In horses inbreeding is the term given when the negative traits are reinforced in the offspring.

Inbreeding is the mating of horses of the same breed which are more closely related than the average of the breed. Over the years, an element of fear has developed over the dangers of inbreeding, however, inbreeding does have its uses and it plays a large part in the establishment of some degree of uniformity in breeds today.

Inbreeding can be divided into two types:

Close breeding - the mating of father to daughter, mother to son or brother to sister. (Brothers and sisters have, on average, half their genes in common. Fathers and daughters or mothers and sons must have half the genes in common)

Line breeding mating half-brother to half-sister, half cousins, grandfather to granddaughter, cousin to cousin, or grandson to grand daughter

The Genetic Effect of Inbreeding
The genetic effect of inbreeding is that it makes more pairs of genes homozygous. This means that there will be no recessives in the genetic makeup of the breed.

By increasing the degree of homozygosity, inbreeding increases the chances that recessive genes will come together and be homozygous. Some of these recessives may control lethal detrimental characteristics. It is very important to understand that inbreeding does not increase the chances that they will come together.

Inbreeding does not uncover dominant genes because they always show themselves. However, it increases the degree of homozygosity among the dominant genes. Thus inbreeding can help to fix characteristics within a breed, but at the same time as fixing good characteristics it may help to fix bad ones.

The effects of inbreeding on quantitative (measurable) traits have been studied. These studies have shown conclusively that increasing inbreeding produces a decline in those traits related to physical fitness. Examples of these traits include; fertility; viability and early growth rate. Inbred animals within a specific inbred line are more likely to be alike genotypically than phenotypically for traits of economic importance.

Line Breeding
Line breeding is a less intensive form of inbreeding than close breeding. The object of line breeding is to keep the offspring related to an outstanding animal, usually male, which had desired characteristics. It aims to avoid some of the dangers attendant with close breeding.

Uses of Line Breeding

Line breeding is useful when an outstanding sire has been found and proved by progeny testing. The sire may be used for a second or third time along the line if it is still living and belongs to the same breeder. The line breeding system must be carefully designed by a person who understands the implications.

Summary of Line breeding

• It keeps offspring related to an outstanding horse, usually stallion, which has extremely desirable characteristics.
• It avoids the danger of close breeding.
• It permits the introduction of new genes.
• It must be practised in a herd large enough to support at least two sires to avoid the danger of close breeding. Alternatively, Artificial Insemination can be used.
• It is carried out usually in an outstanding herd where it is not possible to purchase a sire from outside the herd that could provide better genes than those already in the herd.

Advantages of Inbreeding

  • Inbreeding does not create fresh genes but it does sort out the genes already in the gene pool of a herd. It increases the number of genes in the homozygous condition.
  • Most undesirable traits are recessive. These can be discovered and eliminated from the herd by close breeding.
  • Close breeding such as a mating between sire and daughter will test the good and the bad genes in that family. Such matings are used on an experimental basis to discover if there are any undesirable recessives in the family (such as red coat colour in Friesians). Once the results of such a test mating are known, the breeder has a much better idea whether to use the sire for a line breeding program.
  • The transmitting qualities of a sire will be affected if it is inbred. An inbred sire is much more likely to be prepotent than a sire bred from unrelated parents. Some breeders have been fortunate in having few undesirable recessives in their herds, and certain inbred sires and dams have been very prepotent for high production (particularly in the area of milk yield). The development of several lines, or families, within a herd has produced some outstanding animals when these lines have been crossed.
  • There is no point line breeding to a bad sire.
  • There are many advantages in line breeding to a good sire as the inbreeding will emphasise the good traits as well as the bad traits that the sire is transmitting.

Stallion and Mare Complementation
Naturally the purpose of the breeder is to always improve the quality of foals produced in terms of potential performance and conformation. Conformation is perhaps the most important point in the selection process. Avoid breeding with mares or stallions which show poor conformation and remember that most breeds are known for specific conformation characteristics so these need to be taken into account.

Secondly, the breeder needs to consider breeding with others of a similar size. If stallions and mares are mismatched size wise this can lead to conformation faults in the foals as they end up out of balance.

Let’s take thoroughbred breeding as an example here. Breeders compare one stallion to another to check his ability to sire successful racing horses. The Average Earnings Index (abbreviated to AEI) ranks a sire’s offspring as a ratio of earnings to the average earnings of all sires.  Also the Annual Progeny Earning Index (APEX) is a comparison index used to show the racing earnings of the sire’s offspring compared to those earnings of other sires with 10 or more racing offspring. More accurately the APEX is the percentage of a sire’s offspring which are placed in the top 8% of runners that won money.

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