Inbreeding is the production of offspring by the mating of closely related individuals. This practice provides a greater chance for recessive genes to be expressed phenotypically. In humans, the amount of inbreeding in a specific population is largely controlled by traditional and cultural practices.
In Europe and North America, the marriage between close relatives is strongly discouraged by social convention. However, this is not true in many parts of the world. Marriage between close relatives is common in areas of the world in which geographical or ethnic isolation make marriage to unrelated people difficult.
The majority of serious genetic disorders are recessive, which means that an individual must inherit two copies of the abnormal gene (one from each parent) for the disorder to be expressed.
It is much more likely that both parents will carry the same recessive gene if the parents are related. The risk of a serious disease or malformation in a child of such a union is about 1 in 20. However, among married first cousins, the risk increases to about 1 in 11. If the couple are first-degree relatives, the risk is 1 in 2.
Cultural taboos are only loosely related to risk. The marriage of double-first cousins is just as risky (from a genetic standpoint) as it is in the marriage of a half-sister and a half-brother. However, in most places, the former is legal and the latter is not.
In cultures without significant inbreeding, the distribution of recessive disease genes will no longer be random and the genes will be concentrated in the more inbred families. An unexpected observation has been that non-consanguineous marriages in such cultures actually have a lower rate of genetic disorders than couples in cultures where close inbreeding is not socially acceptable.
For example, non-consanguineous Pakistani couples in Britain have fewer disabled children than their British counterparts. At first glance, it might seem that the line of inheritance on an inbred family with a serious disease gene might simply die out. However, there is some evidence to suggest that closely related couples are, in fact, more fertile than unrelated couples. Therefore, although any individual child is less likely to be healthy, more children are produced, and the net result is roughly the same. In such cultures, some social advantages of marrying close relatives, in terms of maintaining family wealth and relations between groups, may outweigh biological disadvantages.
Consanguinity and Birth Defects
Some birth defects can be inherited, meaning that the hereditary information is passed from parent to child by genes located on the chromosomes found in cells.
Normal body cells have 46 chromosomes, except for the reproductive cells (the sperm and egg) which have only 23 chromosomes each. At conception, each parent normally contributes 23 gene-carrying chromosomes to the hereditary make-up of the child.
In some cases, one affected parent has a faulty gene which dominates its normal counterpart, and each child has a 50 percent chance of inheriting the faulty gene and the disorder. This is known as dominant inheritance. Huntington's disease and Marfan syndrome are examples of genetic conditions that are transmitted this way.
In other genetic disorders, both parents (apparently normal) carry the same abnormal gene. Each child that is conceived has:
- a 25 percent risk of receiving that gene from both parents (in which case, the particular birth defect may occur)
- a 25 percent chance of inheriting the two normal genes and remaining unaffected
- a 50 percent chance of inheriting one faulty and one normal gene (becoming a carrier without the disorder, just like the parents). This type of inheritance is called recessive inheritance.
Cystic fibrosis and Tay-Sachs disease are transmitted as recessive disorders. Therefore, the chance of inheriting a recessive disorder is increased in a child whose parents are "blood" relatives (consanguineous).