Resumen En el presente trabajo se estimaron componentes de (co)varianza y parametros geneticos para la caracteristica peso al destete en animales de las razas brahman y cebu comercial. El objetivo general fue estimar y comparar los parametros geneticos incluyendo o no el efecto genetico materno por medio del Modelo Animal, usando el programa MTDFREML. Se consideraron cinco modelos que incluyeron como efectos fijos el grupo genetico materno, el numero de partos y el grupo contemporaneo, y como aleatorios los efectos geneticos directos y maternos y los efectos del ambiente permanente materno, ademas del efecto del error residual. Los coeficientes de heredabilidad directa fueron de medios a bajos (h 2 a , 0.10 a 0.29) en todos los modelos. Los modelos que incluyeron el efecto genetico materno presentaron valores de heredabilidad materna bajos (h 2 m , 0.07 a 0.17). Las correlaciones geneticas entre los efectos directo y materno fueron de baja magnitud y negativas (-0.18 y -0.29). Se compararon los modelos entre si realizando la prueba de maxima verosimilitud (-2LogL) a traves de la prueba de ji cuadrado, obteniendose como resultado que el mejor modelo fue el numero 5 (Y = Xβ + Z 1 a + Z 3 m + Z 2 ep + e), el cual tuvo en cuenta todos los efectos. Palabras Clave: brahman, cebu comercial, efecto genetico directo, efecto genetico materno, parametros geneticos, peso al destete. Summary The component of (co)variance and genetic parameters for weaning weight trait in animals of pure (Brahman) or crossbreed (Zebu cattle) breeds were estimated, with the aim to estimate and to compare the genetic parameters and genetic value with or without including the maternal genetic effects by mean of Animal model, using MTDFREML program. Five models were considered including 1) the maternal genetic group, 2) calving, and 3) contemporary group as fixed effects, 4) direct and maternal genetic effects and 5) maternal permanent environmental effects, as random effects, and finally the residual error effect. Direct heritability coefficients varied form median to low (h 2 a , 0.10 to 0.29) in all models. Models that included the maternal genetic effect presented the lowest values of maternal heritability (h 2 m , 0.07 to 0.17). The genetic correlations between direct and maternal effects were negative and with low magnitude (-0.18 and -0.29). The models were compared between each other using the Maximum Likelihood method (- 2 log L) based on a X2 test, and it gave as result that the best model was the five one (Y = Xb + Z 1 a + Z 3 m + Z 2 ep + e) which included all effects. Key words: brahman, direct genetic effects, genetic parameters, maternal genetic effects, weaning weight, zebu.
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