26 



PHYSIOLOGICAL GENETICS 



homozygous mutant genes act alike in both sexes. Bui there arc 

 also numerous instances in which the effect differs in the two 

 sexes. Some of these facts are of a type that might lead to an 

 understanding of the phenomenon in terms of developmental 



physiology. 



In Lymantria dispar, some of the geographic races are dis- 

 tinguished by different speed of development, different growth 



2000 



1800 



Hichi SH 



^d Hie hi 



n m w v Pu 



Fig. 25. — Growth curves of male Lymantria dispar, race Hichinoe, females 

 with five moults, males with four. Larval instars plotted against weight in milli- 

 grams. (From Goldschmidt, 1933, Arch. Entwicklgmech. 130, Fig. 14.) 



curves, different number of molts; and all these characters are 

 typically different in both sexes (Goldschmidt, 1933a). But also 

 the degree of difference between the sexes might be different in the 

 respective races and might be controlled genetically. Figure 25 

 shows the growth curves for the two sexes of one of the races in 

 which females have five and males four molts; and Fig. 18 gave 

 similar curves for a number of races, showing the genetic differ- 

 ences. But, as is usually the case with growth, the difference is 

 not one of single genes. In regard to the number of molts, how- 



