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What do the terms inbreeding and linebreeding mean?

What is inbreeding?

Inbreeding is the mating of related individuals that have one or more relatives in common. Linebreeding is a form of inbreeding.

What is close inbreeding?

Close inbreeding is the mating of close relatives. The closest form of inbreeding in domestic animals involves matings between full brothers and sisters (full siblings) and between parents and offspring (collectively called first-degree relatives). The offspring from first degree matings have a significantly increased risk of suffering from an inherited disorder.

The second closest form of inbreeding involves matings between grand-parents and grand-offspring, half brothers and sisters (half siblings), uncles/aunts and nephews/nieces, and  double-first cousins (collectively called second-degree relatives). Second degree matings are the second closest possible form of inbreeding. The offspring from second degree matings also have a substantially increased risk of suffering from an inherited disorder.

The more closely related the parents, the greater the chance of their offspring suffering from an inherited disorder. This is because when animals are related to each other they are far more likely to be carrying the same defective genes. When these defective genes pair up in the offspring, inherited diseases can occur. Inherited diseases can cause significant suffering and reduce quality of life particularly where the disease causes pain.

What is linebreeding?

Linebreeding is a term commonly used to describe milder forms of inbreeding. Typically it involves arranging matings so that one or more relatives occur more than once in a pedigree, while avoiding close inbreeding. 

Note that many dog breeders apply the term “inbreeding” only to close inbreeding, despite the fact that linebreeding is a form of inbreeding and has the same effects.

Why should inbreeding be avoided?

It is scientifically proven and widely recognised that, on average, the effect of inbreeding/linebreeding is:

  1. An increase in the prevalence of inherited disorders
  2. A decrease in viability
  3. A decrease in reproductive ability, and
  4. The loss of genetic diversity (i.e. decrease in genetic variation).

Inbreeding can also result in developmental disruption, higher infant mortality, a shorter life span and reduction of immune system function. The immune system is closely linked to the removal of cancer cells from a healthy body, so reduction of immune system function increases the risk of tumour development. Immune system function is also critical for defense against infectious disease. Welfare problems can occur where the immune system is compromised.

Collectively, these effects of inbreeding/linebreeding are called inbreeding depression. Importantly, inbreeding depression increases as the extent of inbreeding/linebreeding increases.

What is RSPCA Australia’s position?

Given the overwhelming scientific evidence, RSPCA Australia’s position is that both first degree (e.g. father  to daughter) and second degree matings (e.g. grandfather to granddaughter) should cease and as a general rule, matings should be arranged between parents that are as unrelated as possible and are also physically and behaviourally sound. By mating unrelated parents, there is a significant reduction in the risk of the offspring suffering from an inherited disorder.

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Updated on April 30, 2019
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