Effects of inbreeding on the production characteristics of poultry undergoing selection | The effects of inbreeding on the production characteristics of poultry undergoing selection

The magnitude of inbreeding occurring in the course of selection in 4 sublines of a White Leghorn population over 4-5 generations was computed using the co-ancestry method of analysis (Cruden, 1949). Manson (1970) had previously reported the realised responses in each of these selected populations designated as follows: 1. Egg Weight (E.W.) (selected for increased egg weight) 2. Body Weight (B.W.) (selected for decreased body weight) 3. Regression (I) (selected for maximum values of an index, (I) = E.W. -bxB.W., in which b was the estimated linear genetic regression of egg weight on body weight) 4. Ratio (r) (selected on maximum values of a ratio of egg weight/body weight). The computed inbreeding within all 4 selected lines was estimated to exceed the predicted level of inbreeding based upon the average population size. The cumulative additional inbreeding (△F) that occurred during selection was calculated as 8.5, 9.0, 11.8 and 8.0 per cent respectively in each of the above lines. Following adjustment for inbreeding, it was concluded that: (a) Only about two thirds of the decline in egg numbers to 60 weeks observed by Manson (1970) as a correlated response in the Egg Weight and Regression lines was due to inbreeding depression. (b) Latent gains in egg numbers, obscured by inbreeding depression were likely to have occurred as a correlated response in the case of Body Weight and Ratio selection. The mean performance of the selected lines was observed following a single generation of relaxed selection but no significant and consistent effects of relaxation were observed in egg numbers, sexual maturity, egg weight and body weight. Crosses were made between the two high egg weight lines (E.W. and I) and between the two low body weight lines (E.W. and I). Examination of the mean performance of the crosses indicated some heterosis in egg weight in the low body weight crosses. As regards reciprocal cross differences a significant sire line effect in body weight and egg weight was observed between the Egg Weight and Regression crosses. No significant dam line effects were however observed. A number of test matings were arranged to test the hypothesis that a major sex-linked gene causing a reduction in body weight was present in the low body weight population. The segregation ratio observed among the progeny supported this hypothesis, the 'dwarf' female progeny weighing 15 per cent less, laying 10 per cent fewer and 5 per cent smaller eggs when compared with their contemporaneous non-dwarfed full-sibs. An examination of the phenotypic effects of this gene suggests that it may be a similar mutant to the 'B' allele (Jaap, 1971) at the Dw locus on the sex chromosome of the fowl (Hutt, 1959).