Autosomal DNA

Autosomal DNA (atDNA) is the DNA inherited from both parents, shuffled and recombined with each generation. Unlike Y-DNA (paternal line only) or mitochondrial DNA (maternal line only), autosomal DNA carries genetic material from all branches of a person’s family tree.

In genetic genealogy, autosomal DNA tests — offered by companies such as AncestryDNA, 23andMe, and MyHeritage — are used to identify relatives across all family lines. Shared segments of autosomal DNA indicate a common ancestor. As a rule of thumb, first cousins share about 12.5% of their DNA; second cousins share about 3.125%.

The usefulness of autosomal DNA for identifying relatives decreases beyond about five to six generations, as the amount of shared DNA becomes statistically insignificant.