GENETICS OF REPRODUCTIVE PHYSIOLOGY 279 



Finally, brief mention should be made of cases of genetic sterility which may be 

 due to a variety of causes, some of them apparently based on endocrine malfunction, 

 although the etiology is not always clear. In the slow-gaining strain of pigs mentioned 

 earlier there was a high incidence of cystic ovaries which, in this particular instance, 

 appears to be associated with an imbalance of the components of the gonadotrophs 

 hormone. The propensity to cystic ovaries also has an hereditary basis in cattle. 

 In the Swedish Highland breed of cattle, ovarian hypoplasia is inherited, being most 

 pronounced in animals carrying two pairs of double recessive genes. The left ovary 

 is most commonly affected but both ovaries may be hypoplastic. In extreme cases of 

 hypoplasia follicles are completely lacking, but in partial hypoplasia only the primary 

 follicles are absent. It is interesting to note that bulls of this genetic strain of cattle 

 show testicular hypoplasia which affects the left testicle first and more severely than it 

 does the right. 323 



Genetically sterile mice affected by the obese-sterility syndrome have ovaries 

 which normally remain infantile throughout life but which are capable of responding 

 to gonadotrophic hormones. With appropriate hormonal therapy, mice of this strain 

 can be made to produce litters. 1226 It appears in this case that the infantilism of the 

 mice is due to a deficiency of gonadotrophic hormones in genetically obese mice of the 

 obob genotypes. The remainder of the reproductive apparatus is unaffected by this 

 genotype and, as far as its ability to provide fertilizable ova and proper uterine environ- 

 ment for normal pregnancy is considered, it retains its ability to respond normally to 

 the proper exogeneous hormones. 



In a recent experiment the site of genie action on hypophyseal malfunction was 

 analyzed. 155 The pituitary glands from genetically dwarfed mice were implanted 

 into the sellae turcicae of hypophysectomized normal mice, and pituitaries from the 

 latter were implanted into sellae of dwarf mice. It was found that the genotype of the 

 hypophysis determined the growth rate of the recipients, allowing dwarf recipients of 

 normal pituitaries to grow normally, while normal recipients of dwarf pituitaries failed 

 to grow. This experiment shows that the primary genie action is on the pituitary 

 gland itself and not on the hypothalamus. Snell 1235 showed that this type of dwarfing 

 is due to a single, autosomal gene which causes deficiency of the growth hormone 

 secreting acidophils in the hypophysis. 403 



CONCLUDING REMARKS 



Data selected from the literature have been presented to show that genes or genie 

 complexes determine (in a general way) the rates at which hormones are secreted by 

 glands as well as the sensitivity of the target organs to their specific trophic hormones. 

 Different sets of genes may affect the chain of reproductive events at several different 

 levels. Some genie effects may be primary, for example, those acting on the regulator 

 of the rate of reproductive events, the adenohypophysis. Other genie effects may be 

 secondary in that they influence reproductive events during the intrauterine life of 



