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inbreeding

  (ĭn'brē'dĭng) pronunciation
n.
  1. The breeding of related individuals within an isolated or a closed group of organisms or people.
  2. The continued breeding of closely related individuals so as to preserve desirable traits in a stock.

 
 
Dental Dictionary: inbreeding

n

The production of offspring by the mating of closely related individuals, organisms, or plants; self-fertilization is the most extreme form, which normally occurs in certain plants and lower animals. The practice provides a greater chance for recessive genes for both desirable and undesirable traits to become homozygous and to be expressed phenotypically.

 
Genetics Encyclopedia: Inbreeding

Inbreeding is defined as mating between related individuals. It is also called consanguinity, meaning "mixing of the blood." Although some plants successfully self-fertilize (the most extreme case of inbreeding), biological mechanisms are in place in many organisms, from fungi to humans, to encourage cross-fertilization. In human populations, customs and laws in many countries have been developed to prevent marriages between closely related individuals (e.g., siblings and first cousins). Despite these proscriptions, genetic counselors are frequently presented with the question "If I marry my cousin, what is the chance that we will have a baby who has a disease?" The answer is that when two partners are related their chance to have a baby with a disease or birth defect is higher than the background risk in the general population.

Increased Disease Risk

Many genetic diseases are recessive, meaning only people who inherit two disease alleles develop the disease. All of us carry several single alleles for genetic diseases. Since close relatives have more genes in common than unrelated individuals, there is an increased chance that parents who are closely related will have the same disease alleles and thus have a child who is homozygous for a recessive disease.

For instance, cousins share approximately one-eighth or 12.5 percent of their alleles. So, at any locus the chance that cousins share an allele inherited from a common parent is one-eighth. The chance that their offspring will inherit this allele from both parents, if each parent has one copy of the allele, is one-fourth. Thus, the risk the offspring will inherit two copies of the same allele is 1/8 × 1/4, or 1/32, about 3 percent. If this allele is deleterious, then the homozygous child will be affected by the disease. Overall, the risk associated with having a child affected with a recessive disease as a result of a first cousin mating is approximately 3 percent, in addition to the background risk of 3 to 4 percent that all couples face.

Inbreeding can be measured by the inbreeding coefficient (often denoted F). This is the probability that two genes at any locus in one individual are identical by descent (have been inherited from a common ancestor). F is larger the more closely related the parents are. For example, the coefficient of inbreeding for an offspring of two siblings is one-fourth (0.25), for an offspring of two half-siblings it is one-eighth (0.125), and for an offspring of two first cousins it is one-sixteenth (0.0625). (This is a different calculation than the calculation of shared alleles between cousins, above.)

In general, inbreeding in human populations is rare. The average inbreeding coefficient is 0.03 for the Dunker population in Pennsylvania and 0.04 for islanders on Tristan da Cunha. Inbreeding occurs in both those populations. Some isolated populations actively avoid inbreeding and have maintained low average inbreeding coefficients even though they are small. For example, polar Eskimos have an average inbreeding coefficient that is less than 0.003.

Beneficial changes can also come from inbreeding, and inbreeding is practiced routinely in animal breeding to enhance specific characteristics, such as milk production or low fat-to-muscle ratios in cows. However, there can often be deleterious effects of such selective breeding when genes controlling unselected traits are influenced too. Generations of inbreeding decrease genetic diversity, and this can be problematic for a species. Some endangered species, which have had their mating groups reduced to very small numbers, are losing important diversity as a result of inbreeding.

Genetic Studies of Inbred Populations

Inbred populations can offer a rich resource for genetic studies. They have the advantage of often being relatively homogeneous in both their genetics and environment. A method that has been used successfully to identify several recessive mutations in inbred groups is homozygosity mapping.

This approach looks for regions of alleles at genetic loci that are linked to one another and are homozygous. With inbreeding, there is an increased chance that, in an affected individual, the two alleles at the disease locus will have descended from a common ancestor. Therefore tightly linked markers (identifiable DNA segments) surrounding the disease locus will also tend to come from the same ancestral chromosome and thus be identical on both homologous chromosomes.

Together with colleagues, Erik Puffenberger, a research scientist and laboratory director at the Clinic for Special Children in Strasburg, Pennsylvania, capitalized on the inbreeding in a large Mennonite kindred to identify the location of a gene for Hirschprung disease on chromosome 13. In this family, parents of an affected child are, on average, related as closely as second or third cousins. The region was located because, true to theory, affected individuals shared alleles that were identical by descent at the region containing the disease gene.

Bibliography

Cavalli-Sforza, Luigi L., and Walter F. Bodmer. The Genetics of Human Populations. Mineola, NY: Dover Publications, 1999.

Puffenberger Erik G., et al. "Identity-by-Descent and Association Mapping of a Recessive Gene for Hirschprung Disease on Human Chromosome 13q22." Human Molecular Genetics 3 (1994): 1217-1225.

—Eden R. Martin and Marcy C. Speer

 
Britannica Concise Encyclopedia: inbreeding

Mating of closely related individuals. The opposite is outbreeding, the mating of unrelated organisms. Inbreeding is useful in keeping desirable characteristics or eliminating undesirable ones, but it often results in decreased vigour, size, and fertility of the offspring because of the combined effect of harmful genes that were recessive in both parents (see recessiveness). The closest type of inbreeding is self-fertilization. In linebreeding, mates are selected on the basis of their relationships to a certain superior ancestor. The backcross (crossing a first-generation hybrid with one of the parental types) is a common method of inbreeding.

For more information on inbreeding, visit Britannica.com.

 
Columbia Encyclopedia: inbreeding,
mating of closely related organisms. Inbreeding is chiefly used as a means of insuring the preservation of specific desired traits among the offspring of purebred animals (see breeding). Continued inbreeding through many generations reduces the chances for diversity of characteristics in the offspring and tends to reduce vigor and fertility. Only in laboratory conditions can the unwanted characteristics that frequently result from inbreeding be selected out of the strain and selection for purely advantageous traits be carried out. The necessarily uncontrolled cases of inbreeding among humans (as in closed societies or within royal families) have generally proved deleterious, and inbreeding is therefore discouraged in most societies (see consanguinity; incest).

 
Veterinary Dictionary: inbreeding

The mating of closely related organisms or of organisms having closely similar genetic constitutions.

  • i. coefficient — the probability that the two genes present at a locus in that individual are identical by descent.
  • i. depression — depression of performance caused by inbreeding.
 
Wikipedia: inbreeding

Inbreeding is breeding between close relatives, whether plant or animal. If practiced repeatedly, it often leads to a reduction in genetic diversity, and the increased gene expression of recessive traits, resulting in inbreeding depression. This may result in inbred individuals exhibiting reduced health and fitness and lower levels of fertility.

Livestock breeders often practice inbreeding to "fix" desirable characteristics within a population. However, they must then cull unfit offspring, especially when trying to establish the new and desirable trait in their stock.

In plant breeding, inbred lines are used as stocks for the creation of hybrid lines to make use of the heterosis effect. Inbreeding in plants also occurs naturally in the form of self-pollination.

Results of inbreeding

Inbreeding may result in a far higher expression of deleterious recessive genes within a population than would normally be expected. As a result, first-generation inbred individuals are more likely to show physical and health defects, including:

Natural selection works to remove individuals who acquire the above types of traits from the gene pool. Therefore, many more individuals in the first generation of inbreeding will never live to reproduce. Over time, with isolation such as a population bottleneck caused by purposeful (assortative) breeding or natural environmental stresses, the deleterious inherited traits are culled.

The cheetah once was reduced by disease, habitat restriction, overhunting of prey, competition from other predators (primarily lions, competition from human land use, etc.) to a very small number of individuals.[1][2] All cheetahs now come from this very small gene pool. Should a virus appear that none of the cheetahs have resistance to, extinction is always a possibility. Currently, the threatening virus is feline infectious peritonitis, which has a disease rate in domestic cats from 1%-5%; in the cheetah population it is ranging between 50% to 60%. The cheetah is also known, in spite of its small gene pool, for few genetic illnesses.

Island species are often very inbred, as their isolation from the larger group on a mainland allows for natural selection to work upon their population. This type of isolation may result in the formation of race or even speciation, as the inbreeding first removes many deleterious genes, and allows expression of genes that allow a population to adapt to an ecosystem. As the adaptation becomes more pronounced the new species or race radiates from its entrance into the new space, or dies out if it cannot adapt and, most importantly reproduce.[3]

The reduced genetic diversity that results from inbreeding may mean a species may not be able to adapt to changes in environmental conditions. Each individual will have similar immune systems, as immune systems are genetically based. Where a species becomes endangered, the population may fall below a minimum whereby the forced interbreeding between the remaining animals will result in extinction.

In the South American sea lion, there was concern that recent population crashes would reduce genetic diversity. Historical analysis indicated that a population expansion from just two matrilineal lines were responsible for most individuals within the population. Even so, the diversity within the lines allowed for great variation in the gene pool that may inoculate the South American sea lion from extinction.[4]

Natural breedings include inbreeding by necessity, and most animals only migrate when necessary. In many cases, the closest living mate is a mother, sister, grandmother, father, grandfather... In all cases the environment presents stresses to select or remove those individuals who cannot survive because of illness from the population.

In lions, prides are often followed by related males in bachelor groups. When the dominant male is killed or driven off by one of these bachelors, a father may be replaced with his son. There is no mechanism for preventing inbreeding or to ensure outcrossing. In the prides, most lionesses are related to one another. If there is more than one dominant male, the group of alpha males are usually related. Two lines then are being "line bred". Also, in some populations such as the Crater lions, it is known that a population bottleneck has occurred. Far greater genetic heterozygosity than what was expected was found.[5] In fact, predators are known for low genetic variance, along with most of the top portion of the tropic levels of an ecosystem.[6] Additionally, the alpha males of two neighboring prides can potentially be from the same litter; one brother may come to acquire leadership over another's pride, and subsequently mate with his 'nieces' or cousins. However, killing another male's cubs, upon the takeover, allows for the new selected gene complement of the incoming alpha male to prevail over the previous male. There are genetic assays being scheduled for lions to determine their genetic diversity. The preliminary studies show results inconsistent with the outcrossing paradigm based on individual environments of the studied groups.[7]

There was an assumption that wild populations do not inbreed; this is not what is observed some cases in the wild. However, in species such as horses, animals in wild or feral conditions often drive off the young of both genders, thought to be a mechanism by which the species instinctively avoids some of the genetic consequences of inbreeding.[8]

Inbreeding in domestic animals

Breeding in domestic animals is assortative breeding primarily (see selective breeding). Without the sorting of individuals by trait, a breed could not be established, nor could poor genetic material be removed.

Inbreeding is used by breeders of domestic animals to fix desirable genetic traits within a population or to attempt to remove deleterious traits by allowing them to manifest phenotypically from the genotypes. Inbreeding is defined as the use of close relations for breeding such as mother to son, father to daughter, brother to sister. Breeders must cull unfit breeding suppressed individuals and/or individuals who demonstrate either homozygosity or heterozygosity for genetic based diseases.[9] The issue of casual breeders who inbreed irresponsibly is discussed in the following quote on cattle...

Meanwhile, milk production per cow per lactation increased from 17,444 lbs to 25,013 lbs from 1978 to 1998 for the Holstein breed. Mean breeding values for milk of Holstein cows increased by 4,829 lbs during this period (http://aipl.arsusda.gov/main/data.html#gtrend). High producing cows are increasingly difficult to breed and are subject to higher health costs than cows of lower genetic merit for production (Cassell, 2001). Intensive selection for higher yield has increased relationships among animals within breed and increased the rate of casual inbreeding. Many of the traits that affect profitability in crosses of modern dairy breeds have not been studied in designed experiments. Indeed, all crossbreeding research involving North American breeds and strains is very dated (McAllister, 2001) if it exists at all.

[10]

Linebreeding, a specific form of inbreeding, is accomplished through breedings of cousins, aunt to nephew, half brother to half sister. This was used to isolate breeds within the companion and livestock industry. For instance an animal with a desirable colour is bred back within the lines with identified selection traits whether it be milk production or adherence to breed standard of appearance or behavior. Breeders must then cull unfit individuals, and in some cases the breeders will then outbreed to increase the level of genetic diversity. Again casual breeding is problematical as it is without the requisite culling of individuals who are either maladaptive, not to breed standard or carriers of poor genetic material that must be removed from a healthy breeding program. [11]

Outcrossing is where two unrelated individuals have been crossed to produce progeny. In outcrossing, unless there is verifiable genetic information, one may find that all individuals are distantly related to an ancient progenitor. If the trait carries throughout a population, all individuals can have this trait. This is called the founder's effect. In the well established breeds, that are commonly bred,a large gene pool is present. For example, in 2004, over 18,000 Persian cats were registered.[12] A possibility exists for a complete outcross, if no barriers exist between the individuals to breed. However it is not always the case, and a form of distant linebreeding occurs. Again it is up to the assortative breeder to know what sort of traits both positive and negative exist within the diversity of one breeding. This diversity of genetic expression, within even close relatives, increases the variability and diversity of viable stock. [13]

The two dog sites above also point out that in the registered dog population, the onset of large numbers of casual breeders has cooresponded with an increase in the number of genetic illnesses of dogs by not understanding how, why and which traits are inherited. The dog sites indicate that the largest percentage of dog breeders in the US are casual breeders. Therefore the investment in a papered animal,with an expected short term profit, motivates some to ignore the practice of culling. Casual breeders in companion animals often ignore breeding restrictions within their contracts with source companion animal breeders. The casual breeders breed the very culls that a genetics based breeder has released as a pet. The casual breeder also was cited in the quotes above on cattle raising.

Inbreeding is also deliberately induced in laboratory mice in order to guarantee a consistent and uniform animal model for experimental purposes.

Inbreeding in humans

The taboo of incest has been discussed by many social scientists. Anthropologists attest that it exists in most cultures. As inbreeding within the first generation often produces expression of recessive traits, the prohibition has been discussed as a possible functional response to the requirement of culling those born deformed, or with undesirable traits.[citation needed] The eugenicists used breeding techniques to promulgate their ideas of human perfection and "illness" on all humans.[citation needed] Some anthropologists like Charles Davenport advocated the traditional forms of assortative breeding to form "better" human stock.[citation needed] Geneticists and other medical professionals have studied the effects of inbreeding in humans, noting considerable medical consequences even in the first generation.[14]

Royalty and nobility

Charles II of Spain was physically and mentally disabled, in large part due to generations of inbreeding.[15]
Enlarge
Charles II of Spain was physically and mentally disabled, in large part due to generations of inbreeding.[15]

The royal and noble families of Europe have close blood ties which are strengthened by royal intermarriage; the most discussed instances of interbreeding relate to European monarchies. Examples abound in every royal family; in particular, the ruling dynasties of Spain and Portugal were in the past very inbred. Several Habsburgs, Bourbons and Wittelsbachs married aunts, uncles, nieces and nephews. Even in the British royal family, which is very moderate in comparison, there has scarcely been a monarch in 300 years who has not married a (near or distant) relative. Indeed, Queen Elizabeth II and her husband Prince Philip, Duke of Edinburgh are second cousins once removed, both being descended from King Christian IX of Denmark. They are also third cousins as great-great-grandchildren of Queen Victoria of the United Kingdom. European monarchies did avoid brother-sister marriages, though Jean V of Armagnac was an exception.

It is not necessarily the case that there was a greater amount of inbreeding within royalty than there is in the population as a whole: it may simply be 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.

There were at times serious long-term health and political consequences to multi-generational interbreeding in royal families between persons who were closely related.[16] Most notable was Charles II of Spain, who had multiple, severe disabilities largely linked to inbreeding.[17] Not only was he developmentally disabled and could not chew his food properly, he also could not produce children,[18] thus leading to the collapse of his bloodline and the War of the Spanish Succession.[19]

Other examples of royal family intermarriage include:

  • Some Egyptian Pharaohs and Peruvian Sapa Incas married their sisters; in both cases we find a special combination between endogamy and polygamy. Normally the son of the old ruler and the ruler's oldest (half-)sister became the new ruler.
  • The Inca had an unwritten rule that the new ruler must be a son of the Inca and his wife and sister. He then had to marry his sister (not half-sister), which ultimately led to the catastrophic Huascars reign, culminating in a civil war and then fall of the empire.
  • Cleopatra VII and Ptolemy XIII, married and named co-rulers of ancient Egypt following their father's death, were brother and sister. Not only this, but all rulers of the Ptolemaic dynasty from Ptolemy II on engaged in inbreeding among brothers and sisters, so as to keep the Ptolemaic blood "pure".
The family-tree of Charles II of Spain shows an extraordinary number of aunt-nephew and cousin unions of varying degrees
Enlarge
The family-tree of Charles II of Spain shows an extraordinary number of aunt-nephew and cousin unions of varying degrees

Intermarriage in European royal families is no longer practiced as often as in the past. This is likely due to changes in the importance of marriage as a method of forming political alliances through kinship ties between nobility, as well as an awareness of modern medical science. These ties were often sealed only upon the birth of progeny within the arranged marriage. Marriage was seen as a union of lines of nobility, not of a contract between individuals as it is seen today. More marry for "love", best illustrated by the second marriage of Prince Charles of the United Kingdom. During the tumult of the removal, sometimes by revolution, of most lines of nobility from state government, it became less important to marry for the good of the respective monarchies and the states they governed.

See also

References

  1. ^ Cheetahs
  2. ^ M Menotti-Raymond and S J O'Brien. "Dating the genetic bottleneck of the African cheetah." Proc Natl Acad Sci U S A. 1993 April 15; 90(8): 3172–3176.
  3. ^ [http://elibrary.unm.edu/sora/JFO/v051n02/p0168-p0173.pdf CHARLES F. LECK. "ESTABLISHMENT OF NEW POPULATION CENTERS WITH CHANGES IN MIGRATION PATTERNS." New Population Centers Vol. 51, No. 2]
  4. ^ http://www.dur.ac.uk/anthropology.journal/vol13/iss1/posters/freilich.pdf
  5. ^ http://services.oxfordjournals.org/cgi/tslogin?url=http://www.oxfordjournals.org%2Fjnls%2Flist%2Fjhered%2Ffreepdf%2F82-378.pdf
  6. ^ http://www.iupac.org/publications/pac/1998/pdf/7011x2079.pdf
  7. ^ http://services.oxfordjournals.org/cgi/tslogin?url=http://www.oxfordjournals.org%2Fjnls%2Flist%2Fjhered%2Ffreepdf%2F82-378.pdf
  8. ^ "ADVS 3910 Wild Horses Behavior," web page accessed June 22, 2007 at http://www.advs.usu.edu/academics/pdf/ADVS3910WildHorses.pdf
  9. ^ http://muextension.missouri.edu/explore/agguides/ansci/g02036.htm
  10. ^ http://www.nimss.umd.edu/homepages/home.cfm?trackID=2354
  11. ^ http://showcase.netins.net/web/royalair/libreeding.htm
  12. ^ http://www.petplace.com/cats/top-cat-breeds-for-2004/page1.aspx
  13. ^ http://www.bulldoginformation.com/breeding-quality.html
  14. ^ "Medical Consequences of Dating a Relative." Web page accessed September 23, 2007
  15. ^ http://www.historyworld.net/wrldhis/PlainTextHistories.asp?groupid=1761&HistoryID=ab50 History of Spain, Habsburg inbreeding and Spanish succession: 17th c. AD]
  16. ^ "Medical Consequences of Dating a Relative." Web page accessed September 23, 2007
  17. ^ http://www.hapsburg.com/menu5.htm "The Imperial House of Hapsburg: Chapter 5. Web page accessed September 23, 2007]
  18. ^ Id.
  19. ^ "English Monarchs-The House of Stuart-The War of the Spanish Succession" Web page accessed September 23, 2007
  20. ^ [http://www.msu.edu/course/lbs/333/fall/hapsburglip.html "The Hapsburg Lip." Topics in the History of Genetics and Molecular Biology, Fall 2000
  21. ^ http://www.hapsburg.com/menu5.htm "The Imperial House of Hapsburg: Chapter 5. Web page accessed September 23, 2007]

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
Translations: Translations for: Inbreeding

Dansk (Danish)
n. - indavl

Nederlands (Dutch)
inteelt

Français (French)
n. - consanguinité, croisement consanguin

Deutsch (German)
n. - Inzucht

Ελληνική (Greek)
n. - ενδογαμία

Italiano (Italian)
incrocio, matrimonio tra consanguinei

Português (Portuguese)
n. - procriação (f) consangüínea (Biol.)

Русский (Russian)
родственное спаривание

Español (Spanish)
n. - endogamia

Svenska (Swedish)
n. - inavel

中文（简体） (Chinese (Simplified))
同系繁殖, 近亲交配

中文（繁體） (Chinese (Traditional))
n. - 同系繁殖, 近親交配

한국어 (Korean)
n. - 동종번식, 근친결혼

日本語 (Japanese)
n. - 近親交配, 同系主義, 派閥人事

العربيه (Arabic)
‏(الاسم) الإستيلاد الداخلي‏

עברית (Hebrew)
n. - ‮זיווג בתוך המשפחה (הרחבה), הרבעה‬

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved.  Read more
Genetics Encyclopedia. Genetics. Copyright © 2003 by The Gale Group, Inc. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/  Read more
Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Inbreeding".  Read more
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