INHERITANCE OF INTENSITY 3' 



ber 1 or horn first egg after November 1 to March 1, a period of 120 days. The 

 number of eggs laid was divided by 120 less the number of pause days in pauses 

 of seven or more days. Winter pause was considered a separate entity due to 

 inherited and environmental factors. Net spring rate was calculated for the 

 period from March 1 to June 28 after deducting spring pauses. A significant 

 correlation was observed between winter and spring rate, but the evidence was 

 not conclusive that winter and spring rates are governed by the same set of genes. 

 The correlation between winter rate and winter production was more significant 

 than that between spring rate and winter production. 



Tomhave (1941) measured winter intensity by percentage production during 

 the period from first egg to February 1, from which period all pauses of five 

 days or more were deducted. He was able by sib-testing breeding cockerels for 

 winter intensity to produce lines that were significantly different in rate of laying. 



Lerner and Taylor (1943) showed that the degree of heritability between fam- 

 ilies for net winter rate of laying was 21.8 percent. 



Hays (1944) reported a correlation in winter clutch size between mothers and 

 daughters with the value +.1365 which was barely significant. He emphasized 

 that the dams used for breeding failed to breed true for intensity. 



MATERIALS AND METHODS 



The stock used in this study were Rhode Island Reds bred for characters 

 associated with high fecundity. Seven generations of pedigreed pullets were 

 included covering the hatching years of 1937 to 1943. All calculations were 

 based on first-year trapnest records of mothers and daughters. The types of 

 matings to produce each generation of daughters consisted of both young and old 

 parents with emphasis on progeny testing and on low mortality rates. The study 

 included 237 mothers producing a total of 2407 daughters. 



Winter clutch size was calculated in the usual manner by counting the num- 

 ber of clutches of eggs from the first pullet egg through to March 1. The num- 

 ber of eggs was then divided by the number of clutches to get the mean clutch 

 size. Spring clutch size was calculated in a similar way for March, April, and 

 May; summer clutch size for June, July, and August; and fall clutch size for 

 September and October. Mean clutch size is considered a good measure of in- 

 tensity because it is not known to be affected by pauses of various kinds. 



The coefficient of correlation may be used as a measure of inheritance. In a 

 homozygous population of mothers there should be an intimate correlation be- 

 tween mothers and daughters for a particular character even though the fathers 

 were more or less heterozygous. In this study consideration is given first to the 

 correlation between all mothers and daughters with no regard to the phenotype 

 of mothers with respect to clutch size. As a partial assurance of homogeneity 

 of mothers, the data are next broken down into daughters from three possible 

 phenotypes of mothers. 



SECTION 1. MEAN CLUTCH SIZE AS A MEASURE OF INTENSITY 



Correlation in Winter Clutch Size between Mothers and Daughters 



The entire population of mothers and daughters was tabulated with respect 

 to winter clutch size. Although the mean clutch size of all dams was high, the 

 actual range in the dams used was from 1.5 to 14.5. This would include some 

 mothers that lacked genes for high intensity. Dams with no genes for high in- 

 tensity might produce mediocre daughters unless all sires transmitted high in- 

 tensity. These conditions operate to reduce the correlation between mothers 

 and daughters in the total population. 



