268 



GENETICS 



equal and the resulting zygotes, therefore, are approxi- 

 mately 50 per cent male and 50 per cent female 

 (Fig. 82). 



A. THE Y-CHROMOSOME 



The foregoing is the simplest case of sex-determina- 

 tion known and, while this is the fundamental type, 

 still there are many variations of the mechanism. For 

 example, the ^-chromosome may have a "y" partner 

 in the male cells, in which case, if n = the haploid, or 



FIG. 82. Diagram to show how numerical equality of the sexes 

 results when one parent is homozygous (the female in this 

 instance) and the other is heterozygous for the sex character. 



halved, set of autosomes in a given animal, then the 

 following formula holds true: 



2n -f- xy = male and 2n + xx = female. 



In the spermatogonia of animals maturating in this 

 manner, half the spermatids receive an ^-chromosome 

 and half a ^-chromosome, the latter being the male- 

 determining spermatozoa. 



In certain other cases the ^-chromosome may be 

 represented by several discrete, i.e., separate, compo- 

 nents, and it may or may not have a ^-chromosome 



