One of the two types of sex determining chromosomes. The unfertilized egg cell always is created with one X chromosome in the haploid of the human diploid in the unfertilized egg cell. The other sex chromosome is provided by the male’s sperm cell. An X chromosome with the haploid in the sperm cell from the male and the fertilized egg produces a female offspring. A Y chromosome with the haploid in the sperm cell from the male and the fertilized egg produces a male offspring. Thus the Y chromosome is passed from fathers to sons. It should be noted that the X chromosome is much larger than its Y chromosome counterpart. Also, when the two X chromosomes, one from each parent, are paired with each other in a fertilized egg cell, and thus ultimately producing a female offspring, these two X chromosomes are of equal size and are thus homologous in the female offspring. Thus the X chromosomes become subject to cross-over effects during subsequent egg cell creation in the offspring, and thus the homologous gene alleles and genetic marker alleles in both those X chromosomes can randomly swap positions in the next generations making it very difficult to track a particular X chromosome over more than just a couple of generations. Determining a common ancestor for an X chromosome is very difficult beyond a couple generations. Thus the X chromosome is not a very useful tool for Genealogy by DNA purposes. However the Y chromosome because of its small and very different size is not homologous with its X chromosome counterpart in a male offspring and is thus not subjected to cross-over effects and thus the Y chromosome is passed down from fathers to sons unchanged over many generations, except for occasionally occurring natural mutations. Compare to Y Chromosome. See Sex Chromosome and Autosome.

This definition is in context to Genetics and Genealogy. See more contextual defintions for X chromosome.