GENOTYPIC AND SEGREG ATION A L STERILITY 



Federley's moths, the crosses reciprocal to those mentioned gave 

 offspring of both sexes that survived to maturity. The males from 

 the Drosophila cross were sterile, no matter which way it was made. 

 But the testes, which were of normal size from psciidoohscum X 

 persimilis, were abnormally small in tlie reciprocal progeny. 



Fig. 55. — Diagram showing the relation of the X/Autosome balance to the sexual 

 character in Drosophila. cJ = supermale; 9 = superfemale ; rf = intersex (all 

 sterile). 



Note. — (i) Y is without effect on sex-grading. 



(2) Haploid is known as ovarian tissue. 



(3) The homogametic balance can be produced in three ways, the 



hcterogametic in only one. 



Genotypic and Segregational Sterility 



Sterility is the only outcome of hybridity in these male flies. 

 Nevertheless it is to be traced to the same kind of disharmony that 

 causes early death of the female moths. It is a direct result of 

 unbalance in the zygote's genotype, and we may therefore refer to 

 it as genotypic sterility. The sterility of the diploid RapJiano-hrassica 

 and of the diploid Primula kewensis might appear at first sight to 

 be due to the same cause, but one significant observation shows that 

 this cannot be so. Both of these plant hybrids recover fertility when 

 the number of their chromosomes is doubled; whereas tetraploidy 

 leads to no such recovery in the male flies. Now tetraploidy does 

 not cause any essential alteration of genie balance: it caimot 

 therefore remedy genotypic sterility. It does, however, suppress 

 segregation, and so it would appear that, in contrast to the 



23 T 



