INHERITANCE OF BROODINESS 9 



Correlation Studies 



The coefficient of correlation may be used to throw further light on 

 the question of sex-linked inheritance of the broody instinct. The correla- 

 tion between sires' sisters and sires' daughters in percentage of broodiness 

 should be higher than between dams' sisters and dams' daughters in per- 

 centage of broodiness, if sex-linked genes are operating. 



These correlations were calculated in the following manner: The per- 

 centage of broodiness in the sisters of each of 72 sires was paired against 

 the percentage of broodiness in the daughters of each of his mates. The 

 percentage of broodiness in the sisters of each of 206 dams was also paired 

 against the percentage of broodiness in the daughters of each dam. No 

 sires or dams were included unless they had at least three full sisters 

 and three daughters with a complete annual record. Regression was found 

 to be linear in both cases. The constants arrived at were as follows: 



Sires Dams 



Number U. 206. 



Mean percentage broodiness in sisters 49.65 46.34 



Standard deviation ±32.17 ±31.91 



Mean percentage broodiness in daughters 54.12 50.03 



Standard deviation ±33.35 ±33.43 



Coefficient of correlation 6906 ± .0364 .7119 ±.0341 



There was a marked similarity in the mean percentage of broodiness 

 in the sisters of sires and of dams, as well as in the percentage of broody 

 daughters from sires and from dams. The two coefficients of correlation 

 are of almost identical magnitude, which again suggests the absence of 

 sex-linked genes for the broody instinct. 



Phenotypes of Daughters from Broody Hens 



A study was made on the daughters of 41 broody dams that had enough 

 daughters to give something of a clue to the proportions that were broody 

 and non-broody. According to the AC theory, these dams must have 

 carried both genes A and C because they exhibited the broody instinct. 

 Since selection had been carried on for a long time to eliminate brood- 

 iness, it is probable that most of these dams were Aa Cc in genetic make- 

 up. Theoretically, when no selection has been practiced, there would be 

 nine different genotypes of males with respect to genes A and C. 



Table 5 shows that 14 of the 41 dams had 25 percent broody daughters, 

 indicating that many of the sires used were pure recessives (aa cc). There 

 were 6 dams with about Y% of their daughters broody, and these may have 

 come from sires heterozygous for a single dominant gene. Ratios of 50-50 

 could come from males that were pure for one dominant factor, but there 

 was not a single dam with a family of such proportions. Doubly 

 heterozygous males and females when mated should have 9/16 of their 

 daughters broody. Table 5 suggests but four such matings. There were 

 12 dams with all their daughters broody. Such proportions could come 

 either from males or from females homozygous for both genes A and C 

 or from broody hens that carried one dominant gene in homozygous state 

 mated to males that were homozygous for the second dominant gene. The 



