Science Fair Project Encyclopedia
If practised repeatedly, it typically leads to a reduction in genetic diversity. Inbreeding often leads to reduced health and fitness (called consanguinity depression); however, livestock breeders often practice inbreeding, then cull unfit offspring, especially when they are trying to establish a new and desirable trait in their stock.
The most commonly cited problem with inbreeding is that two closely related individuals are likely to have very similar genomes, and if one individual has a gene for a given negative trait, then the other is likely to have it as well. However, Gregor Mendel and his plant breeding experiments show that a single instance of inbreeding is statistically very unlikely to result in a flawed individual, as far as inheriting negative traits goes.
Nevertheless, an inbred individual is likely to possess several physical and health defects, in addition to higher incidence of inheriting a poor trait. They include:
- reduced fertility both in litter size and in sperm viability
- increased congenital defects
- fluctuating facial assymetry
- lower birth rate
- higher neo-natal mortality
- slower growth rate
- smaller adult size, and
- loss of immune system function.
The most discussed instances of interbreeding relate to European monarchies. The royal families of Europe have close blood ties which are strengthened by intermarriage. Some examples are:
- Queen Elizabeth II of the United Kingdom and her husband Prince Philip are third cousins - both being descended from Queen Victoria, Philip is the great-great-grandson, Elizabeth II the great-great-granddaughter.
- Many Egyptian Pharaohs married their sisters.
However, it is not necessarily the case that there is a greater amount of inbreeding within royalty than there is in the population as a whole: it is simply better documented. Among genetic populations that are isolated, opportunities for exogamy are reduced. Isolation may be geographical, leading to inbreeding among peasants in remote mountain valleys. Or isolation may be social, induced by the lack of appropriate partners, such as Protestant princesses for Protestant royal heirs. Since the late middle ages, it is the urban middle class that has had the widest opportunity for outbreeding .
Mammals, most other animals, and higher plants as well, have evolved mechanisms to avoid inbreeding of any sort. Some, like sweet cherries, have even evolved elaborate biochemical mechanism to ensure that their flowers can not be fertilized by themselves or by very genetically similar individuals.
Most pack animals (like lions, primates, and dogs) kick young males out of the pack so as to prevent them from mating with female relatives. Humans have very strong taboos against mating with relatives. Even fruit-flies apparently have a sensing mechanism to avoid too close of inbreeding, even in a closed population they maintain more genetic diversity than they ought to by random mating.
The cheetah is a highly inbred species, probably because of a population bottleneck in the species' recent past. Inbreeding is also deliberately induced in laboratory mice in order to guarantee a consistent and uniform animal model. Human genetic diversity is also limited, indicating a population bottleneck some 70,000 years ago.
Where a species is threatened by extinction, the population may fall below a minimum whereby the forced interbreeding between the remaining animals will result in extinction.
- Pedigree collapse
- Compare with: hybrid vigor
- Self-incompatibility in plants (how some plants avoid inbreeding)
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