What Is Inbreeding? Definition and Genetic Effects

Photograph of a strand of DNA

Inbreeding is the process of mating genetically similar organisms. In humans, it's associated with consanguinity and incest, in which close relatives have sexual relationships and children. Inbreeding violates modern social norms, but is fairly common in animals and plants. 

Genetic Effects of Inbreeding

When two closely related organisms mate, their offspring have a higher level of homozygosity: in other words, an increased chance that the offspring will receive identical alleles from their mother and father.

In contrast, heterozygosity occurs when the offspring receives different alleles. Dominant traits are expressed when only one copy of an allele is present, while recessive traits require two copies of an allele to be expressed.

Homozygosity increases with subsequent generations, so recessive traits that might otherwise be masked may start appearing as a result of repeated inbreeding. One negative consequence of inbreeding is that it makes the expression of undesired recessive traits more likely. However, the risk of manifesting a genetic disease, for example, isn't very high unless inbreeding continues for multiple generations.

The other negative effect of inbreeding is reduction genetic diversity. Diversity helps organisms survive changes in the environment and adapt over time. Inbred organisms may suffer from what is called reduced biological fitness.

Scientists have also identified potential positive consequences of inbreeding.

Selective breeding of animals has led to new breeds of domestic animals, genetically suited to specific tasks. The positive consequences are less well studied in humans, but in a study of Icelandic couples, scientists found that marriages between third cousins resulted in a greater number of children, on average, than those between completely unrelated couples.


Disorders From Inbreeding

The risk of a child developing an autosomal recessive disorder increases with inbreeding. Carriers of a recessive disorder may be unaware they possess a mutated gene because two copies of a recessive allele are needed for gene expression. On the other hand, autosomal dominant disorders are seen in the parents, but might be eliminated through inbreeding if the parents carry the normal gene. Examples of defects seen with inbreeding include:

  • Reduced fertility
  • Reduced birth rate.
  • Higher infant and child mortality.
  • Smaller adult size.
  • Reduced immune function.
  • Increased risk of cardiovascular disease.
  • Increased facial asymmetry.
  • Increased risk of genetic disorders.

Examples of specific genetic disorders associated with inbreeding include schizophrenia, limb malformation, blindness, congenital heart disease, and neonatal diabetes.

The House of Habsburg may be the best example of the effects of inbreeding in humans. The Spanish Habsburg dynasty endured for six centuries, largely from consanguinous marriages. The last ruler of the line, Charles II of Spain, displayed a number of physical problems and was unable to produce an heir. Experts believe inbreeding lead to the extinction of the royal line.

Animal Inbreeding

Successive inbreeding of animals has been used to establish "pure" lines for scientific research. Experiments conducted on these subjects are valuable because genetic variation can't skew the results.

In domestic animals, inbreeding often results in a trade-off where a desirable trait is magnified at the expense of another. For example, inbreeding Holstein dairy cattle has let to increased milk production, but the cows are more difficult to breed.

Many wild animals naturally avoid inbreeding, but there are exceptions. For example, banded mongoose females often mate with male siblings or their father. Female fruit flies prefer to mate with their brothers. The male Adactylidium mite always mates with its daughters. In some species, the advantages of inbreeding may outweigh the risks.

Inbreeding Fast Facts

  • Inbreeding occurs when two closely related organisms mate with each other and produce offspring.
  • The two main negative consequences of inbreeding are an increased risk of undesirable genes and a reduction in genetic diversity. 
  • The House of Habsburg may be the best example of the effects of inbreeding in humans.