Close blood relative marriage and genetic risk

Families from all communities can be affected by genetic disorders.

However, some types of genetic disorders – known as recessive disorders - are more common among families where married (reproductive) couples are blood relatives, for example cousins.

Everyone carries at least one changed recessive gene that can be harmful and could cause a disorder, so there is a risk of recessive genetic disorders in all communities.

If partners are not related to each other the chance of them carrying the same changed (faulty) genes is very low because these changed genes are rare.  Therefore, the chance of a baby born to unrelated couples inheriting two copies of the same changed gene is very low.  So, most recessive genetic conditions are rare in populations that marry outside the family.

However, if partners are close blood relatives – such as cousins - then they share a large part of their genetic make-up and there is a higher chance that they we will share the same changed gene, and be a ‘carrier couple’. For carrier couples there is a chance that their children will inherit two copies of the same gene (receiving one each from both the mother and the father) and so be affected by a recessive genetic disorder. So, in populations where many people marry close relatives recessive genetic disorders are found in higher numbers.

Of the UK communities that have cousin couples, the Pakistani origin community is the largest with the highest level of cousin marriage.  Therefore, there are more ‘carrier couples’ among the Pakistani population and a higher chance of babies receiving two copies of the same changed gene than among other populations where close relative marriage is less common.

It is also important to note that in populations where there is a long history of marrying within the family (known as endogamy) that individuals who do not recognise themselves as closely related may nevertheless share a higher proportion of their genetic material than couples in a population who tend to marry outside the family (exogamy). 

Definitions:

Consanguinity = relationship by blood; relationship by descent from a common ancestor

Consanguineous marriage = marriage between individuals related by blood