Inheritance of Characteristics in Domestic Fowl

by Charles Benedict Davenport

• 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.

Pen No.	Parents.		Comb in offspring.
	♀ (F2).	♂ (F2)	I	Y	oo	Absent.
707	366	1378	18	16	9	...
	522	1378	1	1	0	...
763	2250	2247	9	5	4	1
	2700	2247	3	5	3	1
	3799	2247	5	4	3	...
769	1305	911	7	4	6	...
	2254	911	15	15	7	...
Totals (142)			58	50	32	2
Proportions (per cent)			40.8	35.2	22.5	1.4
					23.9

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.