542 



GENETICS IN RELATION TO AGRICULTURE 



TABLE LXXI. RATIO OF SEXES IN HYBRID 

 GUINEA-PIGS (After Detlefsen) 



up of two classes having different chromatin contents might display dif- 

 ferential mortality under the stress of slightly unfavorable conditions. It 

 may easily be seen, therefore, that slight or conceivably wide variations 

 in the sex-ratio may be due to the operation of factors other than those 

 which actually determine sex. 



Causes of Unusual Sex-ratios. In this discussion we desire to 

 treat only of causes of sex-ratio disturbance which may operate in the 

 higher domestic animals. The curious and interesting conditions which 

 are found in some insects and other lower animal forms will not be men- 

 tioned save in so far as they may throw light upon the problem of sex- 

 ratio disturbance in higher animals. 



In some cases hybridity appears to favor a disproportionate produc- 

 tion of one sex or the other. Thus in the bison-cattle crosses, the 



production of males is rel- 

 atively rare. This, how- 

 ever, appears to be due to 

 the fact that physical 

 difficulties make it prac- 

 tically impossible for a 

 cow to bear to maturity a 

 bison-cattle fetus and to 

 give birth to it. There are 

 two reasons for this, the 

 increased size of the hybrid 

 fetus, and the development 

 of a large hump which 

 cannot be accommodated 

 by the normal pelvic con- 

 formation in the cow. Consequently the fact that practically all 

 the animals born of this species cross are females, may simply 

 be due to abortion and death of male fetuses. The amount of 

 trouble is sufficient in this case to give room for a potential equality 

 of sexes in such crosses. Detlefsen reports similar results from a cavy 

 species cross, which gave a disproportionately low ratio of males. The 

 data are given in Table LXXI. The earlier generations show a con- 

 siderable deviation from the normal equality of the sexes. With suc- 

 cessive back-crosses, however, the ratio soon becomes one of practical 

 equality. 



When races are more closely related a different result is produced. 

 Thus Miss King found in crosses between wild and albino rats a, sex-ratio 

 of 119.1 males to 100 females among 425 hybrids representing the 

 first three hybrid generations of such a cross. Among guinea-pigs 

 crossed inter se, Minot found among 410 individuals a sex-ratio almost 



