CHAPTER IV._Inheritance of Characteristics in Domestic Fowl

RUMPLESSNESS.

The tail of vertebrates is, historically, the post-anal part of the

trunk. Containing no longer any part of the alimentary canal, it has

lost much of its primitive importance, so that its disappearance in

any case is a matter of relatively little importance. Accordingly

we find groups of animals in which it is rudimentary or wholly

absent, such as many amphibia and the anthropoid apes and man. In

all recent birds the tail is a distinct but much reduced organ--the

uropygium--which contains several vertebræ in a degenerate condition.

The uropygium supports the tail feathers, which are of much use in

directing the bird in flight, but in ground birds, such as the grouse

and poultry, seem to function only for display in the male and, in

the female, to facilitate copulation.

Now, among various typically tailed vertebrates the tail is sometimes

absent. Tailless dogs, cats, sheep, and horses are known; on the

other hand, several cases of tails in man have been described

(Harrison, 1901). Thus the tail is a part of the body subject to

sporting; and it has also become the differential character for some

specific groups. In other words, it is an organ that has played

an important part in evolution and consequently its method of

inheritance is a matter of great interest.

The origin of the tailless poultry which I have bred has been

twofold. The most important strain is that referred to in an earlier

report[7] as Bantam Games. The second lot consists of rumpless fowl

that have arisen in my yards, spontaneously, from normal blood. Of

these more later.

[7] Davenport, 1906, pages 62 to 64, fig. 46.

The two rumpless Game cocks bore the numbers 117 and 116. Dr. A. G.

Phelps, of Glens Falls, New York, from whom the birds were purchased,

wrote that he had imported No. 117 from England, and No. 116 was its

son. The birds were very closely similar in all external features.

The matings made with No. 117 and their results are given in table 26.

TABLE 26.--_Progeny of tailless cock and tailed hens._

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

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

| No. | No. | No. | No.| Races. |--------+------+---------|rumpless.|

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

| 1 |525 | 117 |114 | Nankin. | 3 | .. | 0 | 0 |

| 2 |526 | 117 | 20A| Frizzle. | 8 | .. | 0 | 0 |

| 3 |532 | 117 | .. |Bl. Coch. | 14 | .. | 0 | 0 |

| 4 |532_a_| 117 |127 |Wh. Legh. | 19 | .. | 0 | 0 |

| 4_a_|653 | 117 |508 |Bl. Coch. × Wh. Legh.| 8 | 3 | 0 | 0 |

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

| | | | | Totals | 52 | 3 | 0 | 0 |

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

In 25 cases of the 52 an oil-gland was looked for and, in every case,

it was found to be missing.

Table 26, the conclusions from which were drawn in my 1906 report,

seemed to indicate the dominance of tail over its absence. On this

hypothesis I suspected that if No. 117 were bred to his (tailed)

offspring about 50 per cent of the progeny would be tailless, and if

the tailed hybrids of the F1 were bred together about 25 per cent of

their progeny should be tailless. The actual result of such matings

is shown in table 27.

TABLE 27.--_Heterozygotes mated with father._

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

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

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

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

| 5 | 653| 117 |Original.| 577 | 532 | 6 | 1 | 0 |

| 6 | 653| 117 | Do. | 587 | 532 | 8 | 2 | 0 |

| 7 | 653| 117 | Do. | 635 | 532 | 7 | 0 | 0 |

| 8 | 653| 117 | Do. | 691 | 532 | 5 | 2 | 0 |

| 9 | 653| 117 | Do. | 652 | 532 | 15 | 0 | 0 |

| 10 | 653| 117 | Do. | 691 | 532 | 5 | 2 | 0 |

| 11 | 653| 117 | Do. | 705 | 532 | 9 | 2 | 0 |

| 12 | 653| 117 | Do. | 713 | 532 | 7 | 2 | 0 |

| 13 | 653| 117 | Do. | 760 | 532 | 13 | 2 | 0 |

| 14 | 653| 117 | Do. | 799 | 532 | 7 | 0 | 0 |

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

| | | Total | 82 | 13 | 0 |

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

TABLE 28.--_Heterozygotes mated inter se._

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

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

| | | | No. | | No. | | | | | | |

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

| 15 | 661| 466| 526 | 401A| 526 | 5 | 0 | 0 | 100 | 0 | 0 |

| 16 | 661| 466| 526 | 635 | 532 | 5 | 0 | 0 | 100 | 0 | 0 |

| 17 | 661| 466| 526 | 691 | 532 | 4 | 0 | 0 | 100 | 0 | 0 |

| 18 | 661| 466| 526 | 799 | 532 | 4 | 1 | 0 | 80 | 20 | 0 |

| 19 | 649| 516| 532A| 521 | 532A| 17 | 4 | 0 | 81 | 19 | 0 |

| 20 | 649| 516| 532A| 565 | 532A| 24 | 7 | 0 | 77 | 23 | 0 |

| 21 | 649| 516| 532A| 665 | 532A| 11 | 4 | 0 | 73 | 27 | 0 |

| 22 | 649| 516| 532A| 692 | 532A| 18 | 1 | 0 | 95 | 5 | 0 |

| 23 | 652| 343| 525 | 344 | 525 | 8 | 2 | 0 | 80 | 20 | 0 |

| 24 | 661| 428| 526 | 635 | 532 | 4 | 0 | 0 | 100 | 0 | 0 |

| 25 | 661| 428| 526 | 691 | 532 | 3 | 0 | 0 | 100 | 0 | 0 |

| 26 | 661| 428| 526 | 799 | 532 | 5 | 0 | 0 | 100 | 0 | 0 |

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

| Total | 108 | 19 | 0 | 85 | 15 | 0 |

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

The results given in tables 27 and 28 are remarkable. Neither in

the DR × R nor the DR × DR crosses did the tail fail to

develop. The tailless condition, that I had strongly suspected of

being recessive and expected in 25 per cent to 50 per cent of the

offspring, never once appeared. The only point of variation in the

uropygium of the chicks derived from the back cross or from F1's

bred _inter se_ was that in some the uropygium seemed distinctly

smaller than in the others. This small uropygium was as a matter

of fact recorded chiefly in chicks that failed to hatch, but it was

occasionally noticed in older birds, being then usually associated

with a slight convexity of the back. In some of the families the

uropygium is recorded as small in suspiciously close to 25 per cent

of the offspring. There is little doubt in my mind that this small

uropygium represents in some way the "absence" of tail that was

expected.

The next step was to cross the other rumpless bantam (No. 116),

to see if he behaved like his father. Accordingly, in pen 653, I

replaced the cock No. 117 by 116, the hens remaining the same, and

got the result shown in table 29.

TABLE 29.--_Heterozygotes mated with No. 116._

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

|Serial| Father's | Mother's +----------+--------+---------+----------+

| No. | No. | No. | Present. | Small. | Absent. | Per cent |

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

| 27 | 116 | 508 | 5 | 2 | 10 | 59 |

| 28 | 116 | 577 | 3 | 0 | 3 | 50 |

| 29 | 116 | 587 | 3 | 1 | 4 | 50 |

| 30 | 116 | 652 | 4 | 0 | 2 | 33 |

| 31 | 116 | 705 | 3 | 1 | 5 | 56 |

| 32 | 116 | 713 | 1 | 0 | 2 | 67 |

| 33 | 116 | 760 | 4 | 0 | 2 | 33 |

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

| Totals (55) | 23 | 4 | 28 | 51 |

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

Here we get a result almost exactly in accord with Mendelian

expectation. Having, now, obtained rumpless hens, it became possible

for the first time to test the inheritance of rumplessness in both

parents. The result is shown in the table 30.

TABLE 30.--_Rumpless fowl mated inter se._

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

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

|Serial| Pen | | | | | | | |

| No. | No. | | From | | From | | | |

| | | | No. | | No. | | | |

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

| 34 | 742 | 2978 | 27 | 2601 | 29 | 0 | 0 | 4 |

| 35 | 854 | 2978 | 27 | 3430 | 27 | 0 | 0 | 9 |

| 36 | 742 | 2978 | 27 | 3430 | .. | [A]2 | 0 | 7 |

| 37 | 854 | 2978 | 27 | 2977 | 27 | [B]1 | 0 | 1 |

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

| Total | 3 | 0 | 21 |

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

[A] Both from chicks that died in shell.

[B] From a hatched chicken.

Table 30 is unfortunately small; one may say, fragmentary. Rumpless

hens are incapable of copulating unless the tail coverts are trimmed;

moreover my birds have been so much inbred that they are very weak;

finally, the chicks are so small that it is impracticable to rear

them in brooders and the eggs are particularly apt to be broken by

the brooding hens. However, it suffices to show that two tailless

fowl are able to throw some tailed offspring.

The second lot of rumpless fowl, namely, those that arose _de novo_

in my yards, must now be considered. In 1906, 2 birds hatched out

from ordinary tailed strains. As one was a cock and the other a

hen these were mated in 1907. The cock (No. 2464) came from No.

71♀ (a pure White Leghorn bred by myself from original White

Leghorn stock described in my 1906 report) and No. 235♂ (an F1

hybrid between one of these White Leghorns and my original Rose-comb

Black Minorca). The hen was No. 1636. Her mother (No. 618) was an F1

hybrid between a Minorca and Dark Brahma of series V, 1906 report,

and her father (No. 637) had the same origin. Thus the parents and

grandparents of both of these new rumpless birds were well known to

me and known to be fully tailed and to throw only tailed birds, with

the exception of these two birds.

The result of the mating of Nos. 2464 and 1636 in pen 736 was 25

chicks, of which 24 had tails and 1 (No. 5335) was without tail or

oil-gland. This, unfortunately, died early, so it was impossible to

breed it. In 1908, the hen No. 1636 having in the meantime died, I

mated No. 2464♂ to 6 of his (tailed) daughters. He was not well

and soon died, leaving no descendants by them, but 5 offspring by a

female cousin, all tailed. Then one of his sons (tailed) was mated

to its own sisters and produced 49 offspring, all tailed. Thus the

strain seems to have died out. The whole history is important both

because an apparently new mutation had taken place and because it

was, in a degree, "hereditary."

How, if at all, can this case and those of the bantams be brought

under known laws of inheritance? First of all, it must be confessed

that the provisional hypothesis, suggested in my earlier report, that

rumplessness is in my strain recessive has not been supported by the

newer facts. In the light of the principle of imperfect dominance

to which the facts of the last two chapters have led us, everything

receives a satisfactory explanation. The only conclusion that meets

all the facts is this: _The inhibitor of tail development--the

tailless factor--is dominant_; _its absence_--permitting a

continuation of the normal development of the tail region--_is

recessive_.

The application of this hypothesis to the various matings may now be

attempted. No. 117 is to be regarded as a heterozygote. The matings

with tailed birds is of the order DR × R, and expectation in the

typical case is 50 per cent DR (interrupted tail) and 50 per cent

RR (non-interrupted). But, owing to the relatively weak potency

of the interrupter derived from No. 117, growth of the tail is not

interrupted in the heterozygous offspring. These offspring are, by

hypothesis, so far as their gametes go, of two equally numerous

sorts, DR and RR. Mated to No. 117, two sorts of families are

to be expected, namely, the products of DR × RR (=50 per cent

DR, 50 per cent RR) and the products of DR × DR (=25 per

cent DD, 50 per cent DR, 25 per cent RR). The first lot of

families might be expected to resemble the preceding generation in

consisting entirely of tailed birds; the latter might be expected to

show in the 25 per cent extracted DD's evidence of the presence

of the undiluted interrupter. Actually in matings of the latter sort

(table 27) 3 families show no trace of the tail-interrupter, but in

7 there is evidence of a disturbance, as shown by the small size of

the uropygium and the bent back. In these families there are 13 cases

of small uropygium to 53 of large, being about 20 per cent of the

affected uropygium where 25 per cent was to be looked for--not a wide

departure, considering the liability of not recognizing the reduced

uropygium as such. This failure even of the extracted dominants

completely to stop the development of the tail gives a measure of the

weakness of the inhibitor in this case. Also, in table 28, matings

are varied. Some are probably matings of two heterozygotes, others of

two recessives, and others still of a recessive with a heterozygote.

On our hypothesis we should expect some of the families of the mated

hybrids to show evidence of the inhibiting factor and others to show

no such evidence. In those families in which small tail appears

it is found in about 19 per cent of the cases. On account of this

weakness of the inhibitor in the germ-plasm of No. 117 that inhibitor

is rarely fully activated. Only in one case out of the 250 or more

in which that germ-plasm is used is the development of the tail

completely stopped. In this case a hybrid cock derived from pen 526

(series 2, table 26) was crossed with various birds of tailed races

(probable RR's), and produced in addition to 20 tailed offspring 1

devoid of uropygium and oil-gland. In this case we may conceive that

an unusually potent condition of the inhibitor wholly stopped the

development of the tail.

The behavior of No. 116 is that of a pure dominant. Mated to

DR (and some RR?) females he produces pure dominants and

heterozygotes. His inhibiting factor is potent enough to be active in

the DD offspring at least; as a matter of fact 47 per cent of his

get have their tails inhibited. Even in the DR's the inhibitor may

sometimes work itself out. Thus No. 116 crossed on No. 508, without

tailless ancestry, had 56 per cent of the progeny without tail.

Since tailless birds may be either pure dominants or DR's, we may

expect families of two sorts when two such are bred together--those

containing only tailless offspring and those containing only 75 per

cent or less of such. Both sorts of families are to be expected in a

table with the composition of table 30, and both appear there.

The case of the rumpless fowl that arose _de novo_ will be explained,

then, as follows: Even in normal RR matings the inhibiting factor

may arise by mutation. But even when two of these inhibiting factors

are paired they show themselves so weak as not to appear in 25 per

cent, much less the typical 75 per cent of cases, but, as in our

case, merely 4 per cent. The strain takes on, indeed, the essential

features of the "eversporting varieties" of De Vries (1905). It seems

probable, therefore, that even in eversporting varieties inheritance

may be Mendelian, modified by variations in "potency" as shown by

irregularities in dominance.