Only men carryaYchromosome. In each generation, some men have no children or only daughters, reducing the number of Y chromosomes in the population, until only one remains. This is why all men in the world carry a Y chromosome inherited from a single individual—the Adam of the Y chromosome—who lived in the ancestral human population. The same is true of mitochondrial DNA and the mitochondrial Eve. By looking at the most informative of the mutations on the Y chromosome, geneticists can assign every man to one lineage or another. Since there is only one Y chromosome, all these lineages or
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