Inheritance of Characteristics in Domestic Fowl - Charles Benedict Davenport

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.

Pen No.	Parents.		Offspring.
Pen No.	♀ (F₂).	♂ (F₂)	I	Y	oo	Absent.
634	298	444	0	15	18	...
634	366	444	5	23	15	...


729	913	936	2	28	37	...
729	935	936	...	13	39	...


756	1043	1390	...	13	11	1
756	1048	1390	...	0	5	...
Totals (214)			7	92	115	1

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.

Pen No.	Mother.			Father.		Comb in offspring.
Pen No.	No.	Comb.	P. ct. split.	No.	Comb.	I	Y	oo	Abs.
704	65 F₁	Y	50	1420 F₂	Absent	...	10	6	8
704	1061 F₂	Y	50	1420 F₂	Do.	...	4	...	1


819	57 F₁	Y	50	1420 F₂	Do.	...	8	6	5
819	65 F₁	Y	60	1420 F₂	Do.	...	1	...	1
Total						0	23	12	15

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.