ENVIRONMENTAL EFFECTS 



As to the nature of dominant lethals it is 

 probable that they are the result of extensive 

 chromosomal alterations rather than to changes 

 in restricted regions or in single genes. Ale- 

 thai effect from a single gamete may be called 

 dominant in contrast to a condition v/hich must 

 be present in both gametes and hence recessive. 



Since in Habrobracon an unfertilized egg pro- 

 vided with a single set of genes develops nor- 

 mally into a male and since the addition of a 

 second complete set by fertilization likewise 

 results in normal development into a female, it 

 might be thought that the addition of a defi- 

 .cient set should not be lethal to the resulting 

 zygote. In other words, if both In and 2n may 

 develop normally, why should not In + (n-x) also 

 develop normally? The explanation is doubtless 

 to be found in genie balance. If n-x is too 

 small to act as a recessive, containing rela- 

 tively extensive deletions for example, the bal- 

 ance should be so disturbed that development 

 would be prevented. Theoretically the genie set 

 in a sperm might become so extremely deleted by 

 X-radiation that the fertilized egg would devel- 

 op as an unfertilized egg into a male. No sex 

 intergrades have ever been found in the treated 

 material that might be interpreted as hyperploid 

 males or hypoploid females. The sex types sur- 

 viving after X-radiation have been fully as nor- 

 mal as in untreated stock. 



Recessive lethal (or semilethal) factors may 

 be a cause of "bad eggs" (P. W. Whiting, 1929a; 

 Maxwell, 1935). Half of the unfertilized eggs 

 of a female heterozygous for a recessive lethal 

 do not hatch. Eggs may likewise be defective in 

 their gross morphological or non-nuclear as- 



135 



