Estimation of Genetic Trend in selected Population with and Without the use of Control Population

Abstract

Data from a selection experiment conducted with sheep at Massey University, New Zealand, were analyzed to obtain an evaluation of selection response. Selection was for heavy 14-mo greasy fleece weight. Approximately seven generations of selection were represented in the data. Three estimates of genetic superiority of the selected line to the control line were obtained. All three estimates were obtained from a mixed model evaluation using the individual animal model for predicting breeding values from own and relatives' records. The estimators were 1) deviation of selected line predicted yearly phenotypes from control line predicted yearly phenotypes, 2) deviation of the predicted yearly phenotype for the selected line from the year estimate in the control line and 3) the mean yearly breeding value from the analysis of the selected line only. The realized heritability using the first approach was .20. However, the control line was found to have a slight positive drift; hence, this estimate was biased downward. Using Approach 2, accounting for drift, the realized heritability was .23. The same realized heritability, .23, was obtained from an analysis of the selected line ignoring the control (Approach 3), when a prior heritability of .30 was assumed for the mixed model evaluation. The estimate of genetic trend from predicted breeding values in the latter approach is, however, quite dependent on the assumed heritability.

On the use of animal models in the analysis of selection experiments 1

“…Using Thompson As with design 2, the situations can be extended to the case of 2sn unrelated candidates in generation 1 and 2s half-sib families of size n in generation 2, or sn candidates of each sex and 2s full-sib families of size n, since the expressions (6), (7) and (8) (4), (5), (7) and (8) It can also be seen that equation (11) [13] and to the Bulmer effect [14]. This increases (see, for instance, [2] or [11] [15] when the information about heritability in the experiment is large enough. The accuracy of BLUP evaluation has also been sometimes presented as an argument in favour of the method for the estimation of genetic trends.…”

Partition of the genetic trend to validate multiple selection decisions

“…After the papers of Blair and Pollak (1984) and Sorensen and Kennedy (1984), the evaluation of the genetic response is usually calculated just by monitoring how the BLUP averages evolve after the selection decisions and therefore it is used to validate the whole selection scheme. However, this analysis does not allow evaluating each of the single parts of the selection scheme, and the effectiveness of the different selection decisions.…”

Genetic trends in the performance and reproductive traits of pigs

“…However, the high cost of maintaining a control population makes these two procedures unfeasible for commercially oriented breeding programs. In a study using a control line and a selected line, Blair and Pollak (1984) used three approaches to estimate the genetic superiority of the selected line: 1) deviation of the predicted annual phenotype mean of the selected line compared to the predicted annual phenotype mean of the control line, 2) deviation of the predicted annual phenotype mean of the selected line compared to the estimated year effect based on the control line records, and 3) yearly means of the predicted breeding values of the selected line. Method 3 depended on the heritability and produced better partitioning of the phenotype trend in the genetic and environmental components.…”