barfurth (1908), as well as myself in my earlier report, have demonstrated the inheritableness of the character in poultry. Bateson and Punnett (1905, p. 114) say: "The normal foot, though commonly recessive, may sometimes dominate over the extra-toe character, and this heterozygote may give equality when bred with recessives, just as if it were an ordinary DR." Altogether, the inheritance of extra-toe diverges so far from typical Mendelian results as to deserve further study.

A. TYPES OF POLYDACTYLISM.

There are two main types of polydactylism: that in which the inner toe (I) of the normal foot is replaced by 2 simple toes, and that in which it is replaced by two toes, of which the mediad is simple and the laterad is divided distally. The former type is characteristic of the Houdans; the latter is usually associated with the Silkies. Both conditions are, however, found in both races. The simplest condition is seen in many Houdans of my strain. It consists of 2 equal, medium-sized toes (I' and I") lying close together and parallel to or slightly convex towards each other. This condition indicates that the 2 toes, together, are to be regarded as the equivalent of the normal single toe occupying the same position. The 2 toes are, I conjecture, derived from the single toe by splitting. The first series of changes consists of the increase in length of the lateral element (I") and a corresponding decrease of the median element (I'). In the last term of the series there are only 4 toes on the foot, but the inner toe is not like the normal inner toe of poultry, but is a much elongated I".

In the Silkie, also, the series begins with 2 small, closely-applied toes (I' and I"). But when there are only 2 toes the lateral one is usually much the larger. Typically this lateral toe is, as stated, split, so that the nail is double, and the degree of splitting is variable, in extreme cases involving half or more than half of the toe. A second series of changes consists of the gradual reduction of toe I' (often concomitantly with an increase in I") which may end in its entire disappearance and thus reduce the number of toes to 5, but these are not equivalent to the 5 toes of the Houdans, since the extra Houdan toes are I', I", and those of the reduced Silkie are I"a and I"b. Finally, in Silkies, the inner toe (I') may split (more or less completely), and thus the 7-toed condition arises. Moreover, in Houdans I have on one or two occasions found the lateral element (I") bifid distally, resembling perfectly the typical condition found in the Silkies.

A simple nomenclature is suggested for these various types of extra-toes. The simple double-toed condition, as found commonly in Houdans, may be called the duplex type (D). The loss of I' gives the reduced duplex (D'). The case of split I", as commonly seen in the Silkie, is the triplex type (T); with the loss of I' this becomes the reduced triplex (T', not duplex!). The 7-toed condition of Silkies may be called the quadruplex type (Q); the combination split I' and single I" gives the reduced quadruplex (Q').[3]

The reduction that leads to the loss of I' consists of a loss of phalanges, as Bateson (1904) has already pointed out. It seems probable that the reduction affects first the proximal phalanges, since the distal nail-bearing phalanx is the last to disappear.

B. RESULTS OF HYBRIDIZATION.

First let us consider the result of mating extra-toed individuals belonging to "pure" extra-toed races. A typical Houdan cock (D type), of the well-known Petersen strain, was mated with 3 hens bred by me, but derived, several generations before, from the same strain. With the first hen he got 29 chicks, all with the extra-toe except one (3.3 per cent) that had 4 toes on both feet and two that had 4 toes on one foot and 5 on the other, i. e., one foot simplex and one duplex. With the second he got 12 chicks, of which one had 4-5 (D) toes. The third, in 26 young, gave one with 4 toes on each foot. Thus, in 67 chicks altogether there were 2, or 3 per cent, with the normal number of toes on both feet (4-4). Unfortunately these birds did not survive, so it is not known whether they would have thrown as large a proportion of extra-toed offspring as 5-toed Houdans. Bateson's Dorkings gave about 4 per cent of 4-toed offspring. Of the 83 offspring of 6-toed Silkies, 3, or 3.6 per cent, had 4 toes on each foot. Even in pure-bred polydactyl races, consequently, the character "extra-toe" does not uniformly appear in the offspring.

Let us consider next what happens when a polydactyl individual is crossed with a normal individual. Table 10 gives the results of all matings of this sort and its most obvious result is that the polydactyl condition reappears in every family, but not, as in typically Mendelian cases, in all of the offspring; at least this is true of the Houdan crosses. In the Silkie crosses the 6 offspring given as having the single thumb may possibly have been of the type D', as that type was not in mind at the time of making the record and was not always distinguished from type S. It is also clear that the offspring of Silkie crosses are more apt to be polydactyl than those of Houdan crosses. For 27 per cent of the latter are non-polydactyl, while, taking the table as it stands, at most only about 4 per cent and (as just stated) probably none of the Silkie offspring were of the typical single-thumbed type. Also the average degree of polydactylism is much greater in the Silkie than in the Houdan crosses. This excess is in part due to the different method of counting toes in the Silkie and the Houdan hybrids; for whereas in the latter the visible toes are counted as equivalent units, in the former in the case of each reduced type one unit more is assigned than appears. The actual number of toes occurring in the Silkie hybrids was also calculated, and it was found that this still averaged higher than that of the Houdans (9.45 as opposed to 9.26).

Table 10.—Frequency of the various types of toes in the first hybrid generation between a normal and an extra-toed parent.

In hybrids of both classes the greatest number of toes occurring on one foot never exceeds the greatest number possessed by its parents; indeed, the most polydactyl hybrids of the F₁ generation of Silkies never have as many as 6 toes on one foot. This result is not to be explained as due to a regression towards the 4-4-toed condition, but rather as due to the intermediate condition of the heterozygote. For 80 per cent of the hybrids show either the typical or the reduced D type on one or both feet, although neither parent exhibits these types.

We have next to consider the results of mating together the F₁ hybrids. Table 11 gives the results of all matings of this sort.

Table 11.—Frequency of the various types of toes in the second hybrid generation between normal and extra-toed races. Lettering as in table 10.

Comparing tables 10 and 11, it is at once clear that in the second hybrid generation the proportion of extra-toed offspring has decreased. This accords with expectation, if extra-toe is dominant, for then only 75 per cent would be of the dominant type in F₂, while 100 per cent would be of that type in F₁.

Table 12 will enable us to analyze the difference of the proportions in tables 10 and 11.

Table 12.—Percentages of the various types of toes in F₁ and F₂ of the polydactyl hybrids compared.

These tables yield several points of interest. First, although the proportions of normal and extra toe in table 12, a and c, are not Mendelian, yet the average increase, from F₁ to F₂ in the proportion of the recessive (4-toed) type is almost exactly what is called for by Mendel's law. That law calls for an increase of 25 per cent. The actual average increase is 23.3 per cent (20.1 and 26.5 in the two cases). It seems fair to conclude, consequently, that Mendel's law does hold here, and that the 4-toed individuals of F₁ are heterozygotes with imperfect dominance. The feet of most of the 4-toed Silkies of this generation belong, indeed, to the reduced 5-toed type (table 10, B), and the reduced condition is prima facie evidence of heterozygotism. In F₁ Silkies of the first hybrid generation, 20 per cent of the feet exhibit "reduced" types of toes, but in F₂ only 5 per cent; and this might have been anticipated, since in F₂ heterozygotes are relatively only half as numerous as in F₁. Again, in F₂ we see reappearing the high ancestral toe-numbers (practically lost in the heterozygotes of F₁, table 12, b). These I interpret as extracted dominants. 6-toed extracts are more numerous among the Silkie than the Houdan hybrids, because the Silkie ancestors were 6-toed and the Houdan ancestors only 5-toed. However, only a small proportion of the extracted Silkie dominants have as many toes as the original Silkie ancestors, and this indicates a permanent regression (through the contaminating influence of hybridization?) toward the normal condition of toes. It will be observed that, although 6 toes are not found in the Silkie hybrids of F₁, many of these heterozygotes are of the reduced triplex type. Classifying them as virtually 6-toed, we find (table 12, c) 14.5 per cent of the 6-toed type in the F₁ generation.

Among the extracted dominants of F₂ are a few showing more toes than appeared in the ancestors (table 12, a; there was also one 7-toed F₂ Silkie hybrid, not recorded in the table). It is this sort of an advance in F₂ that permits the breeder to make a forward step. Theoretically, the appearance of this more aberrant class is probably due to the greater numbers of progeny than of ancestors, since the extracted dominants of F₂ are seven times as numerous as their extra-toed grandparents. Here, as elsewhere, the absolute range of variability depends upon the number of individuals observed.

Table 13.—Distribution of toe-numbers in the offspring of DR × R matings.

Table 14.—Distribution of toe-numbers in the offspring of DR × D matings.

Table 15.—Percentages of the various types of toes in F₁, F₂, DR × R and DR × D matings of the polydactyl crosses compared.

As we have seen, failure of dominance is much more complete in some of the individuals of F₂, namely, those with 4 toes, than others. There is a variation in "potency." Is the degree of potency inherited? Do the 4-toed heterozygotes produce a larger proportion of imperfect dominants in F₂ than the 5-toed heterozygotes? The answer to this question should be given by the correlation between total number of toes in the two parents and average number of toes in their offspring, as given in table 11. In the case of the Houdan crosses there is a strong positive correlation, measured by 0.683 ± 0.092; but the correlation is insignificant in the Silkie crosses (-0.085 ± 0.032). This lack of correlation in the Silkie hybrids is perhaps due to the heavy regression in toe-number characteristic of the second hybrid generation. In general, there seems to be an inheritance of potency.

It now remains to test our conclusions by reference to the mating of the heterozygote with the dominant and with the recessive types, respectively. An examination of tables 13 to 15, particularly the last, reveals several points of interest. Mendelian expectation in the DR × R cross is 50 per cent of the recessive (4-4) type. Actually, in the two crosses, A and B, 68 per cent and 67 per cent, respectively, were obtained. But recalling that of these amounts one-half of 27.3, or 13.71, and one-half of 9.4, or 4.7, are respectively due to failure to develop the extra-toe in heterozygotes, there remain 54 per cent and 62 per cent, respectively, of 4-toed offspring, which doubtless represent the extracted RR type and approach the expected proportions.

Mendelian expectation in the DR × D cross (table 15) is 50 per cent heterozygotes and 50 per cent extracted dominants. Of the heterozygotes some 14 per cent may be expected to show 4-4 toes; that the percentage is much less than that is doubtless due to the small numbers involved. What is striking is the reappearance, in the second generation, of large proportions of the extreme dominant type. These results thus confirm those of the F₂ generation.

Since extra-toe frequently fails to dominate, there should be certain 4-toed heterozygotes which throw extra-toe offspring, and such are found. In table 16 are given six matings of 4-toed DR's. One sees that they produce some 5-toed offspring. On the other hand, extracted 4-toed recessives are obtained, as table 17 shows.

Finally, we must consider whether, among the polydactyl birds of one class, e. g., Houdans or Silkies, there is any difference in the "centgener power" of parents corresponding to the degree of development of their extra toes. This inquiry is suggested by Castle's study (1906, p. 20) of polydactyl guinea-pigs. He finds that when the extra toes of the mothers are graded into the 5 classes, good (G), fair (F), poor (P), normal though of abnormal ancestry (N), and normal of normal ancestry (N'), it follows: "first, that the proportion of polydactylous young produced by a male decreases in the successive classes from G to N'; and, secondly, that the degree of development of the toes produced on those polydactylous young diminishes in the same order." It is possible to test this conclusion in poultry because, inside of any one type of extra-toe, e. g., the triplex type, variation appears in the absolute size of the toes and in the degree of their separateness. Our questions, then, are: (1) does the proportion of polydactyl young produced by a pair of birds of any type diminish with the degree of development of toes inside of that type, and (2) does the degree of development of the toes produced on the polydactylous offspring diminish in the same order?

Table 16.—Distribution of toe-numbers in the offspring of 4-toed heterozygotes.

Table 17.—Distribution of toe-numbers in the offspring of extracted 4-toed parents.

Two sets of data are available for answering these questions. The most direct set includes the data derived from crossing "pure-bred" polydactyl birds and the other includes the data derived from using hybrids between normal-toed and polydactyl ancestors. The latter data have the advantage that the parents offer a greater variability; but they have the disadvantage that the germinal condition of those parents is incompletely known.

The pure races may be considered first. Eight matings of Houdans, each parent with 5 toes, gave 122 offspring, of which 116 had 5-5 toes, 3 had 4-5 toes, and 3 had 4-4 toes. The variability of the toes is not great in the parent Houdans. But, arranging them in the order of development of the toes, the most developed first, the series of table 18 results.

Table 18.

No direct relation here appears between development of the extra toe in the parents and the average number of toes in the offspring.

Of the Silkies, 3 hens were used in 5 matings. The same 6-toed cock (No. 774) was employed throughout (table 19).

Table 19.

In table 19 the series a of observed average numbers of filial toes (10.3, 10.9, 10.0) and the series b obtained by assigning the typical full number to all reduced types (11.4, 11.4, 10.5) are decidedly irregular. There is, however, between the parental and the filial series a correlation of +0.250 ± 0.070. This indicates a slight tendency for the number of toes in the progeny to vary with those of the parentage.

The second set of data is derived from special matings made with hybrids between Houdans and 4-toed races. On the one hand, in pens 728 and 813, cocks with well-developed toes of the duplex type were mated with hens as nearly as possible of the same sort; while in pens 765, 769, and 820 cocks with small, imperfectly separated toes (probably of the duplex type[4]) were mated with hens as far as possible of the same sort.

Tables 20, 21, and 22 give in detail and in summary the distribution of types of polydactylism in the families from well-developed and in those from poorly developed parents. They show a great difference between the offspring of parents with good extra-toe (table 20) and those with poor extra-toe (table 21). The former yield over 80 per cent offspring with 5 toes or more on one or both feet, while the latter yield about 57 per cent of such.

On the other hand, in the former families there are less than half as many offspring with only 4 toes as in the latter. Classifying "reduced" forms with their proper advanced type, we find highly polydactyl parents yielding only 16 per cent non-polydactyl offspring, while slightly polydactyl parents yield 43 per cent non-polydactyl offspring. The percentage of polydactylous young diminishes with the size and distinctness of the extra toes and the grades of the polydactyl offspring are lower (absence in table 22, b, of 6 toes). Both of Castle's conclusions seem to be confirmed.

Table 20.—Distribution of toe-types in the offspring of "good" extra-toed parents.

Table 21.—Distribution of toe-types in the offspring of "poor" extra-toed parents.

But a more critical examination of the parentages of the 5 pens shows that they are not comparable. In matings 6 to 14 of table 20 the cock is almost certainly a dominant in respect to toes; whereas the cocks in table 21 are probably heterozygous. The heterozygous state determines two things: the imperfect nature of the extra-toe and a relative deficiency in the offspring of the higher toe-numbers. In our results we can not say that one of these things is the cause of the other, as Castle does; they are, rather, in all probability, due to a common cause. I think Castle's paper may justly be criticized for not giving sufficient data concerning the ancestry of the individual mothers used. Without such data the paper can not be said satisfactorily to demonstrate his conclusion.

Table 22.—Summary of observed toe-numbers in offspring, percentages.

To summarize: "Potency," as measured by dominance of the extra-toed condition, is inherited, in the Houdan crosses at least. There is some evidence, derived from "pure-bred" Silkies, that differences in the degree of development of the extra-toes are inherited. But the average condition of the toes in the offspring of second or later generation hybrids can not be used as evidence of inheritance of the degree of parental development of the toes, since these are dependent on the same basal cause, namely, the hidden gametic constitution of the parents. Despite the obscuration of imperfect dominance, polydactylism in poultry proves itself to be a unit-character that segregates. [28]

CHAPTER III.
SYNDACTYLISM.

A. STATEMENT OF PROBLEM.

In man, various mammals, and some birds two or more adjacent fingers are sometimes intimately connected by an extension of the web that is normally a mere rudiment at their base. Such a condition is known as syndactylism. A good introductory account of syndactylism is given by Bateson (1904, pp. 356-358). Taking a number of cases of syndactylism together, he says: "A progressive series may be arranged showing every condition, beginning from an imperfect webbing together of the proximal phalanges to the state in which two digits are intimately united even in their bones, and perhaps even to the condition in which two digits are represented by a single digit." He also calls attention to the fact that in the human hand "there is a considerable preponderance of cases of union between the digits iii and iv;" while in the foot the united digits "are nearly always ii and iii." The matter of syndactylism in birds has a peculiar interest because of the fact that among wading and swimming birds syndactylism has become a normal condition of the feet, and, moreover, just this feature is one that has become classical in evolutionary history, because Lamarck thought it well illustrated his idea of the origin of an organ by effort and use.

Concerning the cause of syndactylism little can be said. Both in mammals and birds the digits are indicated before they are freed from lateral tissue connections. The linear development of the fingers is in part accompanied by a cutting back of this primordial web, in part by a growth beyond it. In syndactylism growth of the web keeps pace with that of the fingers. From this point of view syndactylism may be regarded as due to a peculiar excessive development of the web.[5] In some human cases adhesions of the apex of the appendage to the embryonic membranes has stimulated the growth of the interdigital membrane, resulting in syndactylism. But it would be absurd to attempt to explain syndactylism in general on this ground. The more "normal" forms of syndactylism, as seen in poultry, still want for a causal explanation.

Most of the cases of syndactylism whose inheritance is about to be described arose in a single strain of fowl and can, indeed, be traced back to a single bird. This ancestor is No. 121, a Dark Brahma hen described in a previous report.[6] It was only in the search for the origin of the exaggerated forms of syndactylism observed in some of her descendants that an unusually great extension of the web in her feet was noticed. The syndactyl condition of my birds did not, thus, arise de novo, but had its origin antecedent to the beginning of the breeding experiments. In addition to this main strain a slight degree of syndactylism has appeared among some of my Cochin bantams.

Table 23.—Ancestry of syndactyl fowl and the results of various matings involving syndactylism.

[Abbreviations: Abα, Abβ, etc., types of syndactylism ([p. 32]); F, father; FF, father's father; FM, father's mother; M, mother; MF, mother's father; MM, mother's mother; M × P, hybrid of Minorca and Polish races; Synd., syndactyl (type unknown). f, foot. In Nos. 24 to 42 two cocks (Nos. 242 and 3116, and 5399 and 4562, respectively) were at different times used.]

Table 23.—Ancestry of syndactyl fowl and the results of various matings involving syndactylism—Continued.

The types of syndactylism which have appeared in my flock form a rather extensive series. First, (A) the single web, which, in my specimens, always occupies the interspace between digits iii and iv. This is the same interval which is most apt to show the web in syndactylism of the human hand, and, it is suggestive to note, it is this interval that is filled in those wading birds that have the single web only between the toes (e.g., Cursorius, Glareola, Vanellus, Squatarola, Charadrius, Limosa, Machetes, Himantopus); second, there is (B) the double web, one-seventh as common, which always occupies the interspaces between the digits ii-iii and iii-iv.

On another basis, the syndactyl feet may be classified as: (a) toes adherent, web small in extent, and (b) toes distant, web broad. I have found the narrow web only between digits iii and iv. It is one-eighth as common as the broad-webbed type. The broad, double web approaches closely to the type found normally in swans, geese, and ducks.

Finally, the syndactyl feet may be classified as: α, straight-toed, or β, curve-toed. Class α is to class β in frequency as 2:1. In the typical curve-toed syndactyl foot the web between iii and iv is complete to the nails of each; in fact, in extreme cases the nails of the two toes are more or less fused together. From the fused nails the middle toe, being the longer, passes in a curve to the distal end of the metatarsus. The D-shaped interspace between the curved iii and straight iv toe is filled with the web. In other cases the nails are merely approximated and the middle toe is slightly curved. In three instances (4 per cent of all) the outer toe (iv) is curved toward the (straight) median toe (class β´).

As stated, the polydactyl offspring trace back their ancestry to No. 121; her feet both show the double, broad, straight-toed type (Bbα). We shall attempt in the following paragraphs to trace the heredity of her type of polydactylism and of the others that have subsequently arisen.

B. RESULTS OF HYBRIDIZATION.

In taking up the results of breeding experiments to test the method of inheritance of syndactylism, it will be best first to give in a table all pens in which the character showed itself, with the frequency of the different types of foot in them (table 23).

The history of the syndactyl strain begins with No. 121 ♀ and in the matings 1 to 8 are given the results of crossing together some of her progeny derived from a normal-toed father. This father was either No. 8a or 1a, both full-blooded Tosa (Japanese Game) fowl and without suspicion in either soma or offspring of syndactyl taint. There is no record of trace of syndactylism in the progeny of 121 × 8a (or 1a); but a slightly developed condition of syndactylism may very well have been overlooked by me in this F₁ generation (as I had never thought of such an abnormality), even as I at first overlooked the syndactylism visible in No. 121. But when these F₁ hybrids were mated together (pen 627, serial Nos. 1 to 8) I got, in the different families, from 10 per cent syndactyl offspring down to none at all.

At first sight the suggestion arises that, if inheritance is at all Mendelian, the normal condition is dominant and that the heterozygotes throw again, in pen 627, the syndactylous condition. If this hypothesis were true it would follow that syndactyls bred together should, sometimes at least, throw, even in large families, 100 per cent syndactyl offspring. But only 2 families, Nos. 30 and 34, have yielded 100 per cent syndactyls, and these contained 2 and 1 offspring, respectively; so they are not significant. On the other hand, there are numerous matings of 2 extracted normal-toed parents that have produced only normal-toed offspring (families Nos. 14, 15, 21, 22, 23, including 119 individuals). Consequently the conclusion is favored that normal-foot is recessive and syndactyl-foot dominant, and this shall be our working hypothesis.

On our hypothesis, No. 121 is probably a heterozygote. Mated with the recessive normal, expectation is 50 per cent heterozygous, showing syndactylism; the remainder normal-toed. But dominance is here, as in polydactylism, very imperfect. For this reason and because it was not looked for, no syndactylism was noted in the first hybrid generation. The offspring prove to be of two sorts, however. No. 180 ♂ is a pure recessive, and in 8 matings with as many different sisters of his he got 184 normal-toed to 1 syndactyl. These same sisters, mated to another brother, No. 242, in some cases gave 9 per cent and 10 per cent syndactyl. No. 242 is, consequently, probably a DR and, mated to DR sisters (which constitute according to expectation about one-half of all) gives some DD's, part of which constitute the 9 to 10 per cent of syndactyls. Of course, 25 per cent DD is to be expected; the difference gives a measure in this instance of the imperfection of dominance in the "extracted" as well as "heterozygous" condition.

Matings 9 to 15 (pen 747) are instructive in comparison with the foregoing case. Both parents are derived from pen 658, which contained as breeders a heterozygous Dark Brahma male (No. 146) and various females of non-booted races far removed from suspicion of syndactylism; expectation being an equal number of DR and RR offspring. In pen 747 No. 1888 ♂ acts like a DR, and so do the hens in matings 9 to 13, while the hens in the other 2 matings are doubtless RR's. The former give 17 per cent syndactyl offspring, the latter none at all (in 56 individuals).

Matings 16 and 17 (pen 703) are between pure-bred Dark Brahmas that are probably DR's. About 22 per cent of their offspring are syndactyl—a rather higher proportion than we have found before. Matings 18 to 19 are between progeny of pen 627. In mating 20 the normals were not recorded. The cock in this pen, No. 871, is probably heterozygous, as are also the first two hens, so that nearly 30 per cent of their progeny are syndactyl. From the other 3 hens no syndactyl offspring were obtained. Evidently the two sets of hens have a very different gametic constitution. The existence of two sorts of families is one of the strong arguments for the segregation of this character.

We next come to the pens (matings Nos. 24 to 42) which were especially mated to study the inheritance of syndactylism. I had now, for the first time, two parents with syndactylic feet.

On account of imperfection of dominance decision as to gametic composition of any parent must largely rest on the make-up of the progeny. Table 24 gives the most reasonable classification of the parentages.

Table 24.

Summarizing the foregoing, and comparing the totals with Mendelian expectation, we get the result shown in table 25.

A comparison of realization and expectation in table 25 shows that the proportion of syndactyls is always less than expectation, not only for dominants and heterozygotes together, but even for pure dominants alone. The proportion of syndactyls obtained diminishes, to be sure, in accordance with expectation (on the assumption that they are pure dominants), but the numbers lag behind, in the higher proportions 40 to 25 per cent. So we reach the conclusion that, as in polydactylism, so in syndactylism dominance is very imperfect. But there is this difference, that in syndactylism dominance is so imperfect that the dominant condition rarely shows itself in heterozygotes and even fails in many pure dominants. The striking fact, the one that assures us the segregation is nevertheless occurring in this case too, is that some families (whose two parents are extracted recessives) throw 100 per cent recessives.

Table 25.

These studies on syndactylism in poultry may be used for a critical examination of the recent work of Lewis and Embleton (1908) on syndactylism in man. The cases described by them follow the types I have just described in poultry. Their fig. 18 corresponds to my types a and α; figs. 10 and 11 to my type β. The "crossbones" referred to by the authors correspond to bones of the "curved toe." The facts presented by the authors support the idea that syndactylism is dominant rather than recessive, but they deny the application of Mendelian principles to this case. Actually, the foot deformities described by Lewis and Embleton are inherited much like syndactylism in poultry. No extracted normal (recessive) extremity produces the abnormal condition. Heterozygotes show much variation, from very abnormal to slightly abnormal (possibly perfectly normal?) appendages. Dominance is, indeed, much more potent than in poultry.

The authors' denial of the application of Mendelism to this case seems to be based on an all too superficial consideration of the hereditary behavior of the character and a tendency to "mass" statistics—a procedure that tends to obscure the interpretation of the data of heredity.

As to the inheritance of type, my statistics are not extensive enough to give a final answer, but if all types be grouped into those with straight and those with curved toes, then in crosses of straight-toed syndactyl and normal 33 per cent of the offspring were of the curved type, whereas in crosses of curved-toed syndactyls and normal 45 per cent were of the curved type. These averages depend on 22 and 15 individuals, respectively. They lead us to look for an inheritance of type when more extensive data shall be available.

Syndactylism is a typical sport, that is, a rather large mutation having a teratological aspect. The question arises, Does it prove to be prejudicial to the welfare of the species? The breeder who has only a few individuals of a rare sport feels their loss more than that of normals and the general impression left in his mind is that the sport is less capable of maintaining itself than the normal form. Assembling the data, consisting of about 40 individuals of each kind, it appears that the death-rate is not very different in the two lots; the slight excess of that of the syndactyls is sufficiently accounted for by the circumstance that no normals were reared during the period of greatest mortality (the summer), but were destroyed or given away as soon as hatched. It is probable, therefore, that syndactylism, under the conditions of the poultry-yard, has little life and death significance, but is one of those neutral characters whose existence Darwin clearly recognized.

CHAPTER IV.
RUMPLESSNESS.

The tail of vertebrates is, historically, the post-anal part of the trunk. Containing no longer any part of the alimentary canal, it has lost much of its primitive importance, so that its disappearance in any case is a matter of relatively little importance. Accordingly we find groups of animals in which it is rudimentary or wholly absent, such as many amphibia and the anthropoid apes and man. In all recent birds the tail is a distinct but much reduced organ—the uropygium—which contains several vertebræ in a degenerate condition. The uropygium supports the tail feathers, which are of much use in directing the bird in flight, but in ground birds, such as the grouse and poultry, seem to function only for display in the male and, in the female, to facilitate copulation.

Now, among various typically tailed vertebrates the tail is sometimes absent. Tailless dogs, cats, sheep, and horses are known; on the other hand, several cases of tails in man have been described (Harrison, 1901). Thus the tail is a part of the body subject to sporting; and it has also become the differential character for some specific groups. In other words, it is an organ that has played an important part in evolution and consequently its method of inheritance is a matter of great interest.

The origin of the tailless poultry which I have bred has been twofold. The most important strain is that referred to in an earlier report[7] as Bantam Games. The second lot consists of rumpless fowl that have arisen in my yards, spontaneously, from normal blood. Of these more later.

The two rumpless Game cocks bore the numbers 117 and 116. Dr. A. G. Phelps, of Glens Falls, New York, from whom the birds were purchased, wrote that he had imported No. 117 from England, and No. 116 was its son. The birds were very closely similar in all external features.

The matings made with No. 117 and their results are given in table 26.

Table 26.—Progeny of tailless cock and tailed hens.

In 25 cases of the 52 an oil-gland was looked for and, in every case, it was found to be missing.

Table 26, the conclusions from which were drawn in my 1906 report, seemed to indicate the dominance of tail over its absence. On this hypothesis I suspected that if No. 117 were bred to his (tailed) offspring about 50 per cent of the progeny would be tailless, and if the tailed hybrids of the F₁ were bred together about 25 per cent of their progeny should be tailless. The actual result of such matings is shown in table 27.

Table 27.—Heterozygotes mated with father.

Table 28.—Heterozygotes mated inter se.

The results given in tables 27 and 28 are remarkable. Neither in the DR × R nor the DR × DR crosses did the tail fail to develop. The tailless condition, that I had strongly suspected of being recessive and expected in 25 per cent to 50 per cent of the offspring, never once appeared. The only point of variation in the uropygium of the chicks derived from the back cross or from F₁'s bred inter se was that in some the uropygium seemed distinctly smaller than in the others. This small uropygium was as a matter of fact recorded chiefly in chicks that failed to hatch, but it was occasionally noticed in older birds, being then usually associated with a slight convexity of the back. In some of the families the uropygium is recorded as small in suspiciously close to 25 per cent of the offspring. There is little doubt in my mind that this small uropygium represents in some way the "absence" of tail that was expected.

The next step was to cross the other rumpless bantam (No. 116), to see if he behaved like his father. Accordingly, in pen 653, I replaced the cock No. 117 by 116, the hens remaining the same, and got the result shown in table 29.

Table 29.—Heterozygotes mated with No. 116.

Here we get a result almost exactly in accord with Mendelian expectation. Having, now, obtained rumpless hens, it became possible for the first time to test the inheritance of rumplessness in both parents. The result is shown in the table 30.

Table 30.—Rumpless fowl mated inter se.

Table 30 is unfortunately small; one may say, fragmentary. Rumpless hens are incapable of copulating unless the tail coverts are trimmed; moreover my birds have been so much inbred that they are very weak; finally, the chicks are so small that it is impracticable to rear them in brooders and the eggs are particularly apt to be broken by the brooding hens. However, it suffices to show that two tailless fowl are able to throw some tailed offspring.

The second lot of rumpless fowl, namely, those that arose de novo in my yards, must now be considered. In 1906, 2 birds hatched out from ordinary tailed strains. As one was a cock and the other a hen these were mated in 1907. The cock (No. 2464) came from No. 71♀ (a pure White Leghorn bred by myself from original White Leghorn stock described in my 1906 report) and No. 235♂ (an F₁ hybrid between one of these White Leghorns and my original Rose-comb Black Minorca). The hen was No. 1636. Her mother (No. 618) was an F₁ hybrid between a Minorca and Dark Brahma of series V, 1906 report, and her father (No. 637) had the same origin. Thus the parents and grandparents of both of these new rumpless birds were well known to me and known to be fully tailed and to throw only tailed birds, with the exception of these two birds.

The result of the mating of Nos. 2464 and 1636 in pen 736 was 25 chicks, of which 24 had tails and 1 (No. 5335) was without tail or oil-gland. This, unfortunately, died early, so it was impossible to breed it. In 1908, the hen No. 1636 having in the meantime died, I mated No. 2464♂ to 6 of his (tailed) daughters. He was not well and soon died, leaving no descendants by them, but 5 offspring by a female cousin, all tailed. Then one of his sons (tailed) was mated to its own sisters and produced 49 offspring, all tailed. Thus the strain seems to have died out. The whole history is important both because an apparently new mutation had taken place and because it was, in a degree, "hereditary."

How, if at all, can this case and those of the bantams be brought under known laws of inheritance? First of all, it must be confessed that the provisional hypothesis, suggested in my earlier report, that rumplessness is in my strain recessive has not been supported by the newer facts. In the light of the principle of imperfect dominance to which the facts of the last two chapters have led us, everything receives a satisfactory explanation. The only conclusion that meets all the facts is this: The inhibitor of tail development—the tailless factor—is dominant; its absence—permitting a continuation of the normal development of the tail region—is recessive.

The application of this hypothesis to the various matings may now be attempted. No. 117 is to be regarded as a heterozygote. The matings with tailed birds is of the order DR × R, and expectation in the typical case is 50 per cent DR (interrupted tail) and 50 per cent RR (non-interrupted). But, owing to the relatively weak potency of the interrupter derived from No. 117, growth of the tail is not interrupted in the heterozygous offspring. These offspring are, by hypothesis, so far as their gametes go, of two equally numerous sorts, DR and RR. Mated to No. 117, two sorts of families are to be expected, namely, the products of DR × RR (=50 per cent DR, 50 per cent RR) and the products of DR × DR (=25 per cent DD, 50 per cent DR, 25 per cent RR). The first lot of families might be expected to resemble the preceding generation in consisting entirely of tailed birds; the latter might be expected to show in the 25 per cent extracted DD's evidence of the presence of the undiluted interrupter. Actually in matings of the latter sort (table 27) 3 families show no trace of the tail-interrupter, but in 7 there is evidence of a disturbance, as shown by the small size of the uropygium and the bent back. In these families there are 13 cases of small uropygium to 53 of large, being about 20 per cent of the affected uropygium where 25 per cent was to be looked for—not a wide departure, considering the liability of not recognizing the reduced uropygium as such. This failure even of the extracted dominants completely to stop the development of the tail gives a measure of the weakness of the inhibitor in this case. Also, in table 28, matings are varied. Some are probably matings of two heterozygotes, others of two recessives, and others still of a recessive with a heterozygote. On our hypothesis we should expect some of the families of the mated hybrids to show evidence of the inhibiting factor and others to show no such evidence. In those families in which small tail appears it is found in about 19 per cent of the cases. On account of this weakness of the inhibitor in the germ-plasm of No. 117 that inhibitor is rarely fully activated. Only in one case out of the 250 or more in which that germ-plasm is used is the development of the tail completely stopped. In this case a hybrid cock derived from pen 526 (series 2, table 26) was crossed with various birds of tailed races (probable RR's), and produced in addition to 20 tailed offspring 1 devoid of uropygium and oil-gland. In this case we may conceive that an unusually potent condition of the inhibitor wholly stopped the development of the tail.

The behavior of No. 116 is that of a pure dominant. Mated to DR (and some RR?) females he produces pure dominants and heterozygotes. His inhibiting factor is potent enough to be active in the DD offspring at least; as a matter of fact 47 per cent of his get have their tails inhibited. Even in the DR's the inhibitor may sometimes work itself out. Thus No. 116 crossed on No. 508, without tailless ancestry, had 56 per cent of the progeny without tail. Since tailless birds may be either pure dominants or DR's, we may expect families of two sorts when two such are bred together—those containing only tailless offspring and those containing only 75 per cent or less of such. Both sorts of families are to be expected in a table with the composition of table 30, and both appear there.

The case of the rumpless fowl that arose de novo will be explained, then, as follows: Even in normal RR matings the inhibiting factor may arise by mutation. But even when two of these inhibiting factors are paired they show themselves so weak as not to appear in 25 per cent, much less the typical 75 per cent of cases, but, as in our case, merely 4 per cent. The strain takes on, indeed, the essential features of the "eversporting varieties" of De Vries (1905). It seems probable, therefore, that even in eversporting varieties inheritance may be Mendelian, modified by variations in "potency" as shown by irregularities in dominance.

CHAPTER V.
WINGLESSNESS.

The entire absence of appendages is a rare monstrosity, few cases having been cited even for man. In my experience with poultry, out of about 14,000 birds I have obtained one that had no wing on one side of the body, but this unfortunately died before being bred from. A second bird was given to me by a fancier. The bird was an Indian Game, a vigorous cock, which was handicapped by his abnormality in two ways. First, whenever he fell upon his side or back he was unable to get upon his feet without aid. On several occasions he evidently had spent hours upon the ground before he was discovered and picked up. The wings are thus clearly most important to the fowl in enabling it to regain its feet after having become prone. Secondly, he was unable to tread a hen, since this act requires the use of wings as balancers. He was, however, able to copulate with small birds without leaving the ground. Thus in two respects his abnormality would have proved fatal in nature. First, because of the personal risk, the greater since a prone bird must fall an easy prey to predaceous enemies; and secondly, because of the risk to his germ-plasm. Little wonder, then, that this abnormality should not be known among wild ground-birds.

Mated to 6 hens this wingless cock produced 130 chicks in 1907, of which all had two wings. The following year he was mated to his daughters, but died without leaving offspring. So I used a son of his to mate with his own sisters and half-sisters. The progeny in this F₂ generation consisted of 223 chicks, all of which had two wings. Thus, no trace of winglessness appeared in any of the descendants of the wingless cock.

The explanation of this case is not very certain, in view of the limited data. It seems to resemble the behavior of No. 117, the rumpless cock. And following the interpretation given in his case I would conclude that winglessness is dominant to the normal condition, that the original wingless cock was a heterozygote, and that the dominance of winglessness was imperfect in the first generation. On this hypothesis his son may well have been a pure recessive, and then all of his descendants, in turn, would be either recessives or heterozygotes (with imperfect dominance). It is, on the other hand, possible that the wingless cock was a pure dominant, but that the potency of the inhibitor was so slight as not to appear in the heterozygotes or even in extracted dominants.

CHAPTER VI.
BOOTING.

The method of inheritance of the feathering on the feet of some poultry has already been made the subject of much study. Hurst (1905, p. 152) crossed booted and non-booted birds and bred the hybrids together. He concluded that "the Mendelian principles are at work in these aberrant phenomena, but are masked by something not yet perceived." My own conclusion (1906, p. 72) was: "Booting is dominant, but usually imperfectly so." A more extended study has been desirable.

Booting is variable in amount. To indicate its degree I have had recourse to an artificial scale. I recognize 11 grades, running from 0 to 10. The grade 0 implies no feathers whatsoever. Grade 10 implies heavy booting extending over the front half of the shank. Grade 5 implies an extent of only half of the maximum, i. e., the outer front quarter of the shank. Intermediate grades indicate intermediate extension of the feathered area.

A. TYPES OF BOOTING.

The races of booted poultry used have been as follows: First, bantam Cochins of two varieties; second, a bantam Dark Brahma; and third, the Silkie. In my representatives of the first two groups, but particularly in the Dark Brahma, the amount of booting is variable. In one type the outer third of the shank in the newly hatched chick is covered by strong, heavy, specialized feathers, directed outward, while the middle and inner thirds are covered by smaller, finer, imbricating feathers sparsely placed and resembling reduced contour-feathers. In most individuals the transition from the one kind to the other is gradual, while in others it is sharp, and in a few the outer third only of the shank is feathered. In the Silkies, which the standard poultry books describe as being more sparsely feathered on the shank,[8] the outer zone of feathers is the only one developed; and, occasionally, as table 31 shows, even these feathers may be lacking. We have thus two types to distinguish—the extended (Cochin, Brahma) type and the restricted type.

B. NORMAL VARIABILITY.

To appreciate the results of hybridizing we must first examine the variability of pure-blooded races. This is done in table 31.

Table 31.—Distribution of boot-grades in the offspring of Cochin, Dark Brahma, and Silkie parents.