272 PHYSIOLOGIC GENETICS 



of ovulation, but, in defining genetic differences, it must be used cautiously, since litter 

 size is not always determined by rate of ovulation alone. This will be illustrated in a 

 number of examples later. 



The genetic controls of such characteristics as rate of ovulation are probably as 

 complex in mammals as they are known to be in chickens. Thus, the ability to lay 

 many eggs depends on the combined action of five or more component characteristics 

 each of which is known to be controlled by genes and influenced by hormones. Chickens 

 which lay the highest number of eggs must show early sexual maturity, the ability to 

 lay many eggs in succession without rest periods (long clutches), and they must be non- 

 broody. 539, 540 It is highly probable that the original analysis of the numbers of genes 

 involved in the control of the fecundity of fowl is greatly oversimplified and inadequate 

 and that the number of genes governing the expression of each of these characters is 

 larger than originally thought. This possibility does not alter the fact that each of 

 these five characteristics is known to be affected either by single hormones or interacting 

 hormone complexes which are involved in determining the general level of fecundity 

 in fowl. No comparable analysis has been made with regard to the genetic control 

 of a similar array of component parts of the reproductive cycle of mammals. 



Other indirect measures of rates of hormonal flow can be obtained from observa- 

 tions on the live animals in which rate of growth, rate of lactation, onset of puberty 

 or menopause, and the like reflect endocrine states and are indicative of differences in 

 genotypes. 



Still another method of establishing gene-controlled differences between strains of 

 animals involves the measurements of their sensitivity to the injection of exogenous 

 hormones. In some instances these methods require that the animal be sacrificed 

 following treatment in order to obtain the target organ for analysis or weighing, while 

 in others, the necessary measurements can be obtained from the living animal as in 

 the case of the sensitivity of combs of male chickens to androgens, the sensitivity of the 

 vaginae of rats or mice to estrogen, or the ability of the thyroid to take up radioactive 

 iodine. 



Using one or the other of the techniques mentioned, it is possible to obtain reason- 

 ably reliable direct or indirect estimates of the rate at which the different hormones are 

 secreted. Much information has been accumulated in the literature showing conclu- 

 sively that heritable differences in the rate of hormonal secretion exist between strains 

 of animals. It is not the purpose of this paper to make a complete survey of the litera- 

 ture illustrating these points and only a few of the more unusual or interesting examples 

 will be presented. These examples have been selected because they illustrate the 

 problems and principles which confront the scientist interested in this general area of 

 physiologic genetics. Neither is it possible to present an exhaustive review of the 

 gamut of chemical or methods of biological assay which can be used to obtain estimates 

 of the various hormones contained in glands, tissues, or fluids of animals. The 

 interested reader is referred to the current endocrine literature as well as the standard 

 texts on endocrinology. Hormone Assay, 323 which is somewhat out of date, and The 



