Inbreeding

What is inbreeding?

Animals inherit half of their genetic material (DNA) from their mother and the other half from their father. Inbreeding refers to the proportion of an animal’s identical DNA inherited from a common ancestor that existed on both their mother’s and father’s sides of the pedigree.

Inbreeding causes a measurable negative impact on cattle performance. Inbreeding will not exist if the sire and the dam do not share any near or distant ancestors. Therefore inbreeding can be minimised by avoiding matings between related animals.

How is inbreeding measured?

Inbreeding is measured using inbreeding coefficients. These estimates are calculated from an animal’s recorded pedigree and represent the probability of genes being identical due to shared parentage. The coefficient provides a general estimate because it is based on pedigree rather than an exact figure.

Inbreeding is calculated as half the additive genetic relationship between the parents. This is because each parent contributes half their genes to the offspring.

For example, a father and daughter share half their DNA, so have an additive relationship of 50 percent. Offspring generated from a father-daughter mating would have an inbreeding estimate of half of this, that is 25 percent identical DNA.

This is an estimate because in a father-daughter mating the expected inbreeding is 25 percent, but individual offspring might inherit slightly more or less DNA.

Other examples (also shown in the table below):

• A mating between a full brother and sister (both parents are the same) will also share 25 percent identical DNA.
• Animals that have a single great-grandparent in common will share 3.1 percent identical DNA.
• Animals that are half brothers or sisters (one parent the same) will share 12.5 percent identical DNA.

Genetic mutation due to inbreeding

Inbreeding increases the likelihood of genetic mutations being expressed in offspring. When related animals mate, there is a higher chance that both parents carry the same recessive mutated gene from a common ancestor. If the offspring inherit two copies of this recessive gene, it can result in impaired functions and genetic disorders.

Over time, the accumulation of these genes can lead to inbreeding depression, characterised by reduced health, fertility, and overall performance.

Inbreeding depression: is a population’s reduced ability to survive and reproduce.

How is inbreeding accounted for in breeding values?

Inbreeding can impact any trait and produce lower-performing offspring than expected. However, in most cases, the expected performance of future offspring, born from inbred parents, can still perform well if further inbreeding is avoided.

Animal evaluation adjusts breeding values by removing the effects of inbreeding depression on the inbred animal. The adjustment ensures the breeding values reflect the true genetic potential of an animal, not the reduced performance due to inbreeding.

If a daughter has been inbred, her results will not truly reflect the potential performance of her farther. This means that for daughter-proven sires, breeding values are adjusted to prevent bias from underestimation due to poorer performing inbred offspring. Daughter-proven sires are bulls that have proven reliable and consistent performance results based on the performance of their daughters.

Managing inbreeding during mating

Managing genetic relationships within herds can minimise the risks associated with inbreeding and maintain the genetic health and productivity of stock. Inbreeding will not exist if the sire and the dam do not share any near or distant ancestors and can be minimised by avoiding mating between related animals. Your artificial breeding company has programmes to help you avoid inbreeding.