138 SEX INHERITANCE 



female may be daughters. Sex-linked lethals cause 

 this. A sex-linked lethal is inherited in the sarne 

 way as any other sex-linked gene, but causes the 

 death of any male that carries the gene in his single 

 X chromosome. A female, heterozygous for such 

 a recessive lethal, will survive. She produces only 

 half as many sons as daughters. Such 2 : 1 ratio 

 strains can be continued indefinitely by simple 

 breeding methods. A female may even be heter- 

 ozygous for two different recessive lethals. If these 

 lethals are both carried in the same member of the 

 X-pair, and are close together in the chromosome, 

 only a few more than half the sons will be eliminated ; 

 but if they are far apart, so that crossing over be- 

 tween them is frequent, as many as three-fourths 

 of the sons may be killed. If the two lethals are 

 in opposite members of the X-pair and far apart, ' 

 then also about three-fourths of the sons are elimi- 

 nated; but the closer together the loci of these 

 opposed lethals are, the greater is the proportion of 

 sons killed. Thus, two sex-linked lethals may 

 determine any sex-ratio from 2? : Icf to 2? lOd", 

 according to the particular linkage relations. There 

 is also a large class of sex-linked mutations in Dro- 

 sophila that are "semi-lethal." Some of them kill 

 all males except an occasional one, while still others 

 allow more males to come through, the numbers of 

 male survivals being characteristic for each type, 

 and ranging up to the normal 1 : 1 ratio. 



Lethals that kill female zygotes are not as 

 common as lethals that kill males. There are two 



