1921.] 



Eesearch in Animal Breeding. 



327 



Earlier experiiiieiits, conducted in Cambridge, had revealed 

 the existence of a peculiar form of inheritance to which the name 

 sex-linked heredity was given. The nature of this may be 

 illustrated by a case of the sort which was investigated on the 

 University Farm. In discussing the Hamburgh-Sebright cross 

 used for the weight experiments we stated that the Hamburgh 

 was a gold pencilled, and the Sebright a silver. These colours 

 were deliberately chosen as there was some evidence that gold 

 and silver formed an alternative pair, and that the case was one 

 of sex-linked heredity. The experimental work showed that this 

 w^as so. Silver behaves as a simple dominant to gold, but in the 

 hen the transmission of the factor for silver is sex-linked. The 

 silver hen, no matter how bred, is never pure for the silver 

 factor; half of her eggs are " silver " and half are " gold." 

 Moreover she transmits the silver factor to her male-producing 

 eggs, and the gold to her female-producing ones. If we denote 

 silver by S, and gold by s, and maleness and femaleness by M 

 and F respectivelv, then the constitution of the silver hen is 

 MFSs. Such a hen forms two kinds of eggs only, viz., those 

 bearing maleness and silver (MS^>, and those bearing femaleness 

 and gold (Fs) ; and they are formed in equal numbers. This is 

 at once apparent when she is mated with a gold male, il/-¥.«?s. All 

 of the sperms of such a cockerel are of the same kind in respect 

 of these factors, viz., Ms. The male eggs of the silver hen (MS), 

 w^hen fertilised by the sperm of the gold cockerel (Ms) give birds 

 of the constitution MMSs, i.e., silver males. The female eggs 

 of the silver hen (Fs) when similarly fertilised, give birds of 

 the constitution MFss, i.e., gold females. We have bred a great 

 number of birds from the mating of silver hen and gold cockerel, 

 and hsive never met an exception to the rule that the cockerels 

 all come silver, and the pullets all gold. 



This peculiar sex-linked type of inheritance is found in several 

 other characters in poultry. It was demonstrated in America 

 to hold good for the character of barred plumage such as is 

 found in Plymouth Eocks. Bari-ing is dominant to self black, 

 but the barred hen is never pui-e for the barred factor. She 

 transmits barring to her sons and black to her daughters . When 

 mated with a black cockerel she gives only barred cockerels and 

 black pullets. This observation we have been able to confirm 

 in the course of our experiments. 



As has already been pointed out in this Journal,* sex-linked 



" The Earlv Elimination of Smplns Cockerels, liy R. (". Piinnttt. Jour, of 

 the B(L of Afjric, Pebniary. 1*H1>, \k 13111. . 



