INBREEDING IN HUMANS
Inbreeding is simply defined as the mating of relatives. It is a mating system in which individuals carry alleles that originated from a common ancestor. Inbreeding is considered a problem in humans because inbreeding increases the chances of receiving a deleterious recessive allele inherited from a common ancestor. When discussing inbreeding, the level at which is taking place becomes important. Most studies are concerned with close inbreeding, also known as incest, which usually sets a threshold at the level of first-cousin mating (Thornhill 1993).
discussing inbreeding, one of the most important values to be concerned with is
the inbreeding coefficient. The
inbreeding coefficient represents the probability that an offspring will
receive a gene from each parent that is a copy of a single shared ancestral
gene. The inbreeding coefficient is zero
if the parents do not share a common ancestor, and if the inbreeding coefficient
is one than the offspring has a 100% chance of receiving two copies of the
ancestral gene. However, this maximum
inbreeding coefficient of one cannot be achieved in human populations (
In western civilization consanguineous
marriages and human inbreeding have been frowned upon by society for some time
now. In fact, statutes passed in the 19th
and early 20th centuries made inbreeding and marriages to the first
cousin level illegal in the majority of the
REASONS A POPULATION WOULD PRACTICE INBREEDING
concerns surrounding inbreeding and its effects, levels of inbreeding dropped
as low as 0.5% in the 1990s at the first cousin level in the general
Religion and culture can play a large role in the amount of inbreeding that takes place in a population. In many Muslim and Hindu societies in Africa, Asia, and India, consanguious marriages, especially unions of first cousins, account for twenty to fifty-five percent of the total. These religions tend to inbreed because of religious acceptance, preference, and tradition. Moreover, the culture of these societies also plays a large role into increased levels of inbreeding. Consanguineous marriages are thought to be an advantage when considering compatibility of the bride and her husband's family. This is particularly important when discussing the bride's relationship with her mother-in-law and the up-keep of the family's property. Another incentive to close relative marriages concerns bride wealth and dowry. Consanguineous marriages can lead to greatly reduced or no payments at all in unions of this culture. This allows small landowning families to keep their property and land (Bittles 1991).
that are associated with inbreeding because of religion and culture are the
small Anabaptist populations in
isolation and population size play a large role in many populations when concerning
levels of inbreeding and genetic barriers.
Migration rates can also play a large role in inbreeding levels. Furthermore, as the number of generations
since the isolated population was founded increases so should the inbreeding
levels. Several of these factors were
seen in studies in small communities and countries located and isolated in
European mountain ranges. One such
population study was performed in the country of Andora. Andora is on of the smallest countries in
Europe, and it is very isolated by being surrounded by very high mountains in
Inbreeding has also been seen to occur frequently in many royal families' histories. Royal incest was commonly found in Ancient Egyptian, Incan, Hawaiian, and many European royal families. Brother-sister unions become more frequent when royalty is the major factor concerning the incidence of inbreeding. There are several factors that can explain why royalty leads to high levels of inbreeding. One factor is that the king has limitless power in many cultures, and he can do what he wants and marry who he wants. Also, in many cases inbreeding is practiced in royal families to preserve royal blood lines. Another explanation is that a royal family can keep land, material possessions and resources within the family. Moreover, brother-sister royal incest allows succession of the throne to both a male and female blood line. There are also cases in which royal incest is part of a culture and is sometimes linked to legends or myths. One of the best documented cases of this was seen in the Incan culture in the 16th century. The Incan king was to marry his full sister. This was done to emulate the king's mythical ancestor, the Sun, who married his sister, the Moon, and this was thought to preserve the purity of the divine royal blood line (Van Den Berghe 1980).
Royalty also uses inbreeding to try to maximize fitness. One of the royal strategies to maximize fitness by using inbreeding to put as close to a genetic clone as possible on the throne as the heir. Moreover, females tend to maximize fitness by picking the best possible mate, which in this case would mean marrying to a higher social class. This leads to women with the highest-status in a population to being the most inbred in this type of society (Van Den Berghe 1980).
EFFECTS OF INBREEDING
The negative health effects caused by inbreeding are due to the expression of rare, recessive deleterious genes that are inherited from common ancestors or a single shared ancestor. Studies on population in which inbreeding is common have shown increased levels of mortality and morbidity due to a variety of genetic defects. However, inbreeding can also result in the production of perfectly healthy offspring (Bittles 1991).
Study of European Royal Families
Inbreeding was very common among the royal
families of Europe, and it has been linked as the cause of the widespread number
of cases of hemophilia in the families.
The presence of hemophilia in the royalty of
spreads rapidly through the British royal house for several generations, and
inbreeding in the family was seen as a major cause of the deleterious recessive
allele's frequently through the lines of British royalty. Moreover, the disease was also spread into
the royal houses of
was also spread into the Spanish royal family when Victoria Eugenie, a
hemophilia carrier and daughter of Beatrice, married Alfonso XIII, the king of
Study on Japanese Children after WWII
In the study it was seen that inbreeding did not have an adverse effect on the fertility of the marriages, but there were some significant increases seen on childhood mortality in the first year of life. Inbreeding also increased morbidity in the study. There were significant increases in levels of handicapped offspring associated with inbreeding. Inbreeding was associated with a 37.5% increase in offspring with one or two major handicaps, while it also caused a 24.1% increase in offspring with one or two minor defects. Moreover, inbreeding caused an increase of 31.7% in hearing impairments. Development also seemed to be affected by inbreeding. Children of consanguineous marriages were significantly older than the control group when they first walked and talked.
did not only show harmful effects of inbreeding in
Study on an Amish Settlement
Order Amish tend to have high levels of inbreeding, because they are a highly
traditional agricultural and religious group that is very isolated from outside
populations. These high levels of
inbreeding are seen in a settlement in Lancaster,
Study on the Hutterites
Hutterites are a small group of Anabaptists that fled
Little is known about the effects of inbreeding on offspring's fertility, so this study was done to answer some of these questions. There were several measurements used to detect any adverse changes in fertility. Inbreeding coefficients, measurements of birth intervals, and family size were all measured. The offspring of the Hutterite women showed that the intervals between births decreased as levels of inbreeding increased. These intervals not only increased as inbreeding levels increased, they also increased with each generation of Hutterite women. Therefore, the deleterious effects of inbreeding can be tracked through each generation. These significantly longer interbirth intervals were due to lower conception rates or higher losses in embryonic stages of pregnancy. Moreover, this also caused large declines in family size. In fact, average family size shrunk from above nine from 1901 to 1920 to five from 1941 to 1960. This showed a decrease in two family members per generation in the Hutterite population. This study has shown that deleterious recessive alleles received from inbreeding can lower the fertility rates of adult woman (Ober 1999).
Study on a Population in Dammam,
Dammam is the capital city of the oil-rich Eastern Province of Saudi Arabia. The population of Dammam is estimated to close to two-hundred thousand, and the rate of consanguineous marriages is around 52%, which is higher than the average rates of most countries and providences in the Middle East and the surrounding area. First-cousin marriages dominated all forms of marriage in Dammam accounting for 39.3% of the unions. The high rates of consanguineous marriages lead to a relatively high inbreeding coefficient of 0.0312. These high rates of inbreeding were thought to be due to beliefs, culture, and to keep property within the family.
A study was performed on the population from Dammam to see how inbreeding could affect certain aspects of offspring's health. The study first looked at "Reproductive wastage," which was defined as the number of stillbirths, childhood death's in the first month of life, and childhood deaths during the first year of life. This study showed no real significant differences in "reproductive wastage" between consanguineous and non-consanguineous marriages. Moreover, birth weights were also examined and no significant differences were seen between mean birth weights of children from consanguineous marriages and children from out breeding relationships. These results were somewhat different from other studies performed that showed inbreeding to have negative affects on offspring.
This study shows that inbreeding is not always harmful and can produce perfectly normal offspring. In fact, some investigators believe that long-term practice of inbreeding can actually benefit a population and its health by reducing deleterious or harmful genes (Al-Abdulkareen 1998). The reduction of these harmful genes is thought to be a result of an increased frequency of the deleterious gene's presence which can make it more vulnerable to selection. Therefore, selection could eliminate the harmful gene if it is given ample time to "act" on it (Hedrick 1991).
There are several reasons that a population would practice inbreeding that span from religion to geography to royal bloodlines. Many studies have shown that inbreeding can cause increases in mortality and morbidity. As populations become more knowledgeable to these possible effects levels of inbreeding tend to decrease. However, there are other populations that are less knowledgeable to the possible negative outcomes of inbreeding, and it is possible that the effects of inbreeding may not be detectable or visible. Therefore, if there are harmful recessive alleles present in the population, the genes and characteristics still have the possibility of surfacing and negatively affecting a population, but it is very possible that the population will never see any harmful effects due to incest. In fact, some experts believe that in some cases inbreeding can be helpful to a population by constantly exposing harmful recessive genes to selection. By frequently exposing these genes to selection, the harmful alleles can become permanently eliminated from the population. Inbreeding is a very touchy and controversial subject when it concerns humans, and there is still a lot that we do not know about the possible effects of inbreeding. It is very difficult to run experiments to determine all the possible effects of inbreeding in humans, because there are just too many variables to control. Moreover, ethics makes it difficult and many times impossible to perform studies on humans. However, most experts would agree that practicing outbreeding will provide a population with the best opportunity to achieve a high level of health.
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