Inbreeding and genetic diversity
Importance of restricting inbreeding
Inbreeding is the result of mating related individuals. Early in their development most breeds of livestock had a period of inbreeding to establish more genetic uniformity, for example, colour patterns and aspects of physical appearance. Line breeding is a term often used to describe successful mating of related individuals, but it is simply inbreeding.
Inbreeding results from individuals receiving identical genes from each parent. If the parents are related it is more likely that they have identical genes. When an individual receives an identical gene from each parent it is said to be homozygous for that pair of genes. This would be desirable if the gene received from each parent led to superior performance. However, animals can also carry undesirable genes that may remain hidden unless an animal has a pair of them (these are called recessive genes). An inbred individual is more likely to have identical gene pairs, so is more likely to express undesirable characteristics due to undesirable genes.
How is inbreeding measured?
Individuals are considered to be biologically related when they have one or more common ancestors. For practical purposes, if two sheep have no common ancestor within the last five or six generations, they can be considered unrelated.
Genetic relationships are important in animal breeding because the closer the relationship, the higher the percentage of common genes the two individuals carry. Closeness of relationship is determined by three factors:
- How far back in the two animals’ pedigrees the common ancestor appears;
- How many common ancestors they have; and
- How frequently the common ancestors appear.
The relationship is also influenced by the extent of inbreeding of the common ancestor or ancestors. Small closed populations are much more likely to have many common ancestors. In addition, reproductive technologies such as Artificial Insemination (AI), Multiple Ovulation and Embryo transfer (MOET) and Juvenile in Vitro Embryo Transfer (JIVET) have enhanced the use of the best genetic material. This has resulted in obvious gains by rapid genetic improvement. However these technologies have led to population sizes becoming effectively smaller.
What is wrong with inbreeding?
Inbreeding leads to a decline in performance called inbreeding depression. It is the opposite of heterosis, or hybrid vigour, which is the advantage of cross-breeding.
The most common detrimental effects of inbreeding are:
- Poorer reproductive.
- Higher mortality.
- Lower growth rates.
- Higher frequency of hereditary abnormalities.
All animals within a breed are related and most ram breeders are practising some inbreeding. Inbreeding is nearly unavoidable in ram breeder flocks since it is often difficult to find new high performance sires and dams that are unrelated. Genetic evaluations (BLUP) favours selection of related individuals, hence using ASBV’s contributes to the level of inbreeding.
It is important to realise that inbreeding can only be calculated accurately when parentage is known.
The effect of inbreeding
Accumulated inbreeding most often results in reduced performance, although occasional high performance animals are produced.
Being the opposite of hybrid vigour it is not unexpected that the traits which benefit most from heterosis are, generally, the same traits which are most affected by inbreeding depression. The greater the degree of inbreeding, the greater the reduction in performance.
The actual performance reduction is not the same in all traits. Some traits (like meat quality) are hardly influenced by inbreeding while others (like reproductive rate) can be greatly influenced by inbreeding.