276 PHYSIOLOGIC GENETICS 



Table 54 



Comparison of rate of ovulation and reproductive efficiency of two inbred 



strains of mice 



Average no. Average no. Percentage of eggs 



of eggs per of young per developing 



Strain ovulation litter into young 



DBA 8.2 4.8 58.3 



C57BL 6.7 5.6 83.9 



due to an inhospitable uterine environment in which DBA eggs find themselves. A 

 similar situation which results in lowered fertility in cows, and which could be ascribed 

 to an unfavorable uterine environment, has been described by Hawk et a/. 535 



The comparative rates of the efficiency of reproduction of the two strains of mice 

 present a good example of the necessity of analyzing each step of the reproductive 

 process separately before deciding which stage is adversely affected by genetic 

 mechanisms. Obviously, the genetically determined low size of litters of DBA mice 

 must not be ascribed to a genetically low rate of secretion of gonadotrophs hormones. 

 Several other examples of genie action at secondary or tertiary steps are known in 

 reproduction, lactation, and growth. 



The nature of the adverse uterine environment on implanting ova is not known, 

 but it is known that definite differences exist among strains of animals in the intensity 

 with which uteri respond to their trophic hormones and in the types of responses 

 produced. Drasher 288 demonstrated significant differences in sensitivity of uteri of 

 mice of five different inbred strains to a standard dose of estrogen (table 55). She 

 also made comparisons between the immediate effect of estrogen and the residual 



Table 55 



Effect of 3 gamma of estrogen injected into 5 strains of inbred, castrated mice 

 killed 13 days or 73 days after castration 



Days 

 killed 

 after 

 castration Strain 129 C57BL/6 C3H/He C3HeJH DBA/1 



