BREED DIFFERENCES IN HEAT TOLERANCE OF DAY-OLD BABY CHICKS

In the spring of 1950 all chicks were examined on the tenth day for the pres-ence or absence of a tail. From a total of 3,200 chicks, 531 pullets bred for high fecundity were tested for age at first egg. The results have been tabulated to dis-cover if there was any difference in the age at sexual maturity in the birds that carried the recessive gene si for rapid feathering, and those that carried its dominant allele SI. The mean age at first egg in the 258 sl pullets was 205 days and the mean age at first egg in the 273 SI pullets was 204 days. There is, therefore, no evidence in these data that the sex-linked gene sl is associated with genes affecting sexual maturity. The fact was pointed out earlier (Hays, 1936 and 1945) that the sex-linked gene E for early sexual maturity has a low frequency in our Rhode Island Red but that it does appear. Under our condition