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If Siblings Marry, Risk of Genetic Disorders



My best friend wants to marry his biological sister. What is the risk their children will have genetic disorder?


The closer the biological relationship between parents, the greater the chance that any children they have will inherit identical copies of one or more disease-causing recessive genes. Because we all carry some recessive genes that could cause genetic diseases – if we marry a close relative, there is a higher chance that a close relative will carry the same recessive genes. In this case, there would be a higher chance that this couple would have a child with a genetic disease. Your friend would need to know what medical or genetic problems are in both his mother’s and father’s sides of the family.

Couples who are related by blood have a higher chance of having children with genetic diseases or birth defects. It is estimated that couples who are siblings (1st degree relatives) have about a 7-12% risk above the population risk, which is estimated to be about 3-4% of newborns. Because your friend and his biological sister share one-half of their genes in common, they are 1st degree relatives so would have this higher chance to have a child with birth defects or a genetic disease. If there was a genetic disorder in their family, then the risk may be even higher than 7 -12 %.

I would highly recommend that your friend talk to a geneticist or genetic counselor. They would be able to discuss the chances of having a child with a genetic disease based on his family history. Your friend can ask his doctor for a referral to a genetics clinic or locate one through the National Society of Genetic Counselors’ Resource Center website below.

Related Resources:

Genetic Counseling Resources

For more information:

Go to the Inherited Disorders and Birth Defects health topic.