Are babies born to two cousins really at high risk of birth defects? The following basic genetic principles will help us answer this question.
What do Queen Victoria, Charles Darwin and Albert Einstein have in common? They all married cousins in the first generation.
Currently, it is illegal to marry cousins in many countries around the world. However, for a long time in Western history, everyone had to marry anyone who lived in their area, which meant they had to marry people of their lineage.
Between 1650 and 1850, couples were often fourth generation cousins. So they have the same great-grandparents.
Genetically, this means they have a share of 0.20% of their DNA. Not too much when compared to the third generation (0.78%), the second (3.13%) and especially the first generation (12.5%). And the more DNA they share, the more likely their child is to have a genetic condition, such as cystic fibrosis or sickle cell anemia.
But the important thing here is: your child can still have a genetic disease even if you don't have one.
Albert Einstein married his cousin.
Take cystic fibrosis as an example. It is caused by a defect in the CFTR gene. But you need two copies of this gene to really get sick. So, if there is only one copy of the gene, you will still be "unharmed" . However, you will be seen as a "carrier" for the next generation.
Now, when a "carrier" (that is, carrying a copy of a gene) mates with a normal person, the baby will still not get sick. However, when both parents carry a defective copy of CFTR, the child will have a 25% chance of inheriting two copies of the gene and getting sick.
Therefore, to assess how dangerous it is to marry a first-generation cousin, we need to calculate the likelihood of both carrying a copy of the same genetic disease.
Because they have a grandparents, we will start here. Now, the story becomes "if only" game. What if both of them were "carriers" or just one? What if one of their children is a "carrier" or nobody? And if those children were to marry other "carriers" or just ordinary people? Everything will become very complicated.
But scientists have found that the average number and risk of cousins giving birth to a child with a genetic disease is 4-7%, for the general population of 3-4%.
So, things don't seem so terrible as imagined? The big problem is this: It's just an odds for a genetic disease, but there are thousands of other genetic diseases that may be hiding in your family tree.
In addition, if your children are married to first-generation cousins and their children repeat the same thing, it is the formula of a true disaster. Because instead of introducing new, potentially useful genes into your family's gene pool, you're constantly using old and potentially dangerous genes.
So it's best not to turn it into a family tradition.