Inheritance of Characteristics in Domestic Fowl/Chapter I

 CHAPTER I.

THE SPLIT OR Y COMB.

A. INTERPRETATION OF THE V 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.

The first and third hypotheses will give the same statistical result, namely, the products of two Y-combed individuals of F$1$ 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 F$1$, or F$2$, or F$3$, 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 F$1$ 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 F$2$ zygotes (not all different) as shown in table 2.

* This convenient form of zygotic formulæ, using a subscript 2 instead of doubling the letter, is proposed by Prof. W. E. Castle.

It is a consequence of this second hypothesis that, in F$2$, of every 16 young 9 should have the Y comb; 3 the I comb; 3 the oo comb, and I no comb at all. It follows further that the progeny of two F$2$ 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 F$2$ should yield, in F$2$, 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 F$2$ should yield, in F$2$, 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: absent, 25 per cent.
 * 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;

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 F$2$ 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 F$2$ 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 F$2$ progeny of two Y-combed parents, we get the distribution of families shown in 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.

The progeny of two extracted single-combed parents of the F$2$ generation give in 3 families the following totals: Of 95 F$2$ 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.

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 F$2$ parents. The non-median comb, consequently, probably contains only non-median germ-cells.

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.

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 x Y cross, disregarding the generation of the parents, we get a total I comb 5,* 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 ooxY 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.

Finally, we must consider the result of mating a bird without papillae (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.† 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 x 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

 * Excluding 6 doubtful because from too young embryos and not observed by myself. † One is reported as having a I comb; probably the limiting condition, again.

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