A woman seeks to know the risks of dating—and the risk of having offspring with—a relative.
Dear Professor Gates:
I want to date a fourth cousin of mine. Is that wrong? I’m curious to know how related we actually are. We do not plan to have any children together, but if we did, would there be a risk of passing along a genetic defect or disorder? —Sonya
Your question is a surprisingly common one received by genealogy researchers, not only for dating but also for marriage. Generally, questions arise when the couple in question are second cousins (sharing a great-grandparent) or closer.
An estimated 1 billion people worldwide live in communities where marriage between people who are second cousins or closer (known as consanguineous marriages) are preferred by tradition. However, in the United States only 0.2 percent of U.S. marriages are between second cousins or closer, according to FiveThirtyEight, and your question reflects social taboos and legal restrictions against the practice here. For instance, as of 2012, first-cousin marriages were illegal in 25 American states and restricted in six others (e.g., allowable among people who are both over the age of 50, and presumably unable to reproduce).
Occasionally on my genealogy TV series on PBS, Finding Your Roots, guests have found unexpected cousin marriages in their family trees. Married actors Kevin Bacon and Kyra Sedgwick found out that they are ninth cousins once removed, and action star Michelle Rodriguez learned that her Puerto Rican forebears preferred to marry first cousins in order to preserve a high degree of European ancestry (and lighter complexions) in their offspring.
Cousin marriage figured into the life of Henrietta Lacks, the black woman whose cancer cells were taken in 1951 without her knowledge and later used in the development of many medical breakthroughs. Lacks, whose life will be depicted this month in an HBO film starring Oprah Winfrey, had a child at age 14 by her first cousin David “Day” Lacks, whom she later married.
As for your situation, we asked genetic counselor and DNA consultant Brianne E. Kirkpatrick to weigh in. She told us the following in an email:
First cousins share a small amount of DNA inherited from a set of grandparents that they have in common. It is possible the shared DNA might happen to contain the same marker for a genetic condition, thus cousin couples can have a higher chance of pregnancy loss or a child born with special health needs.
The baseline chance for a birth defect or other special health needs in any child is around 2-3 percent. Having parents who are first cousins bumps that risk to 4-6 percent. Second and third cousins have progressively lower risks for their offspring compared to first cousins, and by fourth cousins (sharing great-great-great-grandparents), the chance does not differ much from that of the general population.
Partners who know they are related to one another as second cousins (or more closely) might consider meeting with a genetic counselor if conception of a pregnancy is possible. A genetic counselor would never tell a couple whether to have children or force them to be tested, but instead would provide information and support for decision-making. No matter what choice a couple ultimately makes, many people find it helpful to ask their questions in the open, supportive setting of a genetic counseling session.
I would advise those who are interested in more information on this topic to visit CousinCouples.com. You can also find information about genetic counseling in the resources section of my site, WatershedDNA.com or search for a genetic counselor near you at AboutGeneticCounselors.com.
Based on all of this information, it sounds as if you are in the clear to date your fourth cousin. Good luck!
Henry Louis Gates Jr. is the Alphonse Fletcher University Professor and founding director of the Hutchins Center for African and African American Research at Harvard University. He is also chairman of The Root. Follow him on Twitter and Facebook.
Send your questions about tracing your own roots to TracingYourRoots@theroot.com.