What Is 3rd cousin

Overview

A 3rd cousin is a relative with whom you share a set of great-great-grandparents. This relationship is defined by tracing lineage back four generations to a common ancestral pair.

Understanding cousin relationships helps clarify family connections, especially in genealogical research and DNA testing. The term '3rd cousin' specifically refers to how many generations removed you are from shared ancestors.

• Common ancestors: You and your 3rd cousin share a pair of great-great-grandparents, who are two generations above your grandparents.
• Generational path: The path to your 3rd cousin goes back four generations from you, then down four generations to them, forming a 'cousin' link.
• DNA sharing: On average, 3rd cousins share approximately 0.78% of their autosomal DNA, though this can vary due to genetic recombination.
• Probability of match: About 90% of 3rd cousins will show a detectable DNA match in commercial genetic testing services like AncestryDNA or 23andMe.
• Family size impact: In large families, individuals may have dozens of 3rd cousins, especially if ancestors had many children over multiple generations.

How It Works

Understanding how cousin relationships are calculated involves tracing lineage through shared ancestors and counting generational steps. The 'degree' of cousinship depends on how far back the most recent common ancestor is located.

• Term: Great-great-grandparent: This is the shared ancestor for 3rd cousins. They are four generations back from you and your cousin.
• Term: Generational gap: If you and a relative are both four generations removed from a shared ancestor, you are 3rd cousins to each other.
• Term: DNA inheritance: Each generation inherits about 50% of DNA from each parent, so by the 3rd cousin level, shared DNA is minimal but often detectable.
• Term: Removal: If one person is a generation apart (e.g., 3rd cousin once removed), the relationship includes a one-generation difference in lineage.
• Term: Pedigree collapse: In some families, intermarriage can cause the same ancestor to appear multiple times in a family tree, increasing shared DNA.
• Term: Genealogical records: Census data, birth certificates, and online databases like FamilySearch help confirm 3rd cousin relationships when DNA is inconclusive.

Comparison at a Glance

Here’s how 3rd cousins compare to other familial relationships in terms of shared DNA and generational distance:

The table shows a clear decline in shared DNA as cousinship increases. By the 3rd cousin level, shared DNA is small but still significant enough for genetic testing to identify matches with high accuracy. This makes 3rd cousins valuable in building family trees and confirming ancestral lines.

Why It Matters

Knowing who your 3rd cousins are can have practical and emotional significance, especially in the age of genetic genealogy. Identifying distant relatives helps reconstruct family history and uncover ancestral origins.

• Genetic genealogy: 3rd cousins are often the most distant relatives who consistently appear in DNA match lists, aiding in building accurate family trees.
• Medical history: Sharing health information with 3rd cousins can help identify hereditary conditions like breast cancer or heart disease.
• Adoption searches: Adopted individuals often use DNA matches with 3rd cousins to trace biological families and identify birth parents.
• Immigration research: 3rd cousins may hold records or stories about ancestral migration from Europe, Asia, or Africa.
• Legal inheritance: In rare cases, 3rd cousins may be considered heirs if no closer relatives exist, depending on state laws.
• Cultural identity: Connecting with 3rd cousins can strengthen ties to ethnic traditions, languages, and religious practices.

As DNA testing becomes more common, identifying and connecting with 3rd cousins offers a powerful way to explore personal heritage and expand understanding of family networks.