What is the risk of affected offspring when using a carrier bull/cow for a simple recessive trait?

With simple recessive genetic conditions (DD, AM, NH, CA, OS, etc.), an animal must carry two copies of the defective gene in order to show the condition. Mating a carrier to a free animal would result in half of the progeny being carriers and all the progeny would appear normal. Mating two carrier animals will result in 25% affected calves, 50% normal phenotype but carriers of the defective gene, and 25% free of the genetic defect. With non-lethal homozygotes as is the case with DD and CA, the potential exists to breed homozygous recessive animals. Breeding affected animals will quickly increase the frequency of the defective allele as every progeny will inherit the broken gene. Breeding strategies should attempt to increase the frequency of desirable alleles while decreasing the frequency of the defective alleles.

Expected outcomes of breeding free (both genes are normal, AA), carrier (normal phenotype but carries one copy of the defective gene, Aa), and affected animals (likely display the phenotype and both genes are defective, aa) for a simple recessive trait. Note, for embryonic lethals the aa animals are never seen.

Expected outcomes of breeding free (both genes are normal, AA), carrier (normal phenotype but carries one copy of the defective gene, Aa), and affected animals (likely display the phenotype and both genes are defective, aa) for a simple recessive trait. Note, for embryonic lethals the aa animals are never seen.

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