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Thoroughbred Genetics: How Close is Too Close?

Thoroughbred breeding involves producing winning racehorses and, often more importantly, producing racehorses that will consistently and predictably produce quality offspring. To breed racehorses for these sometimes conflicting goals, it is important to balance the benefits of outcrossing with those of inbreeding or linebreeding. Outcrossing means that the stallion and mare being bred are not related to each other. Outcrossing increases heterozygosity, or the presence of different alleles in a given gene, and generally increases overall vigour, soundness, longevity and fertility. The heterozygosity obtained through outcrossing is one of the reasons why 'mutts' are usually beset by fewer health problems than purebred dogs. Homozygosity or the presence of genes identical-by-descent is realized by inbreeding and has the effect of increasing the predictability of the performance potential of an animal's progeny. Inbreeding is carried out when the stallion and mare being mated have one or more common ancestors within the useful five-generation…

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