Inheritance of Characteristics in Domestic Fowl - Charles Benedict Davenport

^[A] Reduced duplex and triplex toes classified as typical duplex and triplex.
	a.		b.		c.
No. of toes.	Houdan hybrids.		Silkie hybrids (as observed).		Silkie hybrids (as interpreted).^[A]
	F₁.	F₂.	F₁.	F₂.	F₁.	F₂.
4-4	27.3	47.4	9.4	31.7	4.3	30.8
4-5	19.1	12.1	9.4	7.7	2.9	3.8
4-6	...	.3	...	1.0	1.5	1.0
5-5	53.6	39.7	81.2	51.4	76.8	53.4
5-6	...	.5	...	4.3	5.8	5.8
6-6	...	...	...	3.9	8.7	5.3

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.

A. HOUDAN CROSSES
Serial No.	No. of pen	Mother.			Father.			Offspring.
Serial No.	No. of pen	No.	Races involved.	No. of toes.	No.	Races involved.	No. of toes.	4-4 toes.	4-5 toes.	5-5 toes.	4-6 toes.	Average num. of toes per bird.
1	519A	87	Houd. × Wh. Legh.	4-5	71	Wh. Legh.	4-4	17	2	6	...	8.6
2	671	742	Min. × Dk. Brah.	4-4	352	Houd. × Dk. Brah.	4-4	8	2	2	...	8.5
			Totals (37)					25	4	8	...	8.54
B. SILKIE CROSSES.
3	706	10	Wh. Legh.	4-4	1965	Silkie × Spanish	5-5		4	...	4	9.00
4	766	3814	Do.	4-4	834	Blk. Game × Silkie	5-5	10	4	8	1	9.00
5	766	10	Do.	4-4	834	Do.	5-5	7	...	5	...	8.83
6	607	203	Frizzle × Silkie	5-5	15	Frizzle	4-4	15	2	9	...	8.77
7	766	3815	Wh. Legh.	4-4	834	Blk. Game × Silkie	5-5	11	...	7	...	8.77
8	706	3815	Do.	4-4	1965	Silkie × Spanish	5-5	6	...	3	...	8.67
9	706	71	Do.	4-4	3823	Do.	5-5	18	1	8	...	8.63
10	766	3832	Buff Legh.	4-4	834	Blk. Game × Silkie	5-5	7	...	2	...	8.44
11	706	3833	Do.	4-4	1965	Silkie × Spanish	5-5	3	1	...	...	8.25
12	607	230	Frizzle × Silkie	4-4	15	Frizzle	4-4	23	2	2	...	8.22
13	706	71	Wh. Legh.	4-4	1965	Silkie × Spanish	5-5	5	...	...	...	8.00
14	706	3814	Do.	4-4	1965	Do.	5-5	6	...	...	...	8.00
15	706	3832	Buff Legh.	4-4	1965	Do.	5-5	5	...	...	...	8.00
			Totals (179)					120	10	48	1	8.60

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

A. HOUDAN CROSSES
Serial No.	No. of pen	Mother.			Father.			Offspring.
Serial No.	No. of pen	No.	Races involved.	No. of toes.	No.	Races involved.	No. of toes.	4-4 toes.	4-5 toes.	5-5 toes.	5-6 toes.	6-6 toes.	Average num. of toes per bird.
1	803	529	Houdan × Min.	5-5	7522	Houdan	5-5	1	4	13	..	..	9.67
B. SILKIE CROSSES.
2	606	182	Frizzle × Silkie.	4-4	775	Silkie.	6-6	...	3	10	3	5	10.48
3	606	182	Do.	4-4	21A	Do.	6-6	...	...	5	...	1	10.33
4	606	182	Do.	4-4	551	Do.	5-6	...	...	5	...	...	10.00
			Totals (32)					...	2	20	3	6	10.36

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

No. of toes.	a. Houdan crosses.				b. Silkie crosses.				c. Silkie crosses (reduced forms of toe classified as typical).
No. of toes.	Mating F₁.	Mating F₂.	Mating DR × R	Mating DR × D	Mating F₁.	Mating F₂.	Mating DR × R	Mating DR × D	Mating F₁.	Mating F₂.	Mating DR × R	Mating DR × D
	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.	P. ct.
4-4	27.3	47.4	67.6	5.6	9.4	31.7	67.0	...	4.3	30.8	66.7	...
4-5	19.1	12.1	10.8	22.2	9.4	7.7	5.6	9.4	2.9	3.8	3.1	9.4
5-5	53.6	39.7	21.6	72.2	81.2	51.4	26.8	62.5	76.8	53.4	24.6	62.5
4-6	...	.3	...	...	...	1.0	.6	...	...	1.0	1.9	...
5-6	...	.5	...	...	...	4.3	...	9.4	5.8	5.8	1.5	9.4
6-6	...	...	...	...	...	3.9	...	...	8.7	5.3	1.2	18.7
6-7	...	...	...	...	...	...	...	...	...	...	...	...

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.