All modern humans descend in the male line from a particular man, nicknamed "Genetic Adam", who lived about 60,000 years (~3,000 generations) ago. All living men have inherited his y chromosome (yDNA), along with the mutations that have accumulated in their individual family lines. Geneticists discovered this information by comparing mutations in the y chromosomes of modern men. Unlike autosomnal DNA, yDNA does not recombine. It passes intact from a man to his sons. (Women do not have y chromosomes.) The mutation rate for yDNA is very low, so yDNA changes very slowly over many generations. These characteristics make it useful to both geneticists and genealogists. Because mutations are relatively rare, every man whose y chromosome contains a particular mutation probably shares a common paternal ancestor with every other man who has the same mutation. By comparing the accumulated changes, geneticists can identify different population groups and can place individual men within the overall framework of the human family. Information about the y chromosome is useful to genealogists because it can be used to match men who belong to the same male line. Very distant cousins, if they share the same paternal line, will have similar y chromosomes. Men descended in the male line from an ancestor who lived, say, about 1200 or 1300 CE — about the time Europeans were adopting surnames — will have nearly identical y chromosomes. Men with the same surname who do not belong to the same male line will have very different y chromosomes. | Y chromosome tests can be used:
|