How do blood-related genetic tests identify family relationships?

Modern relationship testing typically examines short tandem repeats, or STRs, which are repeating DNA regions that vary in length from person to person. By comparing many STR markers, laboratories can calculate how likely it is that two people are biologically related. A child inherits half of their DNA from each parent, so the child’s marker pattern should match the alleged parent across many sites. These tests became widely used in the 1990s and are now common in legal and family settings.