Inbreeding
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Inbreeding is the mating of two animals that are closely related to each other, and often refers to the continued practice of this, producing inbred animals. It can be used to create offspring with specific desirable characteristics, as in the case of pedigree dogs, cats, or racehorses, but it also occurs naturally in relatively small and isolated populations of animals (including humans, of course). Inbreeding may occur if few people have a disproportionate amount of children in a community, as is common in polygamous societies and those isolated from the wider population. In the case of humans, inbreeding is most commonly associated with incest.
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[edit] Benefits and motivations
[edit] Encouragement of desirable traits
Inbreeding can be used as a method of artificial selection to selectively breed plants and animals with desirable traits. An example of this would be the breeding of pedigree dogs. Inbreeding may lead to a highly specialised genome, such as in the case of the cheetah - a species of cat that is heavily inbred, claimed to have been caused by a catastrophic drop in population that occurred thousands of years ago. The species is highly specialized, yet possesses a relatively weak immune system.[1]
[edit] Political and financial motivations
The ruling classes of some societies have tried to breed with those of a similar high status. In Europe it was common for the royalty of various nations to be related by blood, the House of Windsor being an example of this.[2]
This inbreeding is comparable to eugenics, in that certain bloodlines are deliberately kept "pure", although in practice it often resulted in the opposite of eugenics, producing such benighted creatures as Carlos II of Spain, who is said to have had one of the most deformed jaws ever recorded. The Habsburg inbreeding was an attempt to ensure that family assets and holdings remained within the control of the family, but in the case of Carlos II resulted in a more inbred genome than the average brother-sister mating.
One general result of this inbreeding in the European royal families was the concentration of the disease hemophilia in the males [3]. So much so that hemophilia was known as the Royal disease until after World War II.
[edit] Applications in genetic research
Inbred strains of species are important in genetic research. Inbreeding creates a less varied genome, reducing the chances that a genetic variation among test subjects could skew their results.
[edit] Drawbacks
[edit] Loss of diversity in the genome
A loss of diversity may leave a group less adaptable to changing circumstances - creating a genetic monoculture. In the case of plants, this increases the risk that crops would be eliminated by a pest or disease for which the entire group lacks resistance. An example of this is found in the Irish potato famine, in which two factors led to mass starvation: over-reliance on a single crop (potatoes), and a lack of variety in that crop.[4]
[edit] Recessive traits and genetic abnormalities
Recessive traits are traits coded for by recessive alleles. Recessive alleles are variants of genes that are "switched off" or overridden by the presence of a dominant allele of the same gene: Therefore, a recessive trait will only be expressed if an individual carries only recessive alleles for that trait's gene(s). Close relatives are much more likely than strangers to carry the same alleles, increasing the chances that their offspring could inherit recessive alleles from both parents. Recessive traits may be cosmetic, such as in the cases of blue eyes and red hair,[5] or more serious, carrying mutations leading to genetic abnormalities or diseases such as hemophilia or cystic fibrosis [6].
In animal breeding, the bulldog is commonly cited as an example of inbreeding taking to a point that the animal could not have otherwise reached by natural selection. Bulldogs have been bred for their characteristic floppy face, but in some dogs this has reached point at which dogs may struggle to breathe. The UK Kennel Club, an organization for dog breeders, issued guidelines to limit the breeding of that specific characteristic.[7]
[edit] Modern instances of inbreeding
Western society generally considers inbreeding among humans to be distasteful, most countries having criminalized incest. Inbreeding in the form of sexual relations between direct blood relatives is rare, although isolated communities may fall prone to inbreeding if discouraged to marry outside of their small social group. Immigrant communities, and those practicing a self-imposed isolation from the surrounding culture, are a commonplace source of marriage between blood relations, most commonly cousins.[8]
People living on the remote island of Tristan da Cunha are descended from 15 people, all of whom arrived between 100 and 200 years ago. As a result, there is a fairly high prevalence of genetic disorders. There are an elevated number of people with glaucoma, and in particular there is an exceptionally high rate of asthma. The latter led to scientific inquiry which uncovered the genetic nature of asthma, as three of the original settlers had it.[9]
Amish communities are isolated from their surrounding modern neighbors, particularly when it comes to relationships, though abuse appears prevalent in some Amish communities. Amish communities typically have a higher incidence of birth defects.[10] Genetic diversity can be increased by taking husbands and wives from other Amish communities.
Genetic abnormalities are more pronounced in the Arab world, due to the more commonly accepting intermarrying of blood relatives, most commonly between first cousins. Many Arab societies are tribal in nature, thus combining isolationism and the need to keep it in the family.[11]
The stereotypical inbreeding within small towns and redneck societies was parodied in the Mel Brooks movie, Blazing Saddles. Most of the residents of the fictional town, Rockridge, share the surname "Johnson", and were incredibly dumb - and initially bigoted.[12]
The English county of Norfolk is, at least comedically, notorious for inbreeding. A UK MP said, regarding the high incidence of diabetes there: ""I would imagine it is linked to the fact that people in Norfolk are quite inbred, with many not leaving the county,"[13] (He later apologised).[14]
[edit] See also
[edit] External links
- Angels and insects at IMDB
[edit] Footnotes
- ↑ Cheetahs Appear Vigorous Despite Inbreeding
- ↑ Windsor family tree (PDF)
- ↑ Haemophilia in the Descendants of Queen Victoria
- ↑ Irish Monoculture and the Irish Potato Famine: cases of missing genetic variation
- ↑ Hair Color
- ↑ Cystic Fibrosis at Genetics Home Reference
- ↑ Healthier new bulldog will lose its Churchillian jowl
- ↑ There's nothing wrong with cousins getting married, scientists say (Presumably scientists with attractive cousins)
- ↑ "Worldwide search for asthma clue". BBC News. 9 December 2008. http://news.bbc.co.uk/2/hi/health/7766656.stm. Retrieved 15 March 2012.
- ↑ Ellis-van Creveld syndrome and the Amish, this links to the Abstract
- ↑ Genetic disorders in the Arab world
- ↑ Blazing Saddles at Rotten Tomatoes, Classic Mel Brooks
- ↑ Dr Ian Gibson, the Norwich North MP
- ↑ "It has obviously caused a lot of offence and for that I unreservedly apologise," he said but added: "Words like inbreeding and outbreeding are very professional, genetic terms. ...