Applications Of Mendelian Genetics In Livestock

These principles may be used to predict the results of matings for traits affected by single gene pairs. For example, polled is dominant to horned in cattle. Matings of heterozygous polled individuals are expected to produce 3/4 polled and 1/4 horned offspring. If combined with black (dominant) vs. red (recessive) matings of heterozygous polled, black cattle are expected to produce 9/16 polled black, 3/16 polled red, 3/16 horned black, and 1/16 horned red.

Additionally, these principles may be used to develop test crosses to evaluate individuals suspected of being carriers (heterozygous) for lethal or deleterious conditions. For example, a bull that is suspected of being a carrier for dwarfism (recessive) could be mated to a group of 10 cows that are known to be carriers. The probability of 10 normal calves, if the bull is a carrier, would be (3/4)10=0.056. This is low enough that the bull's owner could state, with a reasonably high degree of confidence, that the bull is not a carrier. Of course, a single dwarf calf would be all it takes to demonstrate that he is a carrier.

It must be remembered that these basic principles also apply to the genetic background for performance traits such as growth rate, egg production, racing speed, or backfat thickness. Such traits may be influenced by hundreds of gene pairs but these genes are also discrete and come in pairs, one from each parent. Even though the effect of any one gene pair may not be clearly observed, the gene pairs do exist and behave according to Mendel's principles.

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