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Journal Article

Perspectivas da avaliação genética multirracial em bovinos no brasil

TL;DR: Os procedimentos de avaliacao genetica multirracial produzem predicoes geneticas aditivas mais acuradas, permitem a comparacao direta de animais de diferente composicao racial e viabilizam o melhoramento genetico aditivo e nao-aditivo em populacoes multirraciais.
Abstract: A disponibilidade de grandes bases de dados em associacoes de diferentes racas e cruzamentos de bovinos,e a demanda crescente dos produtores por avaliacoes geneticas dos animais de racas puras e cruzados tem renovado o interesse na implementacao de procedimentos de avaliacao genetica multirracial. Na maioria das associacoes de criadores dos Estados Unidos esta sendo aplicada a avaliacao genetica de animais puros e cruzados.O Brasil, com sua enorme populacao bovina (167 milhoes),composta amplamente por Zebu e seus cruzamentos (80%),e muitas grandes e bem estruturadas empresas (varias com dez mil animais ou mais), e provavel que possua um expressivo numero de bases de dados apropriadas para ser analisado por meio de procedimentos de avaliacao genetica multirracial. Comumente, no entanto, neste pais sao usados procedimentos de avaliacao genetica intra-racial independentemente da composicao genetica das populacoes bovinas. Os procedimentos intra-raciais ignoram os efeitos geneticos nao-aditivos entre racas, e assumem que os parâmetros geneticos sao os mesmos em todos os grupos raciais. Os procedimentos de avaliacao genetica multirracial incluem ambos os aspectos, assim, eles sao preferiveis aos intra-raciais para avaliar geneticamente populacoes compostas por animais puros e cruzados. Os procedimentos de avaliacao genetica multirracial produzem predicoes geneticas aditivas mais acuradas, permitem a comparacao direta de animais de diferente composicao racial e viabilizam o melhoramento genetico aditivo e nao-aditivo em populacoes multirraciais. No entanto, esses procedimentos sao computacionalmente mais complexos, requerem maior numero de parâmetros geneticos e apresentam problemas de confundimento e multi colinearidade. Neste artigo, caracterizam-se as populacoes multirraciais, discutem-se modelos de avaliacao genetica, procedimentos e tarefas para sua implementacao, e, quando relevante, apresentam-se comentarios gerais sobre a situacao multirracial no Brasil. PALAVRAS-CHAVE: Bovinos, cruzamento, avaliacao genetica, multirracial, predicao.

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References
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Journal ArticleDOI
01 Nov 1981-Genetics
TL;DR: The minimum number of genes involved in producing a large difference between populations in a quantitative trait is typically estimated to be about 5 or 10, with occasional values up to 20, which strongly supports the neo-Darwinian theory that large evolutionary changes usually occur by the accumulation of multiple genetic factors with relatively small effects.
Abstract: A procedure is outlined for estimating the minimum number of freely segregating genetic factors, nE, contributing to the difference in a quantitative character between two populations that have diverged by artificial or natural selection. If certain simple criteria are satisfied approximately on an appropriate scale of measurement, nE can be estimated by comparing the phenotypic means and variances in the two parental populations and in their F1 and F2 hybrids (and backcrosses). This generalizes the method of Wright to genetically heterogeneous (or wild) parental populations, as well as inbred lines. Standard errors of the estimates are derived for large samples. The minimum number of genes involved in producing a large difference between populations in a quantitative trait is typically estimated to be about 5 or 10, with occasional values up to 20. This strongly supports the neo-Darwinian theory that large evolutionary changes usually occur by the accumulation of multiple genetic factors with relatively small effects.

603 citations

Journal ArticleDOI
TL;DR: The inverse of the genotypic covariance matrix given here can be used both to obtain genetic evaluations by best linear unbiased prediction and to estimate genetic parameters by maximum likelihood in multibreed populations.
Abstract: Covariance between relatives in a multibreed population was derived for an additive model with multiple unlinked loci. An efficient algorithm to compute the inverse of the additive genetic covariance matrix is given. For an additive model, the variance for a crossbred individual is a function of the additive variances for the pure breeds, the covariance between parents, and segregation variances. Provided that the variance of a crossbred individual is computed as presented here, the covariance between crossbred relatives can be computed using formulae for purebred populations. For additive traits the inverse of the genotypic covariance matrix given here can be used both to obtain genetic evaluations by best linear unbiased prediction and to estimate genetic parameters by maximum likelihood in multibreed populations. For nonadditive traits, the procedure currently used to analyze multibreed data can be improved using the theory presented here to compute additive covariances together with a suitable approximation for nonadditive covariances.

84 citations

Journal ArticleDOI
TL;DR: The results showed that it is feasible to evaluate sires for additive and nonadditive genetic effects in a structured multibreed population and data from purebred breeders and commercial producers will be needed to accomplish the same goal at a national level.
Abstract: Estimates of covariances and sire expected progeny differences of additive and nonadditive direct and maternal genetic effects for birth and weaning weights were obtained using records from 1,581 straightbred and crossbred calves from the Angus-Brahman multibreed herd at the University of Florida. Covariances were estimated by Restricted Maximum Likelihood, using a Generalized Expectation-Maximization algorithm applied to multibreed populations. Estimates of heritabilities and additive genetic correlations for straightbred and crossbred groups were within the ranges of values found in the literature for these traits. Maximum values of interactibilities (ratios of nonadditive genetic variances to phenotypic variances) and nonadditive correlations were somewhat smaller than heritabilities and additive genetic correlations. Sire additive and total direct and maternal genetic predictions for birth and weaning weight tended to increase with the fraction of Brahman alleles, whereas nonadditive direct and maternal genetic predictions were similar for sires of all Angus and Brahman fractions. These results showed that it is feasible to evaluate sires for additive and nonadditive genetic effects in a structured multibreed population. Data from purebred breeders and commercial producers will be needed to accomplish the same goal at a national level.

78 citations