SYNDACTYLISM.
A. STATEMENT OF PROBLEM.
In man, various mammals, and some birds two or more adjacent fingers
are sometimes intimately connected by an extension of the web that is
normally a mere rudiment at their base. Such a condition is known as
syndactylism. A good introductory account of syndactylism is given by
Bateson (1904, pp. 356-358). Taking a number of cases of syndactylism
together, he says: "A progressive series may be arranged showing
every condition, beginning from an imperfect webbing together of the
proximal phalanges to the state in which two digits are intimately
united even in their bones, and perhaps even to the condition in
which two digits are represented by a single digit." He also calls
attention to the fact that in the human hand "there is a considerable
preponderance of cases of union between the digits III and IV;" while
in the foot the united digits "are nearly always II and III." The
matter of syndactylism in birds has a peculiar interest because of
the fact that among wading and swimming birds syndactylism has become
a normal condition of the feet, and, moreover, just this feature
is one that has become classical in evolutionary history, because
Lamarck thought it well illustrated his idea of the origin of an
organ by effort and use.
Concerning the cause of syndactylism little can be said. Both in
mammals and birds the digits are indicated before they are freed from
lateral tissue connections. The linear development of the fingers
is in part accompanied by a cutting back of this primordial web, in
part by a growth beyond it. In syndactylism growth of the web keeps
pace with that of the fingers. From this point of view syndactylism
may be regarded as due to a peculiar excessive development of the
web.[5] In some human cases adhesions of the apex of the appendage to
the embryonic membranes has stimulated the growth of the interdigital
membrane, resulting in syndactylism. But it would be absurd to
attempt to explain syndactylism in general on this ground. The more
"normal" forms of syndactylism, as seen in poultry, still want for a
causal explanation.
[5] Lewis and Embleton (1908, p. 45) present strong
arguments against the theory that syndactylism is due to
arrested development.
Most of the cases of syndactylism whose inheritance is about to be
described arose in a single strain of fowl and can, indeed, be traced
back to a single bird. This ancestor is No. 121, a Dark Brahma hen
described in a previous report.[6] It was only in the search for the
origin of the exaggerated forms of syndactylism observed in some of
her descendants that an unusually great extension of the web in her
feet was noticed. The syndactyl condition of my birds did not, thus,
arise _de novo_, but had its origin antecedent to the beginning of
the breeding experiments. In addition to this main strain a slight
degree of syndactylism has appeared among some of my Cochin bantams.
[6] Davenport, 1906, page 34, Plate V.
TABLE 23.--_Ancestry of syndactyl fowl and the results of various
matings involving syndactylism._
[Abbreviations: _Ab_α, _Ab_β, etc., types of syndactylism (p. 32);
F, father; FF, father's father; FM, father's mother; M, mother;
MF, mother's father; MM, mother's mother; M × P, hybrid of Minorca
and Polish races; Synd., syndactyl (type unknown). f, foot. In
Nos. 24 to 42 two cocks (Nos. 242 and 3116, and 5399 and 4562,
respectively) were at different times used.]
+-----------+-----+-------------------------------------------------------------++-------------------------------------------------------------------------+
| | | First mating. || Second mating. |
| | +-------------------------------------+-----------------------++--------------------------------------------+---------------+------------+
| | | Ancestry. | Offspring. || Ancestry. | Offspring. | Average |
| Serial | Pen +-----+--------+-------+-----+--------+--------+--------------++-----+--------+-------+-----+--------+------+---------------+ per |
| No. | No. | M's | | | F's | | | Syndactyl. || M's | | | F's | | | Syndactyl. | cent |
| | | No. | MM. | MF. | No. | FM. | FF. +----+---+-----+| No. | MM. | MF. | No. | FM. | FF. +----+----+-----+ syndactyl. |
| | | | | | | | |2f. |1f.| 0f. || | | | | | | 2f.| 1f.| 0f. | |
+-----------+-----+-----+--------+-------+-----+--------+--------+----+---+-----++-----+--------+-------+-----+--------+------+----+----+-----+------------+
| | | | | | | | | | | || | | | | | | | | | |
| 1_a_, _b_ | 627 | 302 | [1]121 | [2]8A | 180 | [1]121 | [2]8A | 0 | 0 | 34 || 302 | [1]121 | [2]8A | 242 | [1]121 |[2]8A | 3 | 0 | 29 | 10.3 |
| 2_a_, _b_ | 627 | 280 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 23 || 280 | 121 | 8A | 242 | 121 | 8A | 2 | 0 | 21 | 9.5 |
| 3_a_, _b_ | 627 | 181 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 20 || 181 | 121 | 8A | 242 | 121 | 8A | 3 | 0 | 33 | 9.1 |
| 4_a_, _b_ | 627 | 354 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 24 || 354 | 121 | 8A | 242 | 121 | 8A | 1 | 0 | 37 | 2.6 |
| 5_a_, _b_ | 627 | 178 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 20 || 178 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 42 | .. |
| 6_a_, _b_ | 627 | 190 | 121 | 8A | 180 | 121 | 8A | 1 | 0 | 24 || 190 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 6 | .. |
| 7_a_, _b_ | ... | 353 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 13 || 353 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 22 | .. |
| 8_a_, _b_ | ... | 300 | 121 | 1A | 180 | 121 | 8A | 0 | 0 | 23 || 300 | 121 | 1A | 242 | 121 | 8A | 0 | 0 | 37 | .. |
| +----+---+-----+| +----+----+-----+ |
| Totals (182) | 1 | 0 | 181 || Totals (236) | 9 | 0 | 227 | |
| Percentages |0.55| 0 |99.45|| Percentages |3.81| 0 |96.19| |
+----------------------------------------------------------------+----+---+-----++--------------------------------------------+----+----+-----+------------+
+------+------+--------------------------+--------------------------+--------------------------------------------------------------------------------------+
| | | Mother. | Father. | Offspring. |
|Serial| Pen +------+-------+-----------+------+-------+-----------+----------------------+---------------------------------------------------------------+
| No. | No. | | Bred | | | Bred | | Syndactyl. | Classification. |
| | | No. |in pen | Toes. | No. |in pen | Toes. +----+---+-----+-------+------------+------------+-----------+------------+------------+
| | | | No. | | | No. | | 2f.|1f.| 0f. | P. ct.| _Aa_α. | _Ab_α. | _Ab_β. | _Ab_β´. | _Bb_α. |
+------+------+------+-------+-----------+------+-------+-----------+----+---+-----+-------+------------+------------+-----------+------------+------------+
| 9 | 747 | 2526 |[3]658 |Normal. | 1888 |[3]658 |Normal. | 9 | 0 | 9 | 50.0 | .. | 2 | 16 | .. | .. |
| 10 | 747 | 2831 | 658 | Do. | 1888 | 658 | Do. | 6 | 0 | 6 | 50.0 | .. | 7 | 5 | .. | .. |
| 11 | 747 | 2652 | 658 | Do. | 1888 | 658 | Do. | 3 | 0 | 25 | 10.7 | .. | 6 | .. | .. | .. |
| 12 | 747 | 3541 | 658 | Do. | 1888 | 658 | Do. | 4 | 0 | 41 | 8.9 | 1 | 4 | 3 | .. | .. |
| 13 | 747 | 1892 | 658 | Do. | 1888 | 658 | Do. | 4 | 0 | 47 | 7.8 | .. | .. | .. | .. | .. |
| 14 | 747 | 1872 | 658 | Do. | 1888 | 658 | Do. | 0 | 0 | 28 | 0.0 | .. | .. | .. | .. | .. |
| 15 | 747 | 1874 | 658 | Do. | 1888 | 658 | Do. | 0 | 0 | 28 | 0.0 | .. | .. | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 26 | 0 | 184 | 12.4 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 16 | 703 | 2353 |D. Br. | Do. | 122 |D. Br. | Do. | 1 | 0 | 6 | 14.3 | .. | 2 | .. | .. | .. |
| 17 | 703 | 2030 |D. Br. | Do. | 122 |D. Br. | Do. | 2 | 1 | 12 | 20.0 | .. | 5 | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 3 | 1 | 18 | 18.2 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 18 | 754 | 3126 |[4]627 |Normal. | 871 |[4]627 |Normal. | 12 | 1 | 30 | 30.2 | .. | 13 | 12 | .. | .. |
| 19 | 754 | 3175 | 627 | Do. | 871 | 627 | Do. | 3 | 0 | 8 | 27.3 | .. | 3 | 3 | .. | .. |
| 20 | 754 | 873 | 627 | Do. | 871 | 627 | Do. | [2]|(?)| (?)| (?) | .. | .. | 4 | .. | .. |
| 21 | 754 | 1052 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 17 | 0.0 | .. | .. | .. | .. | .. |
| 22 | 754 | 853 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 19 | 0.0 | .. | .. | .. | .. | .. |
| 23 | 754 | 862 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 27 | 0.0 | .. | .. | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 15 | 1 | 101 | 13.7 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 24 | 767 | 2526 |[3]658 |Normal. | 3116 |D. Br. | Synd. | 5 | 0 | 22 | 18.5 | 1 | 1 | 6 | .. | 2 |
| 25 | 767 | 872 |[5]627 |_Ab_β | 242 |[5]513 |Normal. | 1 | 0 | 1 | 50.0 | .. | 1 | .. | 1 | .. |
| 25_a_| 767 | 872 | 627 |_Ab_β | 3116 |D. Br. | Synd. | 7 | 1 | 30 | 21.0 | 3 | 5 | 3 | .. | 4 |
| 26 | 767 | 2104 |[7]608 |Normal. | 3116 |D. Br. | Do. | 3 | 0 | 18 | 14.3 | .. | 2 | 2 | .. | 2 |
| 27 | 767 | 2831 |[3]658 | Do. | 3116 |D. Br. | Do. | 3 | 0 | 32 | 8.6 | .. | 6 | .. | .. | .. |
| 28 | 767 | 181 |[6]513 | Do. | 242 | 513 |Normal. | 1 | 0 | 22 | 4.4 | 2 | .. | .. | .. | .. |
| 28_a_| 767 | 181 | 513 | Do. | 3116 |D. Br. | Synd. | 1 | 1 | 60 | 3.2 | .. | 1 | 1 | .. | 1 |
| 29 | 767 | 190 |[5]520 | Do. | 242 | 513 |Normal. | 1 | 1 | 28 | 6.7 | 1 | .. | .. | .. | 2 |
| 29_a_| 767 | 190 | 520 | Do. | 3116 |D. Br. | Synd. | 4 | | 49 | 7.6 | .. | 3 | 4 | .. | 1 |
| +----+---+-----+-------+ | | | | |
| Syndactyl (242 ♂) | 3 | 1 | 51 | 7.3 | | | | | |
| Syndactyl (3116♂) | 23 | 2 | 211 | 9.4 | | | | | |
+-------------------------------------------------------------------+----+---+-----+-------+------------+------------+-----------+------------+------------+
[1] No. 121 is a Dark Brahma.
[2] No. 8A is a Tosa fowl (Game).
[3] (White Leghorn × Rose Comb Black Minorca) × Dark Brahma.
[4] Dark Brahma.
[5] See _supra_.
[6] 121 ♂ Dark Brahma × 8A Tosa.
[7] F2 (White Leghorn × Dark Brahma).
TABLE 23.--_Ancestry of syndactyl fowl and the results of various
matings involving syndactylism_--Continued.
+------+-----+-----------------------------------------------------+-------------------------------------------------------------------------------------+
| | | Mother. | Father. | Offspring. |
|Serial| Pen +------+-------+-----------+------+-------+-----------+---------------------+---------------------------------------------------------------+
| No. | No. | | Bred | | | Bred | | Syndactyl. | Classification. |
| | | No. |in pen | Toes. | No. |in pen | Toes. +----+---+-----+------+------------+------------+-----------+------------+------------+
| | | | No. | | | No. | |2f. |1f.| 0f. |P. ct.| _Aa_α. | _Ab_α. | _Ab_β. | _Ab_β´. | _Bb_α. |
+------+-----+------+-------+-----------+------+-------+-----------+----+---+-----+------+------------+------------+-----------+------------+------------+
| 30 | 801 | 4569 | 767 |_Ab_α | 5399 | 747 |_Ab_α | 2 | 0 | 0 |100.0 | 1 | 0 | 3 | 0 | 0 |
| 30_a_| 801 | 4569 | 767 |_Ab_α | 4562 | 767 |Normal. | 0 | 2 | 2 | 50.0 | .. | 1 | 1 | .. | .. |
| 31 | 801 | 6843 | 767 | Normal. | 4562 | 767 | Do. | 1 | 3 | 2 | 66.7 | .. | 2 | 2 | 1 | .. |
| 32 | 801 | 872 | 627 |_Ab_β | 5399 | 747 |_Ab_α | 12 | 4 | 11 | 59.3 | 3 | 9 | 11 | .. | 5 |
| 32_a_| 801 | 872 | 627 |_Ab_β | 4562 | 767 |Normal. | 7 | 1 | 12 | 40.0 | 2 | 8 | 4 | 1 | .. |
| 33 | 801 | 5515 | 767 |_Bb_α | 5399 | 747 |_Ab_α | 4 | 0 | 7 | 36.4 | .. | 2 | 6 | .. | .. |
| 33_a_| 801 | 5515 | 767 |_Bb_α | 4562 | 767 |Normal. | 1 | 2 | 5 | 37.5 | 2 | 1 | 1 | .. | .. |
| 34 | 801 | 7528 | 767 |_Ab_β | 5399 | 747 |_Ab_α | 1 | 0 | 0 |100.0 | .. | 2 | .. | .. | .. |
| 34_a_| 801 | 7528 | 767 |_Ab_β | 4562 | 767 |Normal. | 2 | 1 | 7 | 30.0 | .. | 1 | 4 | .. | .. |
| 35 | 801 | 6861 | 767 |Normal. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 | .. | 2 | .. | .. | .. |
| 36 | 801 | 6869 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 3 | 25.0 | 1 | .. | .. | .. | .. |
| 36_a_| 801 | 6869 | 767 | Do. | 4562 | 767 |Normal. | 1 | 0 | 4 | 20.0 | .. | .. | 2 | .. | .. |
| 37 | 801 | 2831 | 658 | Do. | 5399 | 747 |_Ab_α | 3 | 1 | 18 | 18.2 | .. | 4 | .. | .. | 3 |
| 37_a_| 801 | 2831 | 658 | Do. | 4562 | 767 |Normal. | 2 | 1 | 11 | 21.4 | .. | 2 | .. | .. | 3 |
| 38 | 801 | 2526 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 5 | 0.0 | .. | .. | .. | .. | .. |
| 38_a_| 801 | 2526 | 658 | Do. | 4562 | 767 |Normal. | 1 | 0 | 2 | 33.3 | .. | 1 | 1 | .. | .. |
| 39 | 801 | 4570 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 5 | 16.7 | 1 | .. | .. | .. | .. |
| 39_a_| 801 | 4570 | 767 | Do. | 4562 | 767 |Normal. | 0 | 2 | 17 | 10.5 | 1 | 1 | .. | .. | .. |
| 40 | 801 | 1892 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 9 | 0.0 | .. | .. | .. | .. | .. |
| 40_a_| 801 | 1892 | 658 | Do. | 4562 | 767 |Normal. | 1 | 0 | 3 | 25.0 | .. | 2 | .. | .. | .. |
| 41 | 801 | 4263 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 4 | 20.0 | .. | 1 | .. | .. | .. |
| 41_a_| 801 | 4263 | 767 | Do. | 4562 | 767 |Normal. | 0 | 0 | 10 | 0.0 | .. | .. | .. | .. | .. |
| 42 | 801 | 6872 | 767 | Do. | 4562 | 767 | Do. | 0 | 0 | 6 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| Syndactyl (5399 ♂) | 22 | 8 | 62 | 32.6 | | | | | |
| Syndactyl (4562 ♂) | 17 |12 | 84 | 25.7 | | | | | |
| +====+===+=====+======+ | | | | |
| 43 | 776 | 2291 |Coch. |Normal. | 2732 |Coch. |Normal. | 2 | 0 | 6 | 25.0 | .. | 2 | .. | .. | 2 |
| 44 | 776 | 2574 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 9 | 10.0 | .. | 1 | .. | .. | .. |
| 45 | 776 | 2570 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 11 | 8.3 | .. | 1 | .. | .. | .. |
| 46 | 776 | 2297 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 12 | 7.7 | .. | .. | .. | .. | 1 |
| 47 | 776 | 2299 |Coch. | Do. | 2732 |Coch. | Do. | 1 | 0 | 16 | 5.9 | .. | 2 | .. | .. | .. |
| 48 | 776 | 2904 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 6 | 0.0 | .. | .. | .. | .. | .. |
| 49 | 776 | 2937 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 7 | 0.0 | .. | .. | .. | .. | .. |
| 50 | 776 | 2300 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 15 | 0.0 | .. | .. | .. | .. | .. |
| 51 | 776 | 2736 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 18 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| | 3 | 3 | 100 | 5.7 | | | | | |
| +====+===+=====+======+ | | | | |
| 52 | 816 | 121 |D. Br.|_Ab_α | 122 |D. Br. |Normal. | 3 | 1 | 10 | 28.6 | .. | 1 | .. | 2 | 4 |
| 52_a_| 816 | 121 |D. Br.|_Ab_α | 4912 |M × P | Do. | 0 | 0 | 13 | 0.0 | .. | .. | .. | .. | .. |
| 53 | 816 | 5835 |D. Br.|Normal. | 122 |D. Br. | Do. | 1 | 0 | 6 | 14.3 | .. | 2 | .. | .. | .. |
| 54 | 816 | 2353 |D. Br.| Do. | 122 |D. Br. | Do. | 0 | 0 | 7 | 0.0 | .. | .. | .. | .. | .. |
| 54_a_| 816 | 2353 |D. Br.| Do. | 4912 |M × P | Do. | 0 | 0 | 4 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| Syndactyl ( 122 ♂) | 4 | 1 | 23 | 17.9 | | | | | |
| Syndactyl (4912 ♂) | 0 | 0 | 17 | 0.0 | | | | | |
+------------------------------------------------------------------+----+---+-----+------+------------+------------+-----------+------------+------------+
The types of syndactylism which have appeared in my flock form a
rather extensive series. First, (_A_) the single web, which, in my
specimens, always occupies the interspace between digits III and
IV. This is the same interval which is most apt to show the web in
syndactylism of the human hand, and, it is suggestive to note, it
is this interval that is filled in those wading birds that have the
single web only between the toes (_e.g._, _Cursorius_, _Glareola_,
_Vanellus_, _Squatarola_, _Charadrius_, _Limosa_, _Machetes_,
_Himantopus_); second, there is (_B_) the double web, one-seventh
as common, which always occupies the interspaces between the digits
II-III and III-IV.
On another basis, the syndactyl feet may be classified as: (_a_) toes
adherent, web small in extent, and (_b_) toes distant, web broad.
I have found the narrow web only between digits III and IV. It is
one-eighth as common as the broad-webbed type. The broad, double web
approaches closely to the type found normally in swans, geese, and
ducks.
Finally, the syndactyl feet may be classified as: α,
straight-toed, or β, curve-toed. Class α is
to class β in frequency as 2:1. In the typical curve-toed
syndactyl foot the web between III and IV is complete to the nails of
each; in fact, in extreme cases the nails of the two toes are more or
less fused together. From the fused nails the middle toe, being the
longer, passes in a curve to the distal end of the metatarsus. The
D-shaped interspace between the curved III and straight IV toe is
filled with the web. In other cases the nails are merely approximated
and the middle toe is slightly curved. In three instances (4 per cent
of all) the outer toe (IV) is curved toward the (straight) median toe
(class β´).
As stated, the polydactyl offspring trace back their ancestry to
No. 121; her feet both show the double, broad, straight-toed type
(_Bb_α). We shall attempt in the following paragraphs to
trace the heredity of her type of polydactylism and of the others
that have subsequently arisen.
B. RESULTS OF HYBRIDIZATION.
In taking up the results of breeding experiments to test the method
of inheritance of syndactylism, it will be best first to give in
a table all pens in which the character showed itself, with the
frequency of the different types of foot in them (table 23).
The history of the syndactyl strain begins with No. 121 ♀ and
in the matings 1 to 8 are given the results of crossing together some
of her progeny derived from a normal-toed father. This father was
either No. 8A or 1A, both full-blooded Tosa (Japanese Game) fowl and
without suspicion in either soma or offspring of syndactyl taint.
There is no record of trace of syndactylism in the progeny of 121 ×
8A (or 1A); but a slightly developed condition of syndactylism may
very well have been overlooked by me in this F1 generation (as I had
never thought of such an abnormality), even as I at first overlooked
the syndactylism visible in No. 121. But when these F1 hybrids were
mated together (pen 627, serial Nos. 1 to 8) I got, in the different
families, from 10 per cent syndactyl offspring down to none at all.
At first sight the suggestion arises that, if inheritance is at
all Mendelian, the normal condition is dominant and that the
heterozygotes throw again, in pen 627, the syndactylous condition.
If this hypothesis were true it would follow that syndactyls bred
together should, sometimes at least, throw, even in large families,
100 per cent syndactyl offspring. But only 2 families, Nos. 30 and
34, have yielded 100 per cent syndactyls, and these contained 2 and
1 offspring, respectively; so they are not significant. On the other
hand, there are numerous matings of 2 extracted normal-toed parents
that have produced only normal-toed offspring (families Nos. 14, 15,
21, 22, 23, including 119 individuals). Consequently the conclusion
is favored that normal-foot is recessive and syndactyl-foot dominant,
and this shall be our working hypothesis.
On our hypothesis, No. 121 is probably a heterozygote. Mated with the
recessive normal, expectation is 50 per cent heterozygous, showing
syndactylism; the remainder normal-toed. But dominance is here, as
in polydactylism, very imperfect. For this reason and because it
was not looked for, no syndactylism was noted in the first hybrid
generation. The offspring prove to be of two sorts, however. No. 180
♂ is a pure recessive, and in 8 matings with as many different
sisters of his he got 184 normal-toed to 1 syndactyl. These same
sisters, mated to another brother, No. 242, in some cases gave 9 per
cent and 10 per cent syndactyl. No. 242 is, consequently, probably
a DR and, mated to DR sisters (which constitute according to
expectation about one-half of all) gives some DD's, part of which
constitute the 9 to 10 per cent of syndactyls. Of course, 25 per
cent DD is to be expected; the difference gives a measure in this
instance of the imperfection of dominance in the "extracted" as well
as "heterozygous" condition.
Matings 9 to 15 (pen 747) are instructive in comparison with the
foregoing case. Both parents are derived from pen 658, which
contained as breeders a heterozygous Dark Brahma male (No. 146) and
various females of non-booted races far removed from suspicion of
syndactylism; expectation being an equal number of DR and RR
offspring. In pen 747 No. 1888 ♂ acts like a DR, and so do the
hens in matings 9 to 13, while the hens in the other 2 matings are
doubtless RR's. The former give 17 per cent syndactyl offspring,
the latter none at all (in 56 individuals).
Matings 16 and 17 (pen 703) are between pure-bred Dark Brahmas
that are probably DR's. About 22 per cent of their offspring are
syndactyl--a rather higher proportion than we have found before.
Matings 18 to 19 are between progeny of pen 627. In mating 20 the
normals were not recorded. The cock in this pen, No. 871, is probably
heterozygous, as are also the first two hens, so that nearly 30
per cent of their progeny are syndactyl. From the other 3 hens no
syndactyl offspring were obtained. Evidently the two sets of hens
have a very different gametic constitution. The existence of two
sorts of families is one of the strong arguments for the segregation
of this character.
We next come to the pens (matings Nos. 24 to 42) which were
especially mated to study the inheritance of syndactylism. I had now,
for the first time, two parents with syndactylic feet.
On account of imperfection of dominance decision as to gametic
composition of any parent must largely rest on the make-up of the
progeny. Table 24 gives the most reasonable classification of the
parentages.
TABLE 24.
+-----------------------------------------------------------------------------------+
| DD × DD (SYNDACTYL × SYNDACTYL). |
+------+--------+-------+-----------+--------+-------+-----------+------------------+
| | | | | | | | Syndactyl. |
|Family|Mother's|Bred in| Toes. |Father's|Bred in| Toes. +---+---+---+------+
| No. | No. |pen No.| | No. |pen No.| |2t.|1t.|0t.|P. ct.|
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 30 | 4569 | 767 |_Ab_α | 5399 | 747 |_Ab_α | 2 | 0 | 0 | 100.0|
| 34 | 7528 | 767 |_Ab_β | 5399 | 747 |_Ab_α | 1 | 0 | 0 | 100.0|
| 32 | 872 | 627 |_Ab_β | 5399 | 747 |_Ab_α |12 | 4 |11 | 59.3 |
| 33 | 5515 | 767 |_Bb_α | 5399 | 747 |_Ab_α | 4 | 0 | 7 | 36.4 |
| +---+---+---+------+
| Totals |19 | 4 |18 | 74.2 |
+----------------------------------------------------------------+---+---+---+------+
| DD × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 31 | 6843 | 767 |Normal. | 4562 | 767 |Normal. | 1 | 3 | 2 | 66.7 |
| 30_a_| 4569 | 767 |_Ab_α | 4562 | 767 | Do. | 0 | 2 | 2 | 50.0 |
| 33_a_| 5515 | 767 |_Bb_α | 4562 | 767 | Do. | 1 | 2 | 5 | 44.4 |
| 32_a_| 872 | 627 |_Ab_β | 4562 | 767 | Do. | 7 | 1 |12 | 42.9 |
| 34_a_| 7528 | 767 |_Ab_β | 4562 | 767 | Do. | 2 | 1 | 7 | 30.0 |
| 36 | 6869 | 767 |Normal. | 5399 | 747 |_Ab_α | 0 | 1 | 3 | 25.0 |
| 25_a_| 872 | 627 |_Ab_β | 3116 | D. Br.|Synd. | 7 | 1 |30 | 21.1 |
| 41 | 4263 | 767 |Normal. | 5399 | 747 |_Ab_α | 0 | 1 | 4 | 20.0 |
| 37 | 2831 | 658 | Do. | 5399 | 747 |_Ab_α | 3 | 1 |18 | 18.2 |
| 39 | 4570 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 5 | 16.7 |
| 40 | 1892 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 9 | 0.0 |
| +---+---+---+------+
| Totals |21 |14 |97 | 26.5 |
+----------------------------------------------------------------+---+---+---+------+
| DR × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 38_a_| 2526 | 658 | Normal. | 4562 | 767 | Normal. | 1 | 0 | 2 | 33.3 |
| 35 | 6861 | 767 | Do. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 |
| 40_a_| 1892 | 658 | Do. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 |
| 37_a_| 2831 | 658 | Do. | 4562 | 767 | Do. | 2 | 1 |11 | 21.4 |
| 36_a_| 6869 | 767 | Do. | 4562 | 767 | Do. | 1 | 0 | 4 | 20.0 |
| 24 | 2526 | 658 | Do. | 3116 |D. Br. | Synd. | 5 | 0 |22 | 18.5 |
| 26 | 2104 | 608 | Do. | 3116 | Do. | Do. | 3 | 0 |18 | 14.3 |
| 39_a_| 4570 | 767 | Do. | 4562 | 767 | Do. | 0 | 2 |17 | 10.5 |
| 27 | 2831 | 658 | Do. | 3116 |D. Br. | Do. | 3 | 0 |32 | 8.6 |
| 29_a_| 190 | 520 | Do. | 3116 |D. Br. | Do. | 4 | 0 |49 | 7.6 |
| 29 | 767 | 190 | Do. | 242 | 513 | Do. | 1 | 1 |28 | 6.7 |
| 28_a_| 181 | 513 | Do. | 3116 | Do. | Do. | 1 | 1 |60 | 3.2 |
| +---+---+---+------+
| Totals |23 | 5 |249| 10.1 |
+----------------------------------------------------------------+---+---+---+------+
| RR × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 42 | 6872 | 767 |Normal. | 4562 | 767 |Normal. | 0 | 0 | 6 | 0.0 |
| 41_a_| 4263 | 767 | Do. | 4562 | 767 | Do. | 0 | 0 |10 | 0.0 |
| +---+---+---+------+
| Totals | 0 | 0 |16 | 0.0 |
+----------------------------------------------------------------+---+---+---+------+
Summarizing the foregoing, and comparing the totals with Mendelian
expectation, we get the result shown in table 25.
A comparison of realization and expectation in table 25 shows that
the proportion of syndactyls is always less than expectation, not
only for dominants and heterozygotes together, but even for pure
dominants alone. The proportion of syndactyls obtained diminishes,
to be sure, in accordance with expectation (on the assumption that
they are pure dominants), but the numbers lag behind, in the higher
proportions 40 to 25 per cent. So we reach the conclusion that, as
in polydactylism, so in syndactylism dominance is very imperfect.
But there is this difference, that in syndactylism dominance is
so imperfect that the dominant condition rarely shows itself in
heterozygotes and even fails in many pure dominants. The striking
fact, the one that assures us the segregation is nevertheless
occurring in this case too, is that some families (whose two parents
are extracted recessives) throw 100 per cent recessives.
TABLE 25.
+-------------+-----+-----------------------------+--------------+
| | | Expectation. | Realization. |
| Nature of | +--------------+--------------+--------------+
| mating. | f | Dominants + | Pure | |
| | |heterozygotes.| dominants. | Syndactyls. |
+-------------+-----+--------------+--------------+--------------+
| | | _P. ct._ | _P. ct._ | _P. ct._ |
| DD × DD | 41 | 100.0 | 100.0 | 56.1 |
| DD × DR | 132 | 100.0 | 50.0 | 26.5 |
| DR × DR | 277 | 75.0 | 25.0 | 10.1 |
| RR × DR | 16 | 50.0 | 0.0 | 0.0 |
| RR × RR | 119 | 0.0 | 0.0 | 0.0 |
+-------------+-----+--------------+--------------+--------------+
These studies on syndactylism in poultry may be used for a critical
examination of the recent work of Lewis and Embleton (1908) on
syndactylism in man. The cases described by them follow the types I
have just described in poultry. Their fig. 18 corresponds to my types
_a_ and α; figs. 10 and 11 to my type β. The "crossbones"
referred to by the authors correspond to bones of the "curved toe."
The facts presented by the authors support the idea that syndactylism
is dominant rather than recessive, but they deny the application of
Mendelian principles to this case. Actually, the foot deformities
described by Lewis and Embleton are inherited much like syndactylism
in poultry. No extracted normal (recessive) extremity produces
the abnormal condition. Heterozygotes show much variation, from
very abnormal to slightly abnormal (possibly perfectly normal?)
appendages. Dominance is, indeed, much more potent than in poultry.
The authors' denial of the application of Mendelism to this case
seems to be based on an all too superficial consideration of the
hereditary behavior of the character and a tendency to "mass"
statistics--a procedure that tends to obscure the interpretation of
the data of heredity.
As to the inheritance of type, my statistics are not extensive enough
to give a final answer, but if all types be grouped into those with
straight and those with curved toes, then in crosses of straight-toed
syndactyl and normal 33 per cent of the offspring were of the curved
type, whereas in crosses of curved-toed syndactyls and normal 45 per
cent were of the curved type. These averages depend on 22 and 15
individuals, respectively. They lead us to look for an inheritance of
type when more extensive data shall be available.
Syndactylism is a typical sport, that is, a rather large mutation
having a teratological aspect. The question arises, Does it prove to
be prejudicial to the welfare of the species? The breeder who has
only a few individuals of a rare sport feels their loss more than
that of normals and the general impression left in his mind is that
the sport is less capable of maintaining itself than the normal form.
Assembling the data, consisting of about 40 individuals of each kind,
it appears that the death-rate is not very different in the two lots;
the slight excess of that of the syndactyls is sufficiently accounted
for by the circumstance that no _normals_ were reared during the
period of greatest mortality (the summer), but were destroyed or
given away as soon as hatched. It is probable, therefore, that
syndactylism, under the conditions of the poultry-yard, has little
life and death significance, but is one of those neutral characters
whose existence Darwin clearly recognized.