Genetic effects on birth weight in reciprocal Brahman-Simmental crossbred calves.

Brahman-cross calves exhibit unusual inheritance of birth weight: Brahman-sired crossbreds out of females are heavier with greater difference between sexes than calves of the reciprocal cross. The objectives of this work were to confirm that unusual inheritance and to investigate non-Mendelian genetic effects that may influence differences in Brahman × Simmental crossbred calves. Crossbred calves were produced by embryo transfer ( = 2,862) and natural service or artificial insemination ( = 2,125) from 1983 to 1991 by a private seedstock producer. Brahman-sired F embryos out of Simmental donors weighed 9.4 ± 1.1 ( < 0.001) kg more at birth than Simmental-sired F embryos out of Brahman donor cows when transferred to comparable recipients. This reciprocal difference was accompanied by sexual dimorphism: within Brahman-sired F calves, males were 5.0 ± 1.4 kg heavier than females, whereas within Simmental-sired F calves, females were 0.7 ± 0.5 kg heavier than males. Covariates were constructed from the pedigree to represent genetic effects: proportion Brahman in calves and dams (direct and maternal breed effects), direct and maternal breed heterozygosity, probability of Brahman mitochondrial origin, probability of Brahman Y chromosome, probability of Brahman X chromosome, genomic imprinting (the difference between the probabilities of Brahman in the genetic dam and in the sire), nonrandom X inactivation by breed of origin (the probability of breed heterozygosity of the X chromosomes of a female), and nonrandom X inactivation by parent of origin (the difference between probabilities of a female inheriting a paternal or maternal Brahman X chromosome). The maternal breed heterozygosity, genomic imprinting, probability of Brahman X chromosome, and genomic imprinting × sex effect covariates from the full model were significant with regression coefficients of 1.1 ± 0.5 ( < 0.05), ‒8.3 ± 2.3 ( < 0.01), ‒3.5 ± 1.3 ( < 0.01), and ‒5.3 ± 2.0 ( < 0.01), respectively. Results suggest that sex-specific genomic imprinting may be contributing to the inheritance of birth weight in crossbred calves, similar to patterns of mouse litter and placental weight in interspecific crosses.