Inheritance of Characteristics in Domestic Fowl

by Charles Benedict Davenport

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.

DD × DD (SYNDACTYL × SYNDACTYL).
Family No.	Mother's No.	Bred in pen No.	Toes.	Father's No.	Bred in pen No.	Toes.	Syndactyl.
							2t.	1t.	0t.	P. ct.
30	4569	767	Abα	5399	747	Abα	2	0	0	100.0
34	7528	767	Abβ	5399	747	Abα	1	0	0	100.0
32	872	627	Abβ	5399	747	Abα	12	4	11	59.3
33	5515	767	Bbα	5399	747	Abα	4	0	7	36.4
Totals							19	4	18	74.2
DD × DR.
31	6843	767	Normal.	4562	767	Normal.	1	3	2	66.7
30a	4569	767	Abα	4562	767	Do.	0	2	2	50.0
33a	5515	767	Bbα	4562	767	Do	1	2	5	44.4
32a	872	627	Abβ	4562	767	Do.	7	1	12	42.9
34a	7528	767	Abβ	4562	767	Do.	2	1	7	30.0
36	6869	767	Normal.	5399	747	Abα	0	1	3	25.0
25a	872	627	Abβ	3116	D. Br.	Synd.	7	1	30	21.1
41	4263	767	Normal.	5399	747	Abα	0	1	4	20.0
37	2831	658	Do.	5399	747	Abα	3	1	18	18.2
39	4570	658	Do.	5399	747	Abα	0	1	5	16.7
40	1892	658	Do.	5399	747	Abα	0	0	9	0.0
Totals							21	14	97	26.5
DR × DR.
38a	2526	658	Normal.	4562	767	Normal.	1	0	2	33.3
35	6861	767	Do.	4562	767	Do.	1	0	3	25.0
40a	1892	658	Do.	4562	767	Do.	1	0	3	25.0
37a	2831	658	Do.	4562	767	Do.	2	1	11	21.4
36a	6869	767	Do.	4562	767	Do.	1	0	4	20.0
24	2526	658	Do.	3116	D. Br.	Synd.	5	0	22	18.5
26	2104	608	Do.	3116	Do.	Do.	3	0	18	14.3
39a	4570	767	Do.	4562	767	Do.	0	2	17	10.5
27	2831	658	Do.	3116	D. Br.	Do.	3	0	32	8.6
29a	190	520	Do.	3116	D. Br.	Do.	4	0	49	7.6
29	767	190	Do.	242	513	Do.	1	1	28	6.7
28a	181	513	Do.	3116	Do.	Do.	1	1	60	3.2
Totals							23	5	249	10.1
RR × DR.
42	6872	767	Normal.	4562	767	Normal.	0	0	6	0.0
41a	4263	767	Do.	4562	767	Do.	0	0	10	0.0
Totals							0	0	16	0.0

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.

Nature of mating.	f	Expectation.		Realization.
		Dominants + heterozygotes.	Pure dominants.	Syndactyls.
		P. ct.	P. ct.	P. ct.
DD × DD	41	100.0	100.0	56.1
DD × DR	132	100.0	50.0	26.5
DR × DR	277	75.0	25.0	10.1
RR × DR	16	50.0	0.0	0.0
RR × RR	119	0.0	0.0	0.0

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.