Inbreeding in humans

from Wikipedia, the free encyclopedia

As inbreeding in humans is procreation relatively near kinsman referred to each other. Incest , on the other hand, is the sexual intercourse of relatively close blood relatives independent of reproduction. Marriage among relatives (regardless of sexual intercourse and procreation) is known as marriage of relatives . It is restricted by law in many countries.

Inbreeding in humans and hereditary diseases

Findings in human genetics and heredity suggest that inbreeding in humans increases the likelihood of hereditary diseases occurring . This could lead to the hypothesis that “genetic health” can be achieved through particularly “partner selection outside the family”.

More than half of the hereditary diseases known in humans are inherited recessively . This means that they do not appear as a characteristic due to the simultaneous presence of the healthy genetic makeup of the other parent . Therefore, without knowing it, many people have inherent traits for a hereditary disease, but are still healthy (people are then conductors) because they also have the healthy genetic information and this is dominant . Such recessive hereditary diseases can only break out in joint offspring if both parents carry the same genetic information for “sick” and both pass this identical genetic information on to the same child.

Theoretical risk of developing a recessive hereditary disease

In full-blown healthy siblings who are both carriers of the same recessive hereditary disease, where the risk that both siblings have the genetic defect of one parent is usually 25% for each genetic defect, the abnormal expression of the gene ( allele ) is found in girls in 50 % of their eggs and, in the case of boys, 50% of the sperm . When these siblings later father children together , the various possible combinations mean that 75% of their children will inherit the gene for the disease ( Mendelian rules ), with 25% receiving it from both parents ( homozygous ) and therefore falling ill. 50% have a healthy and a diseased gene ( heterozygous ), so they stay healthy. 25% have the healthy gene from both parents (homozygous healthy) and therefore do not pass on the disease disposition.

In contrast, the risk of fathering genetically ill children is considerably lower in couples who are not blood related, because the carriers of the same diseased gene come together much less often . The average risk of inheriting a hereditary disease is around 3 percent for partners who are not blood relatives, as the probability is very low that both of them carry the same diseased hereditary information . However, if both partners belong to a family in which the genetic make-up for a hereditary disease is present, the probability increases that a child will be transmitted by both parents with the hereditary disease.

The risk of an existing hereditary disease occurring for couples is:

  • 00000000n2–4% for partners who are not blood relatives (statistical average)
  • 01/64 =  01.56% for blood relatives of 2nd degree cousins
  • 01/16 =  06.25% for blood relatives cousin – cousin (1st degree)
  • 001/8 = 12.50% for blood relatives uncle - niece , aunt - nephew, grandparent - grandchild or half siblings
  • 001/4 = 25.00% for blood relatives father-daughter, mother-son or full brother-sister

This information corresponds to the so-called inbreeding coefficient , which is calculated from half the genetic relationship coefficient between two people (match of hereditary factors ). With regard to the inheritance of hereditary diseases, a distinction must also be made between dominant and recessive hereditary information.

The greatest risk for the occurrence of genetically determined diseases lies with the common descendants of blood relatives, who in turn are descended from each other or are siblings (between these two is a marriage ban ). With partners who are first cousins, the risk is much lower, but twice as high as with unrelated couples. From cousin – cousin 2nd degree (common ancestors are the great-grandparents ) the risk corresponds roughly to that of unrelated couples.

Irrespective of this, the heterozygous carriers of a recessive hereditary disease also pass on the gene for the disease to 50% of their children with an unrelated, homozygous healthy partner, who then also become heterozygous carriers of the genetic make-up ( conductors ).

Assessment of the real risk

Each person carries an average of six recessively inherited disease-causing alleles (state variants of a gene). Between full siblings or between parents and children, the probability of being carriers of the same disease-causing allele is 50%. This means that it can be assumed that the siblings or the parent and child heterozygously carry the same disease-causing allele for about three genes. The probability that each of them will cause a hereditary disease in a jointly conceived child is 25%, 75%, however, the child will not phenotypically embody the hereditary disease. It can therefore be calculated that with 0.75 (Gen 1) x 0.75 (Gen 2) x 0.75 (Gen 3) = 0.421875 , i.e. with a probability of around 42%, a child in such a relationship is not a phenotypic one Carrier of any hereditary disease, with an equally high probability of becoming a carrier of one, about 14% probability of two and about 2% probability of homozygous carrier of all three hereditary diseases. However, such values ​​can only be used as an approximate orientation and indicate the most probable probabilities , since there is a certain probability that individual persons can also be carriers of significantly more or fewer hereditary diseases. Personal risk assessments are therefore only possible after sequencing both potential parents. It is only through this that it is possible to assess the severity and type of illness at all.

With decreasing degrees of kinship, the predictable realistic probability of having a child with one or more hereditary diseases phenotypically decreases: if there is a connection between half-siblings, uncle and aunt or grandparents grandchildren, around 66% of children will not have a phenotypic genetic disease, if there is a connection between cousin and 1st degree cousin 81%, between aunt / uncle and nephew / non 2nd degree 90%, between cousin and 2nd degree 95%, etc., thus tending towards the probability of the average population. It should be noted that there are other causes of disabilities than just genetic inheritance, so at some point the likelihood of having a disabled child through a lack of oxygen during childbirth outweighs, while a hereditary disease does not necessarily mean a disability, for example through a corresponding one Diet prevented the consequences of one of the most common hereditary diseases in Central Europe, phenylketonuria . In addition, the probabilities for the occurrence of hereditary diseases are high insofar as, for example, a combination of several very serious hereditary diseases does not lead to the birth of a child in the majority of cases, since the consequences for the growing organism may already be so great in the womb. that there is a miscarriage - often unnoticed at an early stage .

Human genetic counseling

In the countries concerned, genetic counseling centers also point out that children of blood-related couples have a greater risk of a hereditary disease or disability than children of unrelated couples (see also hereditary disease risks ).

Proportion of related marriages including 2nd cousins ​​( US National Center for Biotechnology Information 2012).

In the discovery of hereditary diseases such as hemophilia , family tree analysis played an important role, since hereditary diseases were very common in family trees with related marriages . A family tree analysis is also helpful in providing genetic advice for those affected or their relatives. In order to be able to assess the risk of hereditary diseases more precisely, lists of ancestors are drawn up from maternal and paternal biological lineages .

Possible positive effects

Data from the study of the genetic and social ramifications of Iceland's population carried out by the pharmaceutical company DeCODE Genetics shows that women in the early 19th century who were married to a third or fourth cousin were clearly at the forefront of the mothers with the largest number of children: for example, women had who were married to a third cousin had a computational average of 4.04 children and 9.17 grandchildren, while women who were married to an eighth (or more distant) cousin had only 3.34 children and 7.31 grandchildren had. If these data can be corroborated, this could mean that there could be an “optimal family distance” for the number of children and, consequently, for the number of grandchildren.

See also


Web links

Individual evidence

  1. a b Jan Murken et al.: Inbreeding and kinship coefficient in different kinship relationships. In: Human Genetics. 7th, completely revised edition. Georg Thieme Verlag, 2006, ISBN 978-3-13-139297-8 , p. 252: Table (there also the exact formulas; side view in the Google book search).
  2. a b Hansjakob Müller u. a .: Medical genetics - family planning and genetics . In: Swiss Medicine Forum . Volume 5, No. 24 . Swiss Academy of Medical Sciences , Basel 2005, p. 639–641, here p. 640 ( (PDF; 123 kB, 3 pages) Archived via WebCite® ( Memento from January 4, 2015 on WebCite )). “Outside of Central Europe, marriages among relatives are relatively widespread, with around 20% of the world population they are even the preferred form of marriage.” Table 2: Genetic risks in related marriages : “First degree relatives (father-daughter, brother-sister ): 50% | 1st cousin – cousin: 6% | 2nd cousin – cousin: 4% […] Studies have shown that the common offspring of relatives carry higher genetic risks than those of non-relatives. In the case of first-degree cousins, the risk of physical and mental disabilities is twice as great as the risk in the normal population. [...] The severe degenerative nervous disease Tay-Sachs occurs more frequently in the Ashkenazim Jewish population than elsewhere. The risk of this disease with autosomal recessive inheritance is correspondingly high in couples of this origin. "
  3. a b Janine Flocke: Migrants: Relatives, engaged, married! In: Zeit Online. March 27, 2007, accessed August 18, 2019 . Quote: “Because the risk of malformation is often higher than expected, even among cousins. This is especially the case if the couple's ancestors were blood relatives. "Some families have only married each other for generations," says [Yasemin] Yadigaroglu. The Berlin gynecologist and expert in prenatal diagnostics Rolf Becker found that around 8 percent of the children of treated migrants were mentally or physically disabled. ”Note: an average of 2–4 percent for parents who are not blood relatives.
  5. H. Hamamy: consanguineous marriages: Preconception consultation in primary healthcare settings. In: Journal of community genetics. Volume 3, number 3, July 2012, pp. 185-192, doi: 10.1007 / s12687-011-0072-y , PMID 22109912 , PMC 3419292 (free full text).
  6. Agnar Helgason et al. a .: An Association Between the Kinship and Fertility of Human Couples . In: Science . tape 319 , no. 5864 , February 2008, p. 813-816 , doi : 10.1126 / science.1150232 , PMID 18258915 (English).
  7. ^ Dpa announcement: Science: marriage between cousins ​​very fruitful. In: Handelsblatt . February 7, 2008, accessed December 14, 2013 .
  8. Audrey Grayson: Iceland's "Kissing Cousins" Breed More Kids. In: ABC News . February 8, 2008, accessed on December 14, 2013 (English): “[…] Dr. Kari Stefansson, senior author of the paper on the study […] "But in spite of the fact that bringing together two alleles of a recessive trait may be bad, there is clearly some biological wisdom in the union of relatively closely related people." "