Role of selection and inbreeding on the incidence of cutaneous malignant melanoma in Sinclair swine

This paper reports the quantitative analysis of the historical database of a herd of Sinclair swine affected by cutaneous malignant melanoma. The herd was under partial and non-systematic selection for melanoma susceptibility (animals having at least one tumour during the first 6 weeks of life). Weighted selection differentials for the number of tumours at birth and the number of tumours at 6 weeks were generally positive and between -0.43 and 4.76 tumours for the number of tumours at 6 weeks. Estimates of the heritability for number of tumours at birth and at 6 weeks using 1934 animals were 0.27 (+/- 0.03) and 0.25 (+/- 0.03), respectively. The estimate of the genetic correlation between these two traits was 0.95 (+/- 0.03). Genetic trends were positive for the number of tumours at birth and at 6 weeks. In spite of positive selection differentials and a moderate heritability, there was a negative phenotypic trend in the number of tumours. Natural selection might be acting in a direction opposite to artificial selection in the Sinclair herd. The slopes of the regression of the number of tumours at birth, at 6 weeks, and melanoma susceptibility on individual inbreeding coefficients were non-significant, indicating no evidence of dominance. The number of live-born pigs was lower in litters from parents susceptible to the disease (p < 0.01).