138 SEX INHERITANCE 
female may be daughters. Sex-linked lethals cause 
this. A sex-linked lethal is inherited in the same 
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 29 :le to 29 :0¢, 
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 
