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Inherited Disorders and Birth Defects

Cousin Marriage & Genetic Problem



I`m (Male) married to my cousin (Daughter of my mother`s identical twin sister). Our fathers are not relative to each other and there was no cousin marriage in our recent incestors. Case: One of my cousin (niece of my father) was married to her cousin (her mother`s sister`s son) and they faced genetic disorder in their children. In the illness her sons are suffering, the boy will be all right until he is 7-8 yrs old but then he will become kind of mentally retarted, unable to properly walk and talk. Doctors say it`s a genetic problem: his mind is not growing the same speed as his body. Would you please suggest me what is there we can do to prevent any such problem in our kids? We don`t have kids but planning to have in future? Thanks & Best Regards


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 first cousins (3rd degree relatives) have about a 2-3% above the population risk, which is estimated to be about 3-4% of newborns.

If your mother’s sister is her identical twin, then your cousin (your wife) shares the same amount of genes as if she were your half sister. In this case, you would be a second degree relative to your wife and have about 1 / 4 of your genes in common. 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. You would need to know what medical or genetic problems are in your mother’s side of the family.

In your father’s family, it would depend on what the genetic disease is that your father’s niece’s son has that is causing the mental retardation and other problems. If your niece’s father is your father’s brother – it could be that the mutation (change) in the gene that has caused the disease is carried on the X chromosome and is coming from your brother’s wife’s side of the family. If the disease is caused by a recessive gene, then there is a chance that your brother is a carrier for it, which means there is a chance that your father could be a carrier and you could be a carrier of the gene as well.

The family relationships you are asking about are very complicated. I would highly recommend that you talk to a geneticist or genetic counselor. They would be able to discuss your chances to have a child with a genetic disease based on your family history. They would also be able to talk to you about what options you have about preventing these problems. You can ask your doctor for a referral to a genetics clinic near you or you can locate one through the National Society of Genetic Counselors’ Resource Center website below.

Related Resources:

Find a Genetic Counselor - National Society of Genetic Counselors

For more information:

Go to the Inherited Disorders and Birth Defects health topic, where you can:

Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University