GENETICS OF REPRODUCTIVE PHYSIOLOGY 273 



Hormones 1 ® 01 contain methods for biological assay as well as chemical determination of 

 hormones in tissues and fluids. 



CORRELATED CHANGES IN RATES OF HORMONAL SECRETION 



It appears to be generally true that genetic selection for the increased or decreased 

 rates of secretion of one hormone results in increased or decreased rates of secretion of 

 other hormones produced by the same gland. There are many examples of this 

 principle in the literature. Body size is generally determined by the cooperative actions 

 of somatotrophic, thyrotrophic, and perhaps adrenotrophic hormones, the rate of 

 somatotrophin secretion probably being the primary determining factor responsible 

 for the differences in the body size between two breeds of rabbits. Similarly, rate of 

 ovulation is a reflection of the rate of secretion of gonadotrophic hormone complex. 

 In one breed (table 51) there is a close association between small body size and small 



Table 51 



Comparison of various hormone-controlled characteristics of large and small 



breeds of rabbits 



litter size, the former presumably due to a low rate of secretion of somatotrophic hor- 

 mone, the latter presumably caused by a deficiency of gonadotrophic hormone. In 

 the large race, the opposite association of the presumed rates of hormone secretion is 

 noted. 



If one compares the rates of twinning, lactation, and growth in cattle, one finds 

 that the highest rates of these characteristics (all of which are due to the highest rates of 

 hormonal secretion) appear to be closely associated. Thus, the Holsteins are highest 

 in all of these characteristics, while the Jerseys are lowest (table 52). The fact that 

 this association does not seem to hold for body weight and lactation in beef cattle in 

 comparison to Holsteins can be explained on the basis that large body weight in Hol- 

 steins was achieved by selection for a high rate of secretion of somatotrophic hormone in 

 euthyroid, and perhaps even hyperthyroid animals, whereas selection in beef cattle 

 was in favor of hypothyroidism, that is, a low basal metabolic rate and a phlegmatic, 

 easily fattened animal. 



One of the most interesting examples of this type is provided by MacArthur's 831 

 experiment. Using mass selection and avoiding inbreeding, he selected mice for only 



