The Y chromosome is a sex chromosome found only in males. It is passed from father to son. Compared to the other chromosomes, the Y chromosome does not contain many genes - it has almost 30 times fewer genes than the X chromosome. Most of the genes on the Y have male-specific functions, such as male sex determination or male fertility.
Unlike the other chromosomes, there is generally only one copy of the Y per cell. But it isn't left without a dance partner: during recombination the Y chromosome pairs up with the X. While other chromosomes recombine along their entire length, only the tips of the Y and X chromosomes recombine. The tips of the Y chromosome that could recombine with the X chromosome are referred to as the pseudoautosomal region. The rest of the Y chromosome is passed on to the next generation intact.
Because most of a son's Y chromosome is inherited only from his father, the Y chromosome is very useful for tracing geographic ancestry. The Y chromosomes of all living males are related through a single male ancestor who lived over 100,000 years ago. By looking at the geographic distribution of sets of closely related Y chromosome lineages (called haplogroups), we learn how our ancestors migrated throughout the world.
The Y differs from other regions of the genome because only males pass on Y chromosomes each generation (and not even all males, since not all men have sons). For this reason the Y chromosome is particularly susceptible to what is known as genetic drift, a process in which the frequencies of genetic variants can vary over time. These changes are more likely to occur because of population-level events such as migrations, new settlements, and replacement. And since the Y chromosome does not recombine, the history of the lineages affected by these events is preserved through the generations. As a result, Y chromosome variation has a strong geographic pattern.