Inheritance of Characteristics in Domestic Fowl - Charles Benedict Davenport

Let us consider next what happens when a polydactyl individual is crossed with a normal individual. Table 10 gives the results of all matings of this sort and its most obvious result is that the polydactyl condition reappears in every family, but not, as in typically Mendelian cases, in all of the offspring; at least this is true of the Houdan crosses. In the Silkie crosses the 6 offspring given as having the single thumb may possibly have been of the type D', as that type was not in mind at the time of making the record and was not always distinguished from type S. It is also clear that the offspring of Silkie crosses are more apt to be polydactyl than those of Houdan crosses. For 27 per cent of the latter are non-polydactyl, while, taking the table as it stands, at most only about 4 per cent and (as just stated) probably none of the Silkie offspring were of the typical single-thumbed type. Also the average degree of polydactylism is much greater in the Silkie than in the Houdan crosses. This excess is in part due to the different method of counting toes in the Silkie and the Houdan hybrids; for whereas in the latter the visible toes are counted as equivalent units, in the former in the case of each reduced type one unit more is assigned than appears. The actual number of toes occurring in the Silkie hybrids was also calculated, and it was found that this still averaged higher than that of the Houdans (9.45 as opposed to 9.26).

Table 10.—Frequency of the various types of toes in the first hybrid generation between a normal and an extra-toed parent.

^[A] s, means type of single thumb; d, duplex type; d', reduced duplex; t', reduced triplex.
^[B] Of the reduced triplex type (t').
A. HOUDAN CROSSES.
Pen No.	Mother.			Father.			Offspring.
	No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.	Types of toes.
	No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.		4-4			4-5			5-5			Average.
504	8 or 11	Houdan	5-5	13	Wh. Leghorn	4-4		0			1			8			9.9
	8	Do	5-5					1			3			8			9.6
	11	Do	5-5					2			2			7			9.5


525	8 or 11	Do	5-5	27	Minorca	4-4		8			3			13			9.2


727	"Y"	Dk. Brahma	4-4	831	Houdan	5-5		3			2			5			9.2
727	121	Do	4-4	831	Houdan	5-5		13			9			18			9.1


504	10-12	Wh. Leghorn	4-4	9	Do	5-5		3			2			0			8.4
Total (110)								30			21			59			9.26
Percentages								27.3			19.1			53.6
B. SILKIE CROSSES.
Pen No.	Mother.			Father.			Offspring.
	No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.	Types of toes.^[A]
	No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.	ss.	sd'.	sd.	d'd'.	d'd.	dd.	st'.	d't'.	dt'.	t't'.	Average.
851	1002	Cochin	4-4	7526	Silkie	6-6	...	...	1	...	1	2	...	...	2	3	10.78
851	3410	Do	4-4	7526	Do	6-6	1?	...	...	...	2	7	...	...	1	3	10.43
815	131	Do	4-4	774	Do	6-6	...	...	...	1	...	8	...	1	1	1	10.33
851	2073	Do	4-4	7526	Do	6-6	...	...	...	...	...	7	1	...	...	1	10.33
734	841	Do	4-4	774	Do	6-6	...	...	...	...	...	3	..	...	1	...	10.25
851	838	Do	4-4	7526	Do	6-6	...	...	1	1	...	11	...	...	...	3	10.25
851	2299	Do	4-4	7526	Do	6-6	...	...	1?	1	...	4	...	...	...	1	10.14
851	5567	Do	4-4	7526	Do	6-6	...	...	...	...	1	10	1	...	1	...	10.08
734	840	Do	4-4	7526	Do	6-6	...	...	...	1	...	7	...	...	...	...	10.00
734	1002	Do	4-4	774	Do	6-6	...	...	...	...	2	8	...	...	...	...	10.00
851	840	Do	4-4	7526	Do	6-6	...	...	...	...	...	4	...	...	...	...	10.00
851	841	Do	4-4	7526	Do	6-6	...	...	...	...	1	1	...	...	...	...	10.00
744	777	Silkie.	^[B]5-6	1176	Wh. Leghorn.	4-4	...	...	...	...	...	6	...	...	...	...	10.00
744	496	Do	6-6	1176	Do	4-4	1?	...	...	...	...	12	...	...	1	...	9.93
851	6956	Cochin	4-4	7526	Silkie	6-6	4?	1	...	2	...	3	...	...	...	...	9.50
Total (138)							6	1	3	6	7	93	2	1	7	12	10.13

In hybrids of both classes the greatest number of toes occurring on one foot never exceeds the greatest number possessed by its parents; indeed, the most polydactyl hybrids of the F₁ generation of Silkies never have as many as 6 toes on one foot. This result is not to be explained as due to a regression towards the 4-4-toed condition, but rather as due to the intermediate condition of the heterozygote. For 80 per cent of the hybrids show either the typical or the reduced D type on one or both feet, although neither parent exhibits these types.

We have next to consider the results of mating together the F₁ hybrids. Table 11 gives the results of all matings of this sort.

Table 11.—Frequency of the various types of toes in the second hybrid generation between normal and extra-toed races. Lettering as in table 10.

^[A] Includes 1 case of 3-4 toes.
	A. HOUDAN CROSSES (F₁ × F₁).
Serial No.	Pen No.	Mother.			Father.			Offspring.
		No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.	Types of toes.											Average num. of toes per bird.
		No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.		4-4		4-5		5-5		4-6		5-6		Average num. of toes per bird.
1	631	429	Houd. × Wh. Legh.	5-5	83	Wh. Legh. × Houd.	4-4		14^[A]		7		28		1		...		9.3
2	728	174	Do.	5-5	258	Do.	5-5		11		1		20		...		...		9.3
3	631	448	Do.	5-5	409	Do.	4-4		13		4		18		...		...		9.1
4	637	529	Houd. × Min.	5-5	570	Houd. × Min.	4-4		4		...		5		...		...		9.1
5	631	430	Houd. × Wh. Legh.	4-4	83	Wh. Legh. × Houd.	4-4		20		1		21		...		...		9.0
6	631	504	Wh. Legh. × Houd.	5-5	83	Do.	4-4		27		3		23		...		...		8.9
7	631	174	Houd. × Wh. Legh.	5-5	83	Do.	4-4		14		9		11		...		1		8.9
8	519	85	Do.	4-5	83	Do.	4-4		9		2		4		...		...		8.7
9	637	569	Houd. × Min.	5-5	570	Houd. × Min.	4-4		14		1		4		...		1		8.7
10	637	797	Do.	5-5	570	Do.	4-4		2		...		1		...		...		8.7
11	631	86	Houd. × Wh. Legh.	4-4	83	Houd. × Wh. Legh.	4-4		11		1		6		...		...		8.7
12	637	685	Houd. × Min.	4-4	570	Houd. × Min.	4-4		5		1		2		...		...		8.6
13	631	84	Houd. × Wh. Legh.	4-4	83	Houd. × Wh. Legh.	4-4		17		13		4		...		...		8.6
14	519	84	Do.	4-4	83	Do.	4-4		7		1		2		...		...		8.5
15	519	86	Wh. Legh. × Houd.	4-4	83	Wh. Legh. × Houd.	4-4		12		2		2		...		...		8.4
			Totals (380)						180		46		151		1		2		8.92
			Percentages						47.4		12.1		39.7		0.3		0.5
	B. SILKIE CROSSES (F₁ × F₁).
Serial No.	Pen No.	Mother.			Father.			Offspring.
		No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.	Types of toes.
		No.	Race involved.	No. of toes.	No.	Race involved.	No. of toes.	ss	sd	d'd'	d'd	dd	st	d't'	dt'	dt	t't'	t't	tt
16	753	2071	Min. × Silk.	4-4	2573	Min. × Silk.	4-5	7	...	...	1	19	...	1	...	3	...	1	...
17	753	1966	Do.	4-4	2573	Do.	4-5	12	2	...	...	15	1	...	...	2	..	..	4
18	753	2575	Do.	4-5	2573	Do.	4-5	18	...	1	...	16	...	...	1	...	...	...	1
19	709	3827	Silk. × Span.	4-4	1578	Silk. × Span.	6-5	3	...	...	...	2	...	...	...	...	...	...	...
20	709	1963	Do.	4-4	1578	Do.	6-5	12	5	...	1	15	1	...	...	1	...	...	1
21	821	7413	Silk. × Coch.	5-5	6095	Silk. × Coch.	5-5	1	...	...	1	7	...	...	...	2	...	...	...
22	821	7423	Do.	5-5	6095	Do.	5-5	3	...	...	...	7	...	...	...	...	1	...	1
23	821	7428	Do.	5-5	6095	Do.	5-5	5	...	1	4	13	...	...	2	...	...	...	1
24	821	7408	Do.	5-5	6095	Do.	5-5	3	1	...	...	8	...	...	...	1	1	...	...
			Total (208)					64	8	2	7	102	2	1	3	8	2	1	8

Comparing tables 10 and 11, it is at once clear that in the second hybrid generation the proportion of extra-toed offspring has decreased. This accords with expectation, if extra-toe is dominant, for then only 75 per cent would be of the dominant type in F₂, while 100 per cent would be of that type in F₁.

Table 12 will enable us to analyze the difference of the proportions in tables 10 and 11.

Table 12.—Percentages of the various types of toes in F₁ and F₂ of the polydactyl hybrids compared.