Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne First Cousins and Marriage
Name: James
Status: Other
Age: 30s
Location: N/A
Country: N/A
Date: August 2004

It's a genetics concern about my girlfriend's side of the family. When we first started to date she mentioned that her parents were first cousins. I was taken back a little, but her good nature makes up for a lot. She is perfectly fine meaning that she is not physically or mentally challenged. My parents are not related in any way. In the future, if we decide to have kids will the chances be greater for birth defects?

Although the closer animals mate in their heritage the more chances there are for recessive genes that are bad for development and health to occur, there are many thousands (probably millions) of people in the world who marry relations closer than we might think wise. I would expect that if a single generation had a close marriage ("in-breeding") followed by a generation of "out-breeding" there would be much less to worry about than successive generations of inbreeding. That said, there is a well-documented increased risk of still births and birth defects from "consanguineous marriages". Consanguineous marriages are quite common in India (uncle to niece usually) and some studies have been done on it...if you do a search on the issue look up "consanguineous marriages and birth defects" for example...

I will put in a word (or words should I say) for love (since the question did relate to a personal amorous relationship)..."amor vincit omnia" ancient Latin conquers all


First of all, let's review genetics a little. Remember that you need a copy of a trait from both parents-ie. you will give one copy and your girlfriend will give a copy. When these two are paired together, sometimes one dominates over the other, which means the trait will "show" even if only one copy is present. The trait that is hidden is called recessive. If each parent gives a copy of the recessive trait however, there is no trait to dominate over it and the recessive trait will show. This is why we say that sometimes traits "skip a generation" (not always true!) or there is a hidden trait in a family. With that said, let's say that you and your girlfriend were first cousins (like her parents). IF (and I say IF) there happened to be one of these recessive traits being carried in people in the family, with every generation there is a greater chance that two carriers could come together and each donate a recessive trait. In other words, there is a greater PROBABILITY that this trait will show up in a close family mating than in the general population. If there are no traits like these in the family however, the likelihood that first cousins will have problems is no greater than the general population. So it depends on a lot of factors. In your case, the fact that your girlfriend's parents are first cousins shouldn't affect you, because remember you also have to contribute a copy of the recessive trait. You should also be aware that certain disease have a high incidence of carriers in the general population, such as sickle cell trait in Africans, cystic fibrosis in Europeans, for example. Even if you aren't related, the chance of having a child with a genetic disorder is a possiblilty. The job of a genetic counselor is to take a careful family history and counsel couples on their risk based on their history.


No. The fact that your girlfriend's parents were first cousins just means that approximately 1/16th more of her gene pairs are homozygous than the average person but since apparently none of these genes are harmful, your children should be OK. Another thing it means is that, if you have children, 50% of their genes will come from your girlfriend's grandparents but only 25% of their genes will come from your father's parents (your paternal grandparents) and the other 25% from your mother's parents (your maternal grandparents).

Regards, Ron Baker, Ph.D.

Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory