If we want to see if two people are cousins through the maternal line (ie. Mothers were sisters), but we don’t have DNA from their mother’s, the Mitochondrial DNA test is a powerful way to prove or disprove that relationship.
Nuclear DNA is found in the nucleus and contains all the genes that make up who we are. Nuclear DNA is the type of DNA that police use to profile a person, and what DNA labs use to perform a paternity test.
Sperm cells have mitochondria in the tail of the sperm, and not in the head of the sperm. When the sperm fertilizes a egg only the sperm head enters the egg, and the tail is dropped off. Thus, children will only receive mitochondrial DNA from their mother. Testing the sequence of mitochondrial DNA in siblings will show if they are biologically related via the maternal line.