Inheritance of Characteristics in Domestic Fowl

by Charles Benedict Davenport

CHAPTER II.
POLYDACTYLISM.

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.