278 



PHYSIOLOGIC GENETICS 



assay animals one would obviously select White Rock chicks for the assay of gonado- 

 trophs hormones and Leghorn chicks for the assay of androgen. The decision 

 whether to choose an inbred or outbred strain of animals for certain assays would depend 

 on many factors such as the hormone to be assayed and the availability of strains. 

 Some F x hybrids are more sensitive to hormonal stimulus and less variable in their 

 response than either or both of the inbred parental strains while with other hormones 

 and other strains the opposite may be true. 



The relationship of the genotype to sensitivity to stimuli is shown in an analysis 

 of the endocrinology of the broody instinct in chickens. 931 Normally, cocks do not 

 show maternal instincts toward chicks and when confined in close quarters with chicks 

 they actually become aggressive toward them. Broodiness is thought to be controlled 

 by complementary autosomal genes and by a sex-linked gene which appears to be 

 present only in chickens of the Cornish breed. Although it is probable that this genetic 

 scheme is an oversimplification of the actual genetic control of the highly complex 

 physiologic characteristic of broodiness, it will serve to illustrate a principle. In the 

 experiments in question it was found that the injection of the hormone prolactin can 

 induce cocks to become broody and to exhibit complete maternal behavior toward 

 chicks. This includes the whole complicated ritual of clucking, tidbitting, hovering, 

 and defending the chicks. It was further noted that genetically nonbroody breeds of 

 cocks required much larger doses of prolactin and a longer period of injection before 

 exhibiting the full maternal responses (table 57). The dose of prolactin required to 



Table 57 



Comparison of presumed phenotypes and doses of prolactin required to induce 

 maternal behavior in cocks 



induce broodiness in cocks correlated rather well with the observed phenotypes of hens 

 of the three breeds studied, as well as with the presumed genotypes of these breeds. 

 In some White Leghorn cocks it was difficult to induce the full broody responses even 

 with very large doses of prolactin and after prolonged periods of injection. It is 

 not known whether the greater sensitivity to prolactin of geno typically "broody" 

 cocks is due to the possibility that their pituitary glands secrete endogenous prolactin 

 which, added to the exogenous prolactin, raises the hormonal level enough to permit 

 expression of maternal behavior in males. 



