Once more to make it clear about X, Y, and mitochondrial DNA: The father of a son passes on a copy of his only Y Chromosome, and his mother passes on a copy of one of her two X Chromosomes, to their son. The father of a daughter passes on a copy of his only X Chromosome, and her mother passes on a copy of one of her two X Chromosomes, to their daughter. But almost 100% of our mitochondrial DNA comes from the mother alone. Here is how that happens: "In mammals, 99.99% of mitochondrial DNA (mtDNA) is inherited from the mother. This is because the sperm [of the male parent] carries its mitochondria around a portion of its tail and has only about 100 mitochondria compared to 100,000 in the oocyte [egg of the female parent]." The above quote [with my insertions] from 1998 Gwen V. Childs, Ph.D. "The Mitochondrial Life cycle", located on a page of The University of Texas Medical Branch website. So, mtDNA can carry the genetic signature of either of the two X chromosomes which a child would inherit from his or her mother. That's whY, and now... on to the neXt Liz J