40 INHEBITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



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 ordi- 

 nary 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 9 (a pure White 

 Leghorn bred by myself from original White Leghorn stock described in 

 my 1906 report) and No. 235 c? (an F^ 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 Fi 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 fuUy 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. 2464d' 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 pro- 

 visional hypothesis, suggested in my earlier report, that rumplessness is 

 in my strain recessive has not been supported by the newer facts. In the 

 Ught 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 — ike 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 X R, and expectation in the tjrpical 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 nu- 

 merous sorts, DR and RR. Mated to No. 117, two sorts of famiUes are to be 

 expected, namely, the products of DR X RR (=50 per cent DR, 50 per cent 

 RR) and the products of DR X 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 pres- 



