Studies on genetic resistance to Marek's disease in the domestic chicken

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Abstract
Inoculation trials using the HPRS‐B14 and HPRS‐16 strains of classical and acute Marek's disease (MD) respectively, were undertaken on single sire families of commercial chickens. It was demonstrated that both sires and dams had a significant effect on the susceptibility of their progeny to Marek's disease. It is suggested that the genetic constitution of the chicken plays a rôle in controlling its susceptibility to MD. The susceptibility to MD of females was significantly greater than males. Members of some of the single sire families used in one Marek's disease trial became naturally exposed to Marek's disease in a trial designed to examine the effect of sire families on the incidence of leukosis. There was a significant positive correlation between the incidence of Marek's disease in members of single sire families infected by inoculation and by natural exposure. There was no correlation between incidence of Marek's disease and the incidence of tumours of the leukosis/sarcoma complex. The range in incidence of MD in single sire families suggested that there was adequate heterogeneity in the commercial lines of chicken to warrant a selection programme for resistance to Marek's disease. The absence of any significant correlation between resistance to Marek's disease and a number of production traits suggested that such a programme should have little effect on these traits. A negative correlation between incidence of Marek's disease in single sire families and mean latent period was shown. The use of a parameter including a measure of mean latent period as well as incidence of MD resulted in greater sire variance ratios than the use of incidence alone.