Charles Benedict Davenport

The possession of extra toes is a character that crops out again and again among the higher, typically 5-toed vertebrates. Many cases have been cited in works on human and mammalian teratology (cf. Bateson, 1904, and Schwalbe, 1906), and it is recognized that this abnormality is very strongly inherited in man. Bateson and Saunders, and Punnett (1902 and 1905), Hurst (1905), and Barfurth (1908), as well as myself in my earlier report, have demonstrated the inheritableness of the character in poultry. Bateson and Punnett (1905, p. 114) say: "The normal foot, though commonly recessive, may sometimes dominate over the extra-toe character, and this heterozygote may give equality when bred with recessives, just as if it were an ordinary DR." Altogether, the inheritance of extra-toe diverges so far from typical Mendelian results as to deserve further study.

A. TYPES OF POLYDACTYLISM.

There are two main types of polydactylism: that in which the inner toe (I) of the normal foot is replaced by 2 simple toes, and that in which it is replaced by two toes, of which the mediad is simple and the laterad is divided distally. The former type is characteristic of the Houdans; the latter is usually associated with the Silkies. Both conditions are, however, found in both races. The simplest condition is seen in many Houdans of my strain. It consists of 2 equal, medium-sized toes (I' and I") lying close together and parallel to or slightly convex towards each other. This condition indicates that the 2 toes, together, are to be regarded as the equivalent of the normal single toe occupying the same position. The 2 toes are, I conjecture, derived from the single toe by splitting. The first series of changes consists of the increase in length of the lateral element (I") and a corresponding decrease of the median element (I'). In the last term of the series there are only 4 toes on the foot, but the inner toe is not like the normal inner toe of poultry, but is a much elongated I".

In the Silkie, also, the series begins with 2 small, closely-applied toes (I' and I"). But when there are only 2 toes the lateral one is usually much the larger. Typically this lateral toe is, as stated, split, so that the nail is double, and the degree of splitting is variable, in extreme cases involving half or more than half of the toe. A second series of changes consists of the gradual reduction of toe I' (often concomitantly with an increase in I") which may end in its entire disappearance and thus reduce the number of toes to 5, but these are not equivalent to the 5 toes of the Houdans, since the extra Houdan toes are I', I", and those of the reduced Silkie are I"a and I"b. Finally, in Silkies, the inner toe (I') may split (more or less completely), and thus the 7-toed condition arises. Moreover, in Houdans I have on one or two occasions found the lateral element (I") bifid distally, resembling perfectly the typical condition found in the Silkies.

A simple nomenclature is suggested for these various types of extra-toes. The simple double-toed condition, as found commonly in Houdans, may be called the duplex type (D). The loss of I' gives the reduced duplex (D'). The case of split I", as commonly seen in the Silkie, is the triplex type (T); with the loss of I' this becomes the reduced triplex (T', not duplex!). The 7-toed condition of Silkies may be called the quadruplex type (Q); the combination split I' and single I" gives the reduced quadruplex (Q').[3]

The reduction that leads to the loss of I' consists of a loss of phalanges, as Bateson (1904) has already pointed out. It seems probable that the reduction affects first the proximal phalanges, since the distal nail-bearing phalanx is the last to disappear.

B. RESULTS OF HYBRIDIZATION.

First let us consider the result of mating extra-toed individuals belonging to "pure" extra-toed races. A typical Houdan cock (D type), of the well-known Petersen strain, was mated with 3 hens bred by me, but derived, several generations before, from the same strain. With the first hen he got 29 chicks, all with the extra-toe except one (3.3 per cent) that had 4 toes on both feet and two that had 4 toes on one foot and 5 on the other, i. e., one foot simplex and one duplex. With the second he got 12 chicks, of which one had 4-5 (D) toes. The third, in 26 young, gave one with 4 toes on each foot. Thus, in 67 chicks altogether there were 2, or 3 per cent, with the normal number of toes on both feet (4-4). Unfortunately these birds did not survive, so it is not known whether they would have thrown as large a proportion of extra-toed offspring as 5-toed Houdans. Bateson's Dorkings gave about 4 per cent of 4-toed offspring. Of the 83 offspring of 6-toed Silkies, 3, or 3.6 per cent, had 4 toes on each foot. Even in pure-bred polydactyl races, consequently, the character "extra-toe" does not uniformly appear in the offspring.

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.
								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
	121	Do	4-4					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]
							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.
									4-4		4-5		5-5		4-6		5-6
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.
								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.

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.

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.

It now remains to test our conclusions by reference to the mating of the heterozygote with the dominant and with the recessive types, respectively. An examination of tables 13 to 15, particularly the last, reveals several points of interest. Mendelian expectation in the DR × R cross is 50 per cent of the recessive (4-4) type. Actually, in the two crosses, A and B, 68 per cent and 67 per cent, respectively, were obtained. But recalling that of these amounts one-half of 27.3, or 13.71, and one-half of 9.4, or 4.7, are respectively due to failure to develop the extra-toe in heterozygotes, there remain 54 per cent and 62 per cent, respectively, of 4-toed offspring, which doubtless represent the extracted RR type and approach the expected proportions.

Mendelian expectation in the DR × D cross (table 15) is 50 per cent heterozygotes and 50 per cent extracted dominants. Of the heterozygotes some 14 per cent may be expected to show 4-4 toes; that the percentage is much less than that is doubtless due to the small numbers involved. What is striking is the reappearance, in the second generation, of large proportions of the extreme dominant type. These results thus confirm those of the F₂ generation.

Since extra-toe frequently fails to dominate, there should be certain 4-toed heterozygotes which throw extra-toe offspring, and such are found. In table 16 are given six matings of 4-toed DR's. One sees that they produce some 5-toed offspring. On the other hand, extracted 4-toed recessives are obtained, as table 17 shows.

Finally, we must consider whether, among the polydactyl birds of one class, e. g., Houdans or Silkies, there is any difference in the "centgener power" of parents corresponding to the degree of development of their extra toes. This inquiry is suggested by Castle's study (1906, p. 20) of polydactyl guinea-pigs. He finds that when the extra toes of the mothers are graded into the 5 classes, good (G), fair (F), poor (P), normal though of abnormal ancestry (N), and normal of normal ancestry (N'), it follows: "first, that the proportion of polydactylous young produced by a male decreases in the successive classes from G to N'; and, secondly, that the degree of development of the toes produced on those polydactylous young diminishes in the same order." It is possible to test this conclusion in poultry because, inside of any one type of extra-toe, e. g., the triplex type, variation appears in the absolute size of the toes and in the degree of their separateness. Our questions, then, are: (1) does the proportion of polydactyl young produced by a pair of birds of any type diminish with the degree of development of toes inside of that type, and (2) does the degree of development of the toes produced on the polydactylous offspring diminish in the same order?

Two sets of data are available for answering these questions. The most direct set includes the data derived from crossing "pure-bred" polydactyl birds and the other includes the data derived from using hybrids between normal-toed and polydactyl ancestors. The latter data have the advantage that the parents offer a greater variability; but they have the disadvantage that the germinal condition of those parents is incompletely known.

The pure races may be considered first. Eight matings of Houdans, each parent with 5 toes, gave 122 offspring, of which 116 had 5-5 toes, 3 had 4-5 toes, and 3 had 4-4 toes. The variability of the toes is not great in the parent Houdans. But, arranging them in the order of development of the toes, the most developed first, the series of table 18 results.

No direct relation here appears between development of the extra toe in the parents and the average number of toes in the offspring.

Of the Silkies, 3 hens were used in 5 matings. The same 6-toed cock (No. 774) was employed throughout (table 19).

Table 19.