If size matters, the Y chromosome may be a source of little merriment to men. The male sex chromosome is the smallest in the genome. But proportions aside, it is nothing short of a surprising archive of human history, and the sharpest genetic tool in the genealogy shed. Passed down through the ages from fathers to sons, the Y chromosome is like the genetic signature of a male line.
All other chromosomes in the human genome have a matching pair. But in the chromosomal square dance that precedes reproduction, the Y dances alone. This means it has little chance to change or mutate by swapping and shuffling its code with another chromosome. Instead, it is passed down through generations of fathers to sons virtually unchanged. The mutations it does harbour spring up randomly in the man who carries it, and the sons who inherit it from him. Find two men with the same sets of mutations on the Y chromosome, and it suggests they share a common male ancestor. If they also happen to share the same last name, that common forefather likely lived in the recent past.
But even without finding a match, all males carry mutations on their Y chromosome that can reveal the ancient, geographic origins of a paternal ancestry. Its limitations: Only males can have their Y chromosomes tested, and as the genetic hallmark of fathers alone, it really is only one side of any family story.