SEX DETERMINATION AND SEX-LINKED HEREDITY 415 



While in the birds the chromosomal condition has never been 

 completely worked out, on account of inherent technical difficulties 

 likely to be overcome any day, the case of the currant moth. Abraxas, 

 has been thoroughly analyzed. The sex linkage follows the poultry 

 plan and the gametes of the male have been found to be all alike, while 

 those of the female are of two types, one containing an X-chromosome 

 (W-chromosome) and the other a Y-chromosome (Z-chromosome). 

 The striking parallelism between the reversal in sex-linked heredity 

 and in the visible reversal of chromosome composition in these two 

 groups of animals (Drosophila and man, on the one hand, and the 

 butterflies, moths, and birds, on the other) offers one of the most 

 cogent proofs of the validity of the chromosome theory of heredity, 

 which we have already come to rely upon and shall have further 

 occasion to make use of later on. 



To bring the facts of sex-Hnked heredity sharply into focus by 

 way of summary, let us quote from D. F. Jones a genetic formulation 

 of the whole matter: 



"Rules for sex-linked inheritance. — From this series of facts tlie 

 following rules governing the transmission of sex-linked characters 

 can be deduced. 



"i. When the homozygous sex transmits the dominant factor, all 

 of the offspring in the first generation exhibit the dominant character 

 and the second generation is composed of three dominants to one reces- 

 sive, the latter having the same sex as the recessive grandparent. 



"2. When the homozygous sex transmits the rece<;sive factor, both 

 dominant and recessive characters are exhibited in the first generation, 

 but exclusively upon the opposite sexes, and in the second generation 

 both sexes show the sex-linked characters in equal numbers." 



