SEX-CHROMOSOMES AND INHERITANCE 177 



factor from their father, the sons get in addition to a light 

 Z from their father a dark dominating Z from their 

 mother. When the F^^s are bred together four classes 

 result in the proportion of 1:1:1:1, when sex is taken 

 into consideration, or in the ratio 1 : 1 for the color differ- 

 ences alone. 



According to Doncaster, the male and the female 

 Abraxas have each 56 chromosomes, i.e., the female is ZW 

 rather than ZO; but as yet the sex-chromosomes as such 

 have not been identified. That sex is connected with such 

 chromosomes is not only established by sex-linked in- 

 heritance, but is also indicated by an aberrant race of 

 Abraxas found by Doncaster. The males of the race had 

 the normal number of chromosomes (56), but the females 

 had only 55 chromosomes. Doncaster found that in these 

 females an unpaired chromosome, presumably the Z-chro- 

 mosome, was more often thrown out into the polar body 

 than left in the egg, so that most of the resulting eggs had 

 only 27 chromosomes. Any egg of this kind fertilized 

 by a spermatozoon should give a 55-chromosome indivi- 

 dual, i.e., a female. The few eggs that retained the 

 unpaired Z-chromosome, fertilized by a Z-spermatozoon, 

 would be expected to give rise to the rare males, which 

 like normal males have 56 chromosomes. The excess of 

 females is thus accounted for, and incidentally the results 

 show that the TF-chromosome carries no essential factors 

 for the life of the individual, since females without it 

 develop and look like normal females. Probably it is 

 empty as is the Y of Drosophila. 



In poultry there are several cases of sex-linked inheri- 

 tance that follow the Abraxas^ type. One of the most 

 striking cases is the cross between Barred Plymouth Rock 

 and Langshan. When a barred male is crossed to a black 

 female, the sons and daughters are barred (Fig. 76). 

 Barring is dominant to black. Two such F\ 's, inbred, give 

 all barred males ; half the hens are barred, half are black. 

 It may be said here that the black grandmother transmits 



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