Pedigree analysis of selected lines of Merino sheep. 2. Gene contributions

Abstract

Four pairs of selection lines of Merino sheep consisting of 100 ewes and 5 rams were established at the Trangie Agricultural Research centre in 1951. Each line was selected (mass selection) for the appropriate character and direction (Plus or Minus) for a period of 18 years (5 generations). Pedigree analysis of these selected lines in terms of gene contributions revealed different patterns in means and standard deviations of gene contributions of male and female founders. Female founders showed a consistent pattern in all of the selected lines, with an increasing trend for the first 5-6 years in both parameters, and then stabilised. Fleece Plus showed the smallest variation, with an average coefficient of variation of 163%, whereas Folds Plus showed the highest variation with 213%. In the mean contributions, male founders showed a declining pattern for the first 5-6 years and finally stabilised. In the standard deviations, Weight and Folds flocks showed an increasing pattern in the first 1-2 years, whereas Fleece and Crimp flocks showed a declining pattern for the first 3-6 years before stabilising. Male founders showed smaller variation in mean contribution than female founders. In the Weight and Folds flocks, coefficients of variation were in the range of 55-88%, whereas in the Fleece and Crimp flocks they ranged from 98% to 156%. On average, 71% of genes were contributed by female founders and 29% by male founders. A good agreement (>90%) was found between observed and estimated inbreeding from Wray and Thompson (1990) expression in all flocks except Weight Plus and Fleece Minus lines. Effective numbers of male, female, and both male and female founders showed a consistent pattern of early fluctuation, with a trend towards increasing, and eventually stabilised in all the lines. Fleece Plus and Folds Plus lines showed the smallest and highest effective numbers for male, female, and both male and female founders, respectively, which is consistent with the pattern of observed inbreeding. During the first and second year there was a major reduction in the effective number of female founders, which is an indication of marked decrease in the genetic variability at the beginning.