Many individuals wonder about the implications of having children with a third cousin, particularly regarding genetic considerations. While such unions are generally accepted, understanding the nuances of genetic relatedness is helpful for any couple considering starting a family.
Defining Third Cousins
A third cousin is a relative who shares a common set of great-great-grandparents. Your third cousin is, for instance, the child of your parent’s second cousin. This genealogical distance means the amount of shared genetic material, or DNA, is significantly reduced compared to closer relatives.
On average, third cousins share approximately 74 to 79 centimorgans (cM) of DNA, although this can range from 0 to 234 cM. Third cousins typically share about 0.78% of their genes. Due to the random nature of genetic inheritance, it is possible for some third cousins to share no detectable DNA, occurring in about 10% of cases.
Genetic Considerations for Offspring
The primary genetic concern in unions between related individuals, known as consanguineous marriages, revolves around recessive genetic disorders. These disorders occur when an individual inherits two copies of a mutated gene, one from each parent. Both parents must be carriers of the same recessive mutation for their child to be at risk. Related individuals have an increased probability that both parents carry the same harmful recessive gene due to their shared ancestry.
If both parents are carriers of the same recessive mutation, each child has a 25% chance of inheriting both mutated alleles and developing the disorder. The risk of birth defects in the general population is approximately 3-4%. For comparison, children of first-cousin marriages face an estimated 4-6% risk of autosomal recessive genetic disorders, or a 1.7-2.8% increased risk of congenital defects above the general population. Given that third cousins share significantly less DNA than first cousins, the genetic risk for offspring from third-cousin unions is generally considered low and only slightly elevated above that of the general population.
Navigating Legal and Health Questions
From a legal standpoint, marrying a third cousin is permissible in most jurisdictions. While laws regarding marriage between closer relatives like first cousins vary by state, unions between third cousins are almost universally legal across the United States.
For any couple with shared ancestry considering having children, genetic counseling offers valuable insights. It involves assessing potential risks and may include carrier screening to determine if both partners carry mutations for specific recessive genetic disorders. This process provides personalized information, clarifies potential risks, and helps couples make informed reproductive decisions.