SEX-LINKAGE 



any family. Instead of all the Y-linked genes of the father passing 

 to the son, a proportion pass to his daughters. The proportion 

 depends on the crossing-over distance between the gene in question 

 and the segment with complete sex linkage (Fig. 12). 



64,3 



35.6 



32.5 



67.7 



Fig. 12. — Segregation in a cross in the fish Xtphophorus helleri. Taken over the 

 whole progeny the gene determining spotted v. spotless gives a good i : i ratio; 

 but the two types are unequally distributed between the sexes. This is the type of 

 segregation characteristic of partial sex-li:ikage, though the validity of this inter- 

 pretation has not been fully established in the present case (from Kosswig, 1939). 



Finally, in Drosophila and in man, there are completely Y-linked 

 genes without any allelomorph in the X. In man such genes have 

 been recognized by their transmission only from father to son ; in 

 Drosophila by the effects of absence of the whole or part of the 

 Y chromosome. Flies are sometimes hatched without any Y. They 

 are perfect-looking males but their sperm cannot swim, so that they 

 are sterile. The Y chromosome is therefore doing something, although 

 what it does cannot be shown by its relationship with mendelian 

 differences in the X. 



There are thus three kinds of parts in the sex chromosomes: 



51 



