In short, yes, it is legal for second and third cousins to marry in the US. The risk of birth defects in children born to first cousins is increased from a baseline of 3-4 percent to 4-7 percent, according to the National Society of Genetic Counselors.
First cousins share a grandparent (2 generations) Second cousins share a great-grandparent (3 generations) Third cousins share a great-great-grandparent (4 generations) Fourth cousins share a 3rd-great grandparent (5 generations)
Can 3rd cousins share no DNA?
According to Family Tree DNAs figures there is a 90% chance that third cousins will share enough DNA for the relationship to be detected, but there is only a 50% chance that you will share enough DNA with a fourth cousin for the relationship to be identified.
What happens if you marry your cousin?
They say there is no biological reason to discourage cousins from marrying. First cousins are somewhat more likely than unrelated parents to have a child with a serious birth defect, mental retardation or genetic disease, but their increased risk is nowhere near as large as most people think, the scientists said.
How much DNA do you share with 3rd cousins?
Percent DNA Shared by RelationshipRelationshipAverage % DNA SharedRange2nd Cousin3.13%2.85% - 5.04%2nd Cousin once removed1.5%0.57% - 2.54%3rd Cousin0.78%0.3% - 2.0%4th Cousin0.20%0.07% - 0.5%7 more rows