One of the two types of sex determining chromosomes, which are the X and Y chromosomes. Men have an X and a Y chromosome. The Y chromosome is the chromosome that determines a person is male. Since a female has two X chromosomes, the unfertilized egg cell of the mother is always created with one X chromosome in its haploid, i.e., the ½ set of the human diploid contributed by the mother in the unfertilized egg cell. The other sex chromosome required to fertilize the egg cell is provided by the male’s sperm cell’s ½ set of the human diploid. If another X chromosome is provided to the egg cell via the male sperm cell, then the fertilized egg cell will develop into a female child. If a Y chromosome is provided to the egg cell via the sperm cell from the male, then the fertilized egg cell will develop into a male child. Thus the Y chromosome is passed down from fathers to sons. The Y chromosome is very unique. The Y Chromosome is much smaller and very different from its X chromosome partner in the 23rd chromosome sex determining pair. These two chromosomes are not homologous like the other 22 autosome chromosome pairs are. Thus during the metaphase of meiosis (sex cell creation process) because the Y chromosome is so relatively small and very different from the X chromosome, they do not line up very well during the metaphase of meiosis. Synapsis does not occur. Thus, except for the very ends of the chromosomes, they do not exchange genetic material readily via cross-over like the homologous autosome chromosomes do. Thus during creation of male, Y chromosome bearing, sperm cells the Y chromosome is passed to the Y sperm cell virtually unchanged. Thus the genes and genetic markers in the Y chromosome are passed from father to son over many generations virtually unchanged, except for an occasional natural mutation. The Y chromosome is not subject to genetic material mixing by cross-over effects over the generations like all the other chromosomes are. Thus the Y chromosome is unique and is a valuable and useful tool for determining common ancestry in the male population since if two men share a recent common ancestor, in a genealogical time frame, their Y chromosomes will be virtually identical. The Y chromosome is thus a very useful tool for Genealogy by DNA purposes. Compare to X Chromosome. See Sex Chromosome, Autosome, and Genetic Marker.

This definition is in context to Genetics and Genealogy. See more contextual defintions for Y Chromosome.