Inbreeding

Inbreeding involves the mating of animals related to each other. More specifically, the mating of animals that is more closely related to each other than the average relationship in the flock, and even closer than any two animals drawn from the flock at random. The most intensive form of breeding is the repeatedly mating of a full brother with a full sister, a sire with a daughter or a son with the dam.

The genetic effect of breeding is that it makes more pairs of genes in the population homozygous.

In general, the effect of any system of inbreeding is an increase in homozygosity and decreasing in heterozygosis. If harmful recessive genes are present in the stock, inbreeding will convert them (homozygous), thus enabling us to eliminate them at a faster rate. It is usually impossible, even by the most rigid selection, to rid an inbred line of all detrimental recessive genes.

Inbreeding Depression

Experience has shown that inbreeding is usually associated with an overall decline in vigour and performance. Some examples of inbreeding depression are given in Table A.

From the results of these and many other studies we can generalize that inbreeding tends to reduce fitness. Thus, characteristics that form an important component of fitness, such as the number of young born, or lactation in mammals show a reduction after inbreeding. In saying that a certain character shows inbreeding depression, we refer to the average change of the mean value in several inbred lines. These separate lines are commonly found to differ to a greater or lesser extent in the change in the show.

Inbreeding is the most powerful instrument at the disposal of the breeder to build up a uniform or similar families or bloodlines. Inbreeding is therefore the only way in which pure-breeding groups can be obtained. However, breeders must realise that purity resulting from inbreeding is attributable to the system of mating related animals, while selection merely gives direction to this process.

In practice “pure breeding” usually means inbreeding with a specific breed and crossbreeding with other breeds thus being excluded.

The most important reasons for the application of inbreeding in a flock are the following:

• It is necessary when the relationship with a specific good ancestor must be maintained at a high level.
• It helps to reveal undesirable recessive characters and eliminate them from the flock.
• It results in uniform and separate families so that the selection between family groups can be applied effectively.
• It increases pre-potency.
• Sometimes it is necessary because of financial considerations, particularly when another good sire must succeed a specific ram in use, which is of excellent quality and has been purchased at great expense.

The danger of intensive inbreeding is that the accumulation of undesirable homozygous genes takes place at such a rate that it is impossible to remove all the animals showing weaknesses as a result, from the flock. Some of these undesirable genes will therefore be fixed in the whole flock.

Prepotency = Power is superior to that of the other parent in transmitting inheritable characters to the offspring.

The phenomenon of degeneration resulting from inbreeding may easily make its appearance in a flock without being identified as such. This problem probably arises when farmers breed their own rams by using a few purchased stud rams on a nucleus flock. A nucleus flock is usually small, which means that the successive sets of rams bred from it will therefore practically be regarded as half-brothers. Lambs of successive years will therefore be cousins, and this causes an increase in inbreeding of approximately six per cent in the flock. Dams can also be regarded as of poor quality. The danger of artificial insemination can also bring about severe inbreeding. The degeneration caused by inbreeding is often greater than the estimated inbreeding improvement resulting from the higher selection intensity. An unqualified recommendation can therefore not be made for the application of artificial insemination with a small stock, particularly not in cases where the flock consists of less than 2 000 breeding ewes.