GENETICS OF REPRODUCTIVE PHYSIOLOGY 275 



it was found that the anterior pituitary glands of the rapidly gaining strain contained 

 significantly more somatotrophin per unit pituitary tissue than did the glands from the 

 slowly gaining strain, it was possible to account for the differences in the rates of gain 

 between the two strains. However, even though there was also a clear-cut difference 

 between the fertility (that is, litter size) of the two strains (table 53), the assay of the 

 hypophyses for concentration of gonadotrophic hormone showed an inverse relationship 

 to rate of ovulation and to size of litter. Since the assay method used measured 

 the total gonadotrophic hormone content rather than the two component parts of the 

 gonadotrophic complex (consisting of the follicle-stimulating hormone FSH, and the 

 luteinizing hormone LH), it is conceivable that the poorer reproductive performance 

 of the slow strain was due to an abnormal ratio between FSH and LH which was 

 perhaps different from the ratio usually found in normal animals and which was 

 incompatible with optimum reproductive performance. From the genetic point of 

 view it is clear that the two hormones under discussion were present in the pituitary 

 glands of the slow and fast strains in significantly different concentrations; but the 

 endocrine interpretation of the inverse relation between gonadotropin and fecundity 

 of these two strains is not obvious. The slow strain showed a significantly higher 

 incidence of cystic ovaries and generally impaired fertility in comparison to the fast 

 strain. In this case, then, the higher gonadotrophic hormone potency of hypophyses 

 of the slow strain is indicative of abnormal reproductive performance rather than of 

 greater fecundity. This example illustrates the fact that estimates of hormonal 

 concentrations are not always reliable indicators of the efficiency with which the 

 animal utilizes its hormones. Such estimates are useful only when considered in 

 conjunction with the overall performance of the animal. 



It is interesting to speculate on the possibility of producing strains of animals in 

 which a high rate of secretion of one hypophyseal hormone is associated with a low 

 rate of secretion of another trophic hormone. This would create genetic strains in 

 which, for example, large body size is combined with small litter size, or small body 

 size is associated with a litter size significantly larger than that observed in the larger 

 strain. As a general rule, breeds of chickens with large body size are poorer egg 

 producers than are breeds of chickens with a smaller body size. Although this suggests 

 an inverse relationship between the hormones concerned with the control of these 

 physiologic differences, it is possible that the principle cited earlier to explain 

 differences between beef and dairy breeds may apply. 



There are examples which show that beneficial effects of genie complexes control- 

 ling hormonal action at one step of the reproductive process may be partly or completely 

 offset by adverse genie effects acting at another step. The work of Fekete 350 shows 

 that between two strains of mice ovulation rate is inversely correlated with reproductive 

 efficiency and litter size. Despite a significantly higher rate of ovulation, the litter 

 size of mice of the DBA strain is significantly lower than the litter size of mice of the 

 C57BL strain (table 54). Analysis of this situation has shown that DBA ova are as 

 viable as the ova of the other strain, but that the higher loss of ova in DBA mice is 



