CHAPTER II._Inheritance of Characteristics in Domestic Fowl

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]

[3] _E. g._, Pen 813, 935 ♀, embryo from egg of May 13.

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. HOUDAN CROSSES. |

+-----+--------------------------------------+---------------------------------+------------------------------+

| No. +-----------+----------------+---------+---------------------------------+---------------------+--------+

| | | | toes. | | | toes. | 4-4 | 4-5 | 5-5 | |

+-----+-----------+----------------+---------+-------+----------------+--------+-------+------+------+--------+

| | {8 or 11 | Houdan. | 5-5 | } | | | { 0 | 1 | 8 | 9.9 |

| 504 | { 8 | Do. | 5-5 | } 13 | Wh. Leghorn. | 4-4 | { 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. |

+----+--------------------+-------------------------+---------------------------------------------------------+

| No.+-----+-------+------+-----+------------+------+-------------------------------------------------+-------+

| | | | | | | |ss.|sd'.|sd.|d'd'.|d'd.|dd.|st'.|d't'.|dt'.|t't'.| |

+----+-----+-------+------+-----+------------+------+---+----+---+-----+----+---+----+-----+----+-----+-------+

| 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 |

+---------------------------------------------------+---+----+---+-----+----+---+----+-----+----+-----+-------+

[A] Of the reduced triplex type (t').

[B] s, means type of single thumb; d, duplex type; d', reduced duplex; t', reduced triplex.

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. HOUDAN CROSSES (F1 × F1). |

+------+-----+------------------------------+------------------------------+---------------------------------------------+

| | +-----+------------------+-----+-----+------------------+-----+----------------------------------+----------+

| No. | No. | | | No. | | | No. | Types of toes. | num. of |

| | | No. | Races involved. | of | No. | Races involved. | of +------+------+------+------+------+ toes per |

| | | | |toes.| | |toes.| 4-4 | 4-5 | 5-5 | 4-6 | 5-6 | 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|Pen| | | No. | | | No. | Types of toes. |

| No. |No.| No.| Races. | of | No.| Races. | of +-----------------------------------------------------------+

| | | | |toes.| | |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 |

+------+---+----+--------------------------------------------+----+----+----+----+----+----+----+----+----+----+----+----+

[A]: Includes 1 case of 3-4 toes.

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._

+------+---------------+------------------+------------------+

| | _a_. | _b_. | _c_. |

|toes. +---------------+------------------+------------------+

| | 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 |

+------+-------+-------+-------+----------+-------+----------+

[A] Reduced duplex and triplex toes classified as typical duplex

and triplex.

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 | | | No. | | | | |Average |

| No. |of | No.| Races involved. | of | No.| Races involved. | of | 4-4 | 4-5 | 5-5 | 4-6 |num. of |

+------+----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+

| 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|Pen|No.| Races involved. | No. | No.|Races involved.| No.| 4-4 |4-5 | 5-5 | 5-6 | 6-6 |Average |

+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+ 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) | .. | .. | 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._

+--------+-----------------------------------+-----------------------------------+-----------------------------------+

| toes. +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+

| | F1. | F2. | DR × R | DR × D | F1. | F2. | DR × R | DR × D | F1. | F2. | DR × R | 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.

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?

TABLE 16.--_Distribution of toe-numbers in the offspring of

4-toed heterozygotes._

+-----+-------------------------+-----------------------+-----------------+--------+

| Pen +----+-------------+------+----+-----------+------+-----+-----+-----+ Nature |

| No. | No.| Races. |No. of| No.| Races. |No. of| 4-4 | 4-5 | 5-5 | of |

+-----+----+-------------+------+----+-----------+------------+-----+-----+--------+

| 637 | 685|Houd. × Min. | 4-4 | 570|Houd.×Min. | 4-4 | 5 | 1 | 2 |DR × DR |

| 729 | 913|Houd. × Min. | 4-4 | 936|Houd.×Legh.| 4-4 | 38 | 13 | 19 |DR × DR |

| 729 |2269| Do. | 4-4 | 936| Do. | 4-4 | 15 | 5 | 10 |DR × DR |

| 729 |2324| Do. | 4-4 | 936| Do. | 4-4 | 30 | 5 | 3 |DR × R |

| 642 | 750|Min. × Polish| 4-4 | 647| Do. | 4-4 | 10 | .. | 3 | R × DR |

| 671 | 742|Min. × Brah. | 4-4 | 352|Houd.×Brah.| 4-4 | 8 | 2 | 2 | R × DR |

+-----+----+-------------+------+----+-----------+------+-----+-----+-----+--------+

TABLE 17.--_Distribution of toe-numbers in the offspring of

extracted 4-toed parents._

+----+------------------------+--------------------------+-----------------+-------+

| +----+-------------+-----+---+----------------+-----+-----+-----+-----+Nature |

|Pen | | | No. | | | No. | | | | of |

|No. | No.| Races. | of |No.| Races. | of | 4-4 |4-5 | 5-5 |mating.|

+----+----+-------------+-----+---+----------------+-----+-----+-----+-----+-------+

| {|2011|Polish × Min.| 4-4 |444|F2 Houd. × Legh.| 4-4 | 10 | .. | .. | R × R |

| {|2614| Do. | 4-4 |444| Do. | 4-4 | 6 | .. | .. | R × R |

|762{|2333| Do. | 4-4 |444| Do. | 4-4 | 16 | .. | .. | R × R |

| {|2618| Do. | 4-4 |444| Do. | 4-4 | 2 | .. | .. | R × R |

| {|3776| Do. | 4-4 |444| Do. | 4-4 | 2 | .. | .. | R × R |

+----+----+-------------+-----+---+----------------+-----+-----+-----+-----+-------+

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.

TABLE 18.

+------+---------+-------+--------------------------+

|Serial| Pen No. |No. of +-----+-----+-----+--------+

| No. | |mother.| 4-4 | 4-5 | 5-5 |Average.|

+------+---------+-------+-----+-----+-----+--------+

| 1 | 727 803 | 2457 | 1 | 2 | 34 | 9.89 |

| 2 | 727 803 | 3105 | 1 | 0 | 45 | 9.95 |

| 3 | 803 | 2579 | .. | 1 | 12 | 9.92 |

| 4 | 727 | 3106 | .. | .. | 4 | 10.00 |

| 5 | 727 | 2494 | 1 | 0 | 5 | 9.67 |

| 6 | 727 | 2459 | .. | .. | 16 | 10.00 |

+------+---------+-------+-----+-----+-----+--------+

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.

+------+---------+----------+-------+---------------------------------+

|Serial| Pen No. +---+------+ f +---+---+---+---+---+---+---------+

| No. | |No.|No. of| |4-4|5-4|5-5|4-6|5-6|6-8| Average.|

+------+---------+---+------+-------+---+---+---+---+---+---+---------+

| 1 | 734 815 |499| 6-6 |21{_a_ | 2 | 1 | 7 | 0 | 3 | 8 | 10.3 |

| | | | | {_b_ | 1 | 0 | 3 | 0 | 0 |17 | 11.4 |

| | | | | | | | | | | | |

| 2 | 734 815 |773| 6-5 |13{_a_ |.. |.. | 6 | 0 | 3 | 4 | 10.9 |

| | | | | {_b_ |.. |.. | 2 | 1 | 1 | 9 | 11.4 |

| | | | | | | | | | | | |

| 3 | 734 |500| 5-5 | 8{_a_ |.. | 2 | 4 | 0 | 2 |.. | 10.0 |

| | | | | {_b_ |.. |.. | 3 | 2 | 2 | 1 | 10.5 |

+------+---------+---+------+-------+---+---+---+---+---+---+---------+

In table 19 the series _a_ of observed average numbers of filial

toes (10.3, 10.9, 10.0) and the series _b_ obtained by assigning

the typical full number to all reduced types (11.4, 11.4, 10.5) are

decidedly irregular. There is, however, between the parental and the

filial series a correlation of +0.250±0.070. This indicates a slight

tendency for the number of toes in the progeny to vary with those of

the parentage.

The second set of data is derived from special matings made with

hybrids between Houdans and 4-toed races. On the one hand, in pens

728 and 813, cocks with well-developed toes of the duplex type were

mated with hens as nearly as possible of the same sort; while in

pens 765, 769, and 820 cocks with small, imperfectly separated toes

(probably of the duplex type[4]) were mated with hens as far as

possible of the same sort.

[4] I say probably of the duplex type because the cock

of pen 769 had a distally split toe on the right foot, reminding

somewhat of the _reduced_ triplex type. But as the left foot had a

typical duplex thumb, and the triplex is not common in Houdans, it

should probably be classed as duplex.

Tables 20, 21, and 22 give in detail and in summary the distribution

of types of polydactylism in the families from well-developed and in

those from poorly developed parents. They show a great difference

between the offspring of parents with good extra-toe (table 20) and

those with poor extra-toe (table 21). The former yield over 80 per

cent offspring with 5 toes or more on one or both feet, while the

latter yield about 57 per cent of such.

On the other hand, in the former families there are less than half

as many offspring with only 4 toes as in the latter. Classifying

"reduced" forms with their proper advanced type, we find highly

polydactyl parents yielding only 16 per cent non-polydactyl

offspring, while slightly polydactyl parents yield 43 per cent

non-polydactyl offspring. The percentage of polydactylous young

diminishes with the size and distinctness of the extra toes and the

grades of the polydactyl offspring are lower (absence in table 22, _b_,

of 6 toes). Both of Castle's conclusions seem to be confirmed.

TABLE 20.--_Distribution of toe-types in the offspring of "good"

extra-toed parents._

+------+---+---------------------------+---------------------------+---------+--------------------------------------+-------------------------------------------------------+

|Serial|Pen+----+----+-----------------+----+----+-----------------+ Mating. +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+

| No. |No.| No.|Gen.| Races. | No.|Gen.| Races. | | 4-4 | 4-5 | 5-5 | 5-6 | 6-6 |Average.| ss.| sd.|d'd'.|d'd.| dd.|d't'.|dt'.|dt.|t't'.|tt.|q't.|

+------+---+----+----+-----------------+----+----+-----------------+---------+-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+

| 1 |728|2271| F2 |Wh. Legh. × Houd.| 258| F1 |Houd. × Wh. Legh.| DD × DR | 4 | 1 | 21 | .. | .. | 9.65 | 3 | .. | 1 | 1 | 21 | .. | .. | ..| .. | ..| .. |

| 2 |728| 912| F2 | Do. | 258| F1 | Do. | DR × DR | 5 | 3 | 21 | .. | .. | 9.55 | 5 | 3 | .. | .. | 20 | .. | 1 | ..| .. | ..| .. |

| 3 |728|2248| F2 | Do. | 258| F1 | Do. | DD × DR | 8 | 3 | 22 | .. | .. | 9.42 | 8 | 3 | .. | .. | 21 | .. | .. | ..| 1 | ..| .. |

| 4 |728|2272| F2 | Do. | 258| F1 | Do. | DR × DR | 17 | 4 | 34 | .. | .. | 9.31 | 17 | 1 | .. | 3 | 34 | .. | .. | ..| .. | ..| .. |

| 5 |728| 174| F1 | Do. | 258| F1 | Do. | DR × DR | 10 | 1 | 15 | .. | .. | 9.19 | 10 | 1 | .. | .. | 14 | .. | 1 | ..| .. | ..| .. |

| | | | | +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+

| | | | | Totals (169) | 44 | 12 | 113 | .. | .. | 9.41 | 43 | 8 | 1 | 4 |110 | 0 | 2 | 0 | 1 | ..| .. |

| | | | | Percentages | 26.0| 7.1| 66.9| .. | .. | .. |25.4| 4.7| 0.6| 2.4|65.2| .. | 1.2| ..| 0.6 | ..| .. |

| | | | | +=====+=====+=====+=====+=====+========+====+====+=====+====+====+=====+====+===+=====+===+====+

| 6 |813|2271| F2 |Wh. Legh. × Houd.|3904| F3 |Houd. × Wh. Legh.| D × D | .. | 2 | 32 | .. | .. | 9.94 | .. | .. | .. | 2 | 32 | .. | .. | ..| .. | ..| .. |

| 7 |813|5113| F2 | Do. |3904| F3 | Do. | D × D | 2 | 1 | 32 | 1 | .. | 9.89 | .. | .. | 2 | 1 | 32 | .. | .. | 1 | .. | ..| .. |

| 8 |813| 377| F2 | Do. |3904| F3 | Do. | DR × D | 2 | 5 | 17 | .. | 1 | 9.68 | 2 | 2 | .. | 3 | 16 | .. | 1 | ..| .. | 1| .. |

| 9 |813|5122| F3 | Do. |3904| F3 | Do. | D × D | 1 | 3 | 7 | .. | .. | 9.55 | 1 | 3 | .. | .. | 7 | .. | .. | ..| .. | ..| .. |

| 10 |813| 935| F2 | Do. |3904| F3 | Do. | DR × D | 1 | 2 | 25 | 1 | 1 | 9.53 | 1 | 2 | .. | .. | 25 | .. | .. | 1 | .. | ..| 1 |

| 11 |813|2272| F2 | Do. |3904| F3 | Do. | DR × D | 5 | 2 | 18 | .. | .. | 9.52 | 4 | 1 | 1 | .. | 18 | 1 | .. | ..| .. | ..| .. |

| 12 |813| 912| F2 | Do. |3904| F3 | Do. | DR × D | 4 | 5 | 11 | .. | .. | 9.35 | 3 | 5 | 1 | .. | 11 | .. | .. | ..| .. | ..| .. |

| 13 |813|7320| F3 | Do. |3904| F3 | Do. | DR × D | 5 | 1 | 11 | .. | .. | 9.35 | 3 | 1 | 2 | .. | 11 | .. | .. | ..| .. | ..| .. |

| 14 |813|5142| F2 | Do. |3904| F3 | Do. | DR × D | 2 | 1 | 4 | .. | .. | 9.28 | 2 | .. | .. | 1 | 4 | .. | .. | ..| .. | ..| .. |

| | | | | +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+

| | | | | Totals (205) | 22 | 22 | 157 | 2 | 2 | 9.70 | 16 | 14 | 6 | 7 |156 | 1 | 1 | 2| 0 | 1| 1 |

| | | | | Percentages | 10.7| 10.7| 76.5| 1.0 | 1.0 | .. | 7.8| 6.8| 2.9| 3.4|76.2| 0.5 | 0.5|1.0| .. |0.5| 0.5|

+------+---+----+----+-------------------------------------------------------+-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+

TABLE 21.--_Distribution of toe-types in the offspring of "poor" extra-toed parents._

+------+---+----------------------------+----------------------------+--------+-------------------------------+------------------------------------+

|Serial|Pen+----+-----+-----------------+----+-----+-----------------+ Mating.+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+

| No. |No.| No.| Gen.| Races. | No.| Gen.| Races. | | 4-4 | 4-5 | 5-5 |5-6 |Average.| ss.| sd.|d'd'.|d'd.| dd.|t't'.|dq'.|

| | | | | | | | | | | | | | | | | | | | | |

+------+---+----+-----+-----------------+----+-----+-----------------+--------+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+

| 1 |765| 984|F2 |Wh. Legh. × Houd.|1794|F2 |Wh. Legh. × Houd.| DR × DR| 9 | 5 | 11 | .. | 9.08 | 9 | 3 | .. | 2 | 10 | 1 | .. |

| 2 |765|1790|F2 | Do. |1794|F2 | Do. | DR × DR| 18 | 7 | 17 | .. | 8.98 | 18 | 6 | .. | 1 | 17 | .. | .. |

| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+

| | | | | Totals (67) | 27 | 12 | 28 | .. | 9.02 | 27 | 9 | .. | 3 | 27 | 1 | .. |

| | | | | Percentages | 40.3| 17.9| 41.8| .. | .. |40.3|13.4| .. | 4.5|40.3| 1.5 | .. |

| | | | | +=====+=====+=====+====+========+====+====+====+====+====+=====+====+

| 3 |769| 492|F1 |Wh. Legh. × Houd.| 911|F2 |Wh. Legh. × Houd.| DR × DR| 13 | 1 | 14 | .. | 9.04 | 13 | 1 | .. | .. | 14 | .. | .. |

| 4 |769|4976|F2 | Do. | 911|F2 | Do. | DR × DR| 11 | 3 | 9 | .. | 8.91 | 11 | 3 | .. | .. | 8 | 1 | .. |

| 5 |769|2254|F2 | Do. | 911|F2 | Do. | DR × DR| 22 | 6 | 8 | .. | 8.61 | 22 | 4 | .. | 2 | 8 | .. | .. |

| 6 |769|1305|F2 | Do. | 911|F2 | Do. | DR × DR| 12 | 1 | 4 | .. | 8.53 | 12 | .. | .. | 1 | 4 | .. | .. |

| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+

| | | | | Totals (104) | 58 | 11 | 35 | .. | 8.77 | 58 | 8 | .. | 3 | 34 | 1 | .. |

| | | | | Percentages | 55.8| 10.6| 33.7| .. | .. |55.8| 7.7| .. | 2.9|32.7| 1.0 | .. |

| | | | | +=====+=====+=====+====+========+====+====+=====+====+====+=====+====+

| 7 |820| 984|F2 |Wh. Legh. × Houd.|4731|F3 |Wh. Legh. × Houd.| D × DR| 2 | 3 | 27 | .. | 9.78 | 2 | 2 | .. | 1 | 27 | .. | .. |

| 8 |820|2255|F2 | Do. |4731|F3 | Do. | DR × DR| 6 | 1 | 10 | .. | 9.24 | 6 | .. | .. | 1 | 10 | .. | .. |

| 9 |820|6479|F3 | Do. |4731|F3 | Do. | DR × DR| 12 | 2 | 16 | .. | 9.13 | 10 | 1 | 2 | 1 | 15 | 1 | .. |

| 10 |820|2016|F1[A]| Do. |4731|F3 | Do. | DR × DR| 9 | 2 | 2 | .. | 8.45 | 9 | 2 | .. | ..| 2 | .. | .. |

| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+

| | | | | Totals (92) | 29 | 8 | 55 | .. | 9.28 | 27 | 5 | 2 | 3 | 54 | 1 | .. |

| | | | | Percentages | 31.5| 8.7 | 59.8| .. | .. |29.3| 5.4| 2.2| 3.3|58.7| 1.1 | .. |

+------+---+----+-----+-------------------------------------------------------+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+

[A] No. 2016 has 4-4 toes and is a hybrid between a 5-toed White Leghorn × Houdan and a 4-toed Minorca × Polish.

But a more critical examination of the parentages of the 5 pens shows

that they are not comparable. In matings 6 to 14 of table 20 the

cock is almost certainly a dominant in respect to toes; whereas the

cocks in table 21 are probably heterozygous. The heterozygous state

determines two things: the imperfect nature of the extra-toe and a

relative deficiency in the offspring of the higher toe-numbers. In

our results we can not say that one of these things is the cause of

the other, as Castle does; they are, rather, in all probability, due

to a common cause. I think Castle's paper may justly be criticized

for not giving sufficient data concerning the ancestry of the

individual mothers used. Without such data the paper can not be said

satisfactorily to demonstrate his conclusion.

TABLE 22.--_Summary of observed toe-numbers in offspring,

percentages._

+-------------------------------------------++-----------------------------+

| _a._ Parents have "good" extra toes. || _b._ Parents have "poor" |

| || extra toes. |

+--------+------+------+------+------+------++--------+------+------+------+

| | 4-4 | 4-5 | 5-5 | 5-6 | 6-6 || | 4-4 | 4-5 | 5-5 |

| Pen No.| toes.| toes.| toes.| toes.| toes.|| Pen No.| toes.| toes.| toes.|

+--------+------+------+------+------+------++--------+------+------+------+

| 728 | 26.0 | 7.1 | 66.9 | .. | .. || 765 | 40.3 | 17.9 | 41.8 |

| 813 | 10.7 | 10.7 | 76.5 | 1.0 | 1.0 || 769 | 55.8 | 10.6 | 33.7 |

| | | | | | || 820 | 31.5 | 8.7 | 59.8 |

| +------+------+------+------+------++ +------+------+------+

|Average.| 17.7 | 9.1 | 72.2 | 0.5 | 0.5 ||Average.| 43.2 | 11.8 | 44.9 |

+--------+------+------+------+------+------++--------+------+------+------+

To summarize: "Potency," as measured by dominance of the extra-toed

condition, is inherited, in the Houdan crosses at least. There is

some evidence, derived from "pure-bred" Silkies, that differences in

the degree of development of the extra-toes are inherited. But the

average condition of the toes in the offspring of second or later

generation hybrids can not be used as evidence of inheritance of the

degree of parental development of the toes, since these are dependent

on the same basal cause, namely, the hidden gametic constitution

of the parents. Despite the obscuration of imperfect dominance,

polydactylism in poultry proves itself to be a unit-character that

segregates.