Genetic Sources of Variation in the Gestation Length of the Horse

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Abstract
Evidence is presented that in Arabian horses the genotype of the foetus affects the length of gestation. This, of course, implies an influence of the stallion. The magnitude of this effect in records corrected for seasonal effect and level of nutrition of the mare was similar to reports for other breeds of horses and for dairy cattle and sheep. The heritability estimate obtained from these adjusted records was 36 percent. Analysis of the authors' data and other data in the literature suggests the possible importance with respect to gestation length of sex linked genes in both the genotype of the foetus and of the mare. A partitioning of the total phenotypic variance of gestation lengths into genetic and non-genetic factors yielded the following estimates of the relative importance of these sources of variation: Season of breeding accounted for 43.4 percent of the total variance, level of nutrition of the mare 5.2 percent, additive genotype of the foal 18.3 percent, dominance deviations from the additive scheme 7.1 percent, permanent maternal traits 12.2 percent, and residual unanalyzed causes 13.8 percent.