Inbreeding & how to manage it
What is Inbreeding?
Inbreeding is the practice of mating two genetically related animals. To a degree this is inevitable within any long-term selection programme involving a closed population.
Breeders will sometimes deliberately inbreed specific bloodlines to fix certain characteristics in the herd, increasing the frequency of favourable genes – or more rarely to expose recessive genes.
Inbreeding may have a place in a breeding strategy as in some instances it will bring a number of favourable genes together. However, it will also tend to increase the number of recessive or deleterious genes being expressed – leading to a reduction in fitness and productivity; this is referred to as “inbreeding depression”.
The Challenge to Avoid Inbreeding
To make fast genetic progress in any particular attribute a small number of bulls and cows that excel in that attribute tend to be mated together. If this approach is taken to an extreme, it can quickly result in an increase in the level of inbreeding within the herd.
Modern livestock breeding programmes can be susceptible to increases in inbreeding due to the widespread use of AI & ET, fast generation turnover, selective use of specific family lines and the tendency for a relatively small number of different sire families to dominate within certain breeds. Increases in inbreeding are observed in both recording and non-recording herds.
Breeders need to strike a balance to optimise rates of genetic gain, whilst controlling increases in levels of inbreeding.
Calculating Levels of Inbreeding
The level of inbreeding is calculated as the probability of two alleles being identical by descent. This value is called the “inbreeding coefficient”.
Typical inbreeding percentages are as follows, assuming no previous inbreeding between any parents
- Father/daughter, mother/son or brother/sister → 25%
- Grandfather/granddaughter or grandmother/grandson → 12.5%
- Half-brother/half-sister → 12.5%
- Uncle/niece or aunt/nephew → 12.5%
- Great-grandfather/great-granddaughter or great-grandmother/great-grandson → 6.25%
Should Breeders Completely Avoid Inbreeding?
Complete avoidance of inbreeding is almost impossible and a balance has to be struck between the genetic superiority of a specific bull and the level of inbreeding he creates in the future herd. Low levels of inbreeding (<6.25%) are often considered an acceptable compromise. Levels higher than that should be avoided.
The best solution to the inbreeding challenge is to measure the level of inbreeding between individuals and make recommendations based on this information.
Summary
Breeders shouldn’t stop making good use of the reproductive and genetic tools available to them – but they can take steps to minimise increases in inbreeding now and in the future.