Consanguineous Marriage

 by Bianka Anszczak

As perfectly summarised by Journalist Mona Chalabi, many people today, to use a scientific term, would feel ‘icky’ at the thought of marrying a close family member such as their first cousin. But, although it is taboo in some cultures, it is still accepted by many around the world.

While some may view consanguineous marriages as beneficial, the potential consequences can be tragic. But, despite this being common knowledge today, it continues to be common practice.

The differences between individuals relies on the recombination of genes during meiosis. Genes are the code that controls the body, responsible for characteristics such as eye colour, hair colour, height and so on. Many blood relatives share genes from the same gene pool, and this is where the problem lies. Any child, regardless of whether their parents are blood related, could inherit a genetic condition. However, the chance of both parents each carrying a defective recessive gene is increased when they share the same gene pool. These restrictions in the gene pool cause potentially dangerous or defective genes to be reused and recycled for new offspring. The whole point of intermingling our DNA, is so that we don’t reuse these defective genes.

If both parents carry the receive gene for a certain condition, they have a 1/4 risk of having an affected child, each time they conceive.  

Think of it this way: if X and Y share grandparents who had cystic fibrosis, it is much more likely that they have a carrier gene for cystic fibrosis. On the other end, let’s say X and Y do not share grandparents, but X’s grandparents had cystic fibrosis. Although X is likely to be a carrier of cystic fibrosis, by marrying outside of his family, the chance of Y also being a carrier, is dramatically decreased. In fact, in non-related couples, the risk is as Slim as 2-3%, therefore, decreasing the chance of the offspring inheriting two recessive genes for this disease.

Although it is certainly not as common in Europe and North America now, in the past it used to be not unknown. Some famous names include Albert Einstein, Queen Victoria and even Charles Darwin (who you would think should know better). For many, it was the preferred strategy to ensure power and wealth stayed within the family.

Researchers concluded that roughly from 1650 to 1850 the average family was a result of 4th cousin marriages (meaning they shared great great great grandparents). This was due to people living in such small communities causing them to share 0.2% of their DNA. Now this is a very small amount when comparing it to 3rd cousins, which is 0.78%, 2nd being 3.13% and 1st, a whopping 12.5%, increasing the chance of having offspring with a genetic disease.

 Of course, having children with someone unrelated does not entirely remove the risk of both parents having a dangerous gene. Nor are cousin marriages the sole cause of these different genetic disorders. However, they are responsible for the increased risk of developing these disorders. And so, by lowering these risks, cases of offspring with inherited genetic conditions could potentially be managed.