Why marrying a close relative is bad for your children

Across different religions and cultures, the sanctioned union of marriage has withstood the test of time. Though the reasons may differ ranging from bringing powerful families together, building a legacy, maintaining generational wealth, forging a life-long partnership, or even just avoiding societal pressures many families have been built on this foundation.

Since the beginning of human history, marriages between close relatives have been commonplace throughout the world. This practice is known as inbreeding or consanguinity. These pairings of people with similar ancestries have been a source of major public and scientific interest from the mid-nineteenth century onwards and have contributed to our understanding of the medical genetics of both common and non-communicable diseases.

Research has shown that over the centuries, consanguineous marriages have not been confined to religious isolates or socioeconomic status. Though culturally acceptable in some regions, genetically, offspring of consanguineous unions may be at a higher risk of expressing autosomal recessive disorders.

Blood relatives share a higher proportion of their genetics. Siblings as well as children and their parents share about 50 per cent of the same DNA. Grandchildren share about 25 per cent DNA with their grandparents while first cousins share 12.5 per cent of their DNA. The problem arises when there is an unusual gene at play in the bloodline. The risk of genetic disorders among offspring of consanguineous union is 4-6 per cent while the offspring of unrelated couples is 2-3 per cent.

If both parents have this unusual gene, often referred to as a mutated gene, there is a one in four probability of having a healthy child, a two in four chance of having a child with one mutated copy, and a one in four risk of having a child with two mutated copies.

Hereditary mutation can either be dominant or recessive. When an anomaly is passed down by autosomal dominant inheritance, it typically manifests itself in every generation and only one copy must be inherited for the disease to be expressed, as in the instance of breast cancer linked to the BRACA 1 and BRACA 2 genes. In Kenya, breast cancer is the most often diagnosed cancer, with 6,000 new cases and 2,500 cancer-related deaths per year.

In the case of autosomal recessive inheritance, two copies of the defective gene must be present for the trait to be expressed. Where only one of the required two genes is inherited, then the person is said to be a carrier of the disease, even though they typically don’t display the disease. Sickle cell anemia is an example of a disease inherited using the recessive pattern, which is the most common hemoglobinopathy encountered in Kenya, with an estimated almost 4,000 children in the country born with this disease.

Consanguineous unions have the effect of increasing defects including cognitive difficulties, heart defects, impaired hearing, as well as other genetically inherited diseases. The highest rates of consanguineous marriage are reported from the Middle East, North Africa, and Central Asia where second cousins or closer relatives account for more than fifty per cent of all marriages.

In North African nations like Tunisia, where 40–49 per cent of marriages are consanguineous, and Morocco, where 29–33 per cent are consanguineous unions, diseases including thalassemia, sickle cell disease, and familial Mediterranean fever are frequently exhibited.

It is recommended that couples seek genetic counseling before marriage, not just for consanguineous couples but also for any couples who may have a family history of genetic abnormalities as a preventative precaution for inheritable diseases.