Managing Genetic Defects in Beef Cattle

Over the last five years the beef seedstock sector has had to deal with a number of recessive genetic defects. The use of assisted reproductive technologies including embryo transfer and artificial insemination has allowed breeders to concentrate selection to a relatively small number of animals.

While many breeders avoid matings of half-sibs or sires to daughters to reduce the accumulation of inbreeding, it is not unusual for very prominent sires to appear several generations back in pedigrees of both the sire and dam of a particular individual. In these circumstances there is an increased chance for the appearance of a progeny affected by a recessive genetic defect.

Autosomal recessive genetic defects are inherited congenital abnormalities. These genetic mutations occur on one of the 29 pairs of autosomal (non-sex coding) chromosomes. Animals that inherit a single defective or mutated gene and one normal copy of the gene are called heterozygotes and are not affected by the disease, but are carriers of the defect, passing it on to half of their progeny on average.

Animals that have two copies of the normal or unmutated gene are called homozygous normal. Animals that inherit two copies of the defective gene are homozygous for the recessive forms of the gene and are phenotypically affected by the abnormality.

The carriers (heterozygotes) and homozygous normal animals do not exhibit the deleterious condition and are phenotypically indistinguishable. The fact that the normal animals and heterozygotes are indistinguishable makes phenotypic selection to eliminate defect carriers ineffective.

To illustrate this difficulty in phenotypic selection to eliminate recessive carriers, consider a more common trait like coat color. Both homozygous black animals and heterozygous black (red carriers) are phenotypically black and indistinguishable. Only when two black animals produce a red calf can we infer the genetic makeup of the color parents as both being heterozygotes (red carriers).

All breeds carry some genetic defects. Presumably some of the autosomal recessive lethal genetic conditions affect embryos during gestation and cause early embryonic loss. These defects are difficult to detect

and may simply be diagnosed as 'reproductive failure.' A number of defects have been documented in beef breeds, several quite recently.

A brief description of several genetic defects are listed below. Due to space limitations this is not a complete listing of defects.

Management of genetic defects in seedstock or commercial beef herds can be quite challenging. In the case of seedstock herds, suspect animals or those known to be the progeny of carriers should be tested when DNA diagnostic tools are available and economically practical.

Carrier animals maybe retained in the breeding herd, but breeders should test all progeny to determine carrier status prior to marketing as breeding stock. Carrier calves should only be sold to feeders and designated for slaughter only.

Table 1 details the expected results if a homozygous normal sire is mated to a carrier (heterozygote) cow. The resulting progeny will include, on average, one-half defect free and one-half carrier calves with no affected calves. Also included in Table 1 is the expected proportion of progeny of various genotypes when a carrier sire is mated to a carrier dam.

On average one-quarter of the calves will be free of the defect, one-half will be carriers and one-quarter will be affected. If the animals are affected by a lethal defect, the surviving animals will include one-third defect free calves, and two-thirds will be carriers of the defect.

If you have a calf born with a suspected congenital defect, photograph the affected calf and contact your breed association immediately to arrange for tissue collection and reporting instructions.

In a commercial herd that has had affected calves or is striving to prevent introduction of a known defect, bulls should be DNA tested as the primary means of control. Mating of carrier cows to non-carrier or clean bulls will result in the production of no affected calves.

It is frequently not economically practical to cull commercial cows on the basis of their carrier status. If a DNA test is available, all new sire purchases or semen used for AI (artificial insemination) should be from sires that are not carriers. Non-carriers may also be determined by pedigree if both sire and dam have tested free of the defect. If a DNA test for a defect is not available, the strategic use of a planned crossbreeding system may eliminate the appearance of affected calves.

Care should be taken to select a breed that has not had any calves produced in recent generations that are affected by the same defect one is trying to eliminate. At the commercial level, autosomal recessive defects can be effectively managed through careful sire and/or breed selection without extensive culling of the beef cow herd. 

Table 1. A.) Expected calf genotypic frequencies from mating of non-carrier sire to carrier dam for deleterious recessive genetic defect. B.) Expected calf genotypic frequencies from mating of carrier sire to carrier dam.

A = Normal genotype a = Defective/mutated genotype