In Genetic MythBusters Part 1, I discussed the idea of genetic conditions skipping a generation. If you read it, you know that nothing actually does skip a generation. Another common myth is that there is a really high risk for birth defects when people who are related to each other have children together. In actuality, the risk is increased, but not perhaps not as high as you think. I'm not advocating that first cousins have children together, and if you've grown up with your cousins, you probably wouldn't be so excited about that anyway!But the fact of the matter is that the only time it is dangerous to have a child with someone related to you is when you each carry the same disease-causing recessive genetic alteration. This is not something you would likely know ahead of time. Let me explain!
We have two copies of each of our genes, one from our mothers and one from our fathers. Our egg or sperm cells contain one copy of each gene. Therefore, when we have children, we pass on one copy or the other, not both. You may have heard of dominant and recessive inheritance.
Dominant means that only one of the copies needs to be altered in order for the person to have the disease or condition related to that gene. So a person affected by a dominant condition will have inherited it from mom or dad, and will have a 50% chance with each pregnancy to pass it on to his or her child. A recessive condition is one in which both copies of the gene need to be altered for the disease or condition to be expressed. In this case, both parents have to be carriers.
Put another way … let's go back to high school biology for a few minutes …the example almost every teacher uses to explain inheritance patterns is eye color. Brown eyes are dominant over blue or green. This is why two brown-eyed parents can have a blue-eyed child. Since blue eyes are recessive, the child has two copies of the "blue-eye gene" and no copies of the "brown-eye gene." Two blue-eyed parents will have only blue-eyed children, because both parents only have "blue-eye" genes to pass along.
The risk for an average, unrelated couple is approximately 3 to 5%. The closer the relationship between the two people, the higher the likelihood climbs. First cousins have approximately 1/8 of their genes in common, and that makes the risk for having a child with special needs more like 5 to 8%. So you can see that it's increased, but it still leaves a greater than 90% probability of having a perfectly healthy child.Janice Berliner, 2019
How does this relate to today's topic? Keep reading, I'm getting to it! There are thousands of different recessive disorders, and we all have several abnormal recessive genes that we likely will never know we have.Most recessive disorders are quite rare because the only time they crop up is when a carrier of a recessive disorder has a child with another carrier, and each passes on that gene alteration in the same pregnancy. This means that the fetus/child does not have a normal copy of this gene, and will be affected by the disorder.
So you can see why being a carrier is generally not a problem. In fact, it can actually be an advantage. For example: a carrier of sickle cell disease (someone who has one normal copy of the gene and one altered copy) is unaffected by this serious blood disorder. This person is said to have sickle cell trait. A person with sickle cell trait is more likely to be resistant to malaria. This is why sickle cell disease occurs more frequently in areas where malaria is endemic, such as Africa or the Mediterranean. It is a survival advantage to have sickle cell trait, and the more people in the population who have sickle cell trait, the more who will end up with sickle cell disease, having inherited it from both parents.
The same is true for people who are close relatives and have children together. They are simply more likely to carry the same disease-causing gene alteration, having inherited it from a common ancestor. How closely related they are will dictate the risk for having a child with birth defects or special needs. The risk for an average, unrelated couple is approximately 3 to 5%. The closer the relationship between the two people, the higher the likelihood climbs. First cousins have approximately 1/8 of their genes in common, and that makes the risk for having a child with special needs more like 5 to 8%. So you can see that it's increased, but it still leaves a greater than 90% probability of having a perfectly healthy child. There are never guarantees for having a healthy child, and of course most children with special needs are born to parents who are not related to each other. By the time you get to second cousins, the degree of relationship is not much higher than that of unrelated individuals. Therefore, the probability of having an affected child may be slightly higher than the average couple, but not necessarily high enough to warrant prohibiting the relationship or needing to do any special carrier screening or prenatal testing, beyond what might have been recommended anyway.
The bottom line: If you're second cousins or more remote, go for it. If you're closer relatives than that, please contact a genetic counselor for information and guidance. To find a genetic counselor in your area, go to NSGC.org and click on "find a counselor." And for further reading, try: https://www.nytimes.com/2002/04/04/us/few-risks-seen-to-the-children-of-1st-cousins.html