This report is an attempt to interpret evidence concerning geneticenvironmental interaction as it relates to the effectiveness of selection.Methods of estimating genetic-environmental interaction were investigated with the conclusion that the standard analysis of variance is a satisfactory tool provided (1) the interaction component of variance is adjusted for important variation between environments in the scale of genetic effects and (2) the variance component for the average effects of genetic groups is recognised as equivalent to the average covariance of the same genetic group in different environments (i.e. ) to include the real possibility of negative genetic correlation.It is shown that (1) response to selection for improved average performance over varying environments (ΔGt) is proportional to the genetic correlation between expressions of the same genotype in different environments (rG), that (2) gain in ΔGi, from measuring performance in k environments is proportional to if k times as many animals are tested, but to if total numbers per genotypic class (nk) are held constant, where g2 is heritability of individual variation within environments and rG is the correlation between the phenotypic expressions of the same genotype in different environments.Evidence is presented that (1) variations among poultry-farm environments in California cause important, but largely unpredictable, shifts in ranking of genetic stocks for egg production, and (2) improved accuracy in measuring genetic differences in average performance across environments probably justifies utilising a sample of 5 to 10 farms representing the range of environments for which the stock is bred.
When improvement is desired for several traits that may differ in variability, heritability, economic importance, and in the correlation among their phenotypes and genotypes, simultaneous multiple-trait index selection was more effective than independent culling levels or sequential selection. Such comparisons required definition of aggregate breeding value determined jointly by breeding values and economic importance of the component traits. The economic weight should approximate the partial regression of cost per unit of enterprise output value on breeding value for each trait. These can vary with production and marketing system, with performance of traits, and with breed role (i.e., paternal, maternal, or general) in crossbreeding systems. Genetic gains desired to maintain competitive ranking also may define the relative importance of traits. Because information available to estimate breeding values varies among the ages and categories of individuals under selection and because means are unknown, regressed (BLUP) predictions of trait breeding values are useful. They allow appropriate economic weights to be applied as the last step for predicting aggregate breeding values for individuals of different age classes, and they simplify choosing the proportions of selected breeders from each age class that maximize rate of change in aggregate breeding values. Inappropriate economic weights or errors in the parameters used to predict trait breeding values overestimate realized response in true aggregate breeding value.
Data from 1,909 purebred, F1, backcross and F2 and F3 inter se combinations of Angus and Hereford were used to estimate average individual, maternal and grandmaternal genetic effects, individual and maternal heterosis, dominance and epistatic genetic effects. Models for evaluating heterosis and epistatic or recombination effects were discussed. Average individual effects indicate that Angus, compared with Hereford, had calves that were born earlier, had lighter birth weights, lower pre- and postweaning gains and lower pregnancy rates. Angus also produced lighter weight carcasses with more fat cover and marbling. Maternal effects of Angus were in the direction of reduced birth weight, greater calving ease, higher preweaning but lower postweaning growth rate and increased fatness when contrasted with Hereford. There was a tendency for opposite direction of maternal and grandmaternal effects for traits influenced by preweaning maternal environment. When additive X additive effects were ignored, total heterosis was significant for earlier day born, heavier birth weight, preweaning and postweaning gain, and heavier and fatter carcasses. Heterosis retained in F3 inter se vs F1 generation crosses indicated that net epistatic effects were relatively negligible for date of calving, birth weight, weaning gain and fat cover. There was a greater reduction of heterosis effects than expected from dominance alone for survival, pregnancy and marbling score. Loss of heterosis in F3 was less than expected for postweaning gain, carcass weight and rib eye area. Except for survival, pregnancy and marbling, these deviations from dominance expectations, or lack of them, are favorable for F3 composite populations.
Sources of individual plus maternal effects on lamb mortality were studied in data collected at the U.S. Meat Animal Research Center from 1980 to 1985 for paternal and maternal breeds lambing yearly and for maternal breeds lambing at 8-mo intervals. Records included 16,881 lambs born. Breeds included were Finnsheep (F), Dorset (D), Rambouillet (R), Suffolk (S), Targhee (T), Composite 1 (C1 = F/2, D/4, R/4), Composite 2 (C2 = F/2, S/4, T/4), and Composite 3 (C3 = Columbia/2, S/4, Hampshire/4), Traits analyzed were perinatal, postnatal, and total mortality to 60 d of age and postnatal respiratory, digestive, starvation, injury, and other or unknown causes of mortality. The least squares analyses included breed, year, sire within breed-year, sex, linear and quadratic season and age of dam covariates (Model 1), plus litter size (Model 2), plus birth weight (Model 3), and significant two-way interactions. Age of dam, litter size, and birth weight all had important, often quadratic, effects that differed among breeds. Models 1, 2, and 3, respectively, reduced variation by 8, 10, and 16% for perinatal, 7, 8, and 12% for postnatal, and 9, 11, and 20% for total mortality. In Model 1, breed means ranged from 3.5 to 16.2% for perinatal, 7.2 to 21.1% for postnatal, and 16.7 to 32.8% for total mortality. Respiratory and starvation problems were major causes of postnatal mortality. Heterosis for lamb survival retained in composites was 9% for C1 and 18% for C2. Mortality was 1 to 5% higher for males than for females.(ABSTRACT TRUNCATED AT 250 WORDS)
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