X-chromosome: The female sex chromosome, being the differential sex chromosome carried by half the male gametes (specialised haploid cells produced by meiosis) and all the female gametes in man and many other species. Xenogeneic: In transplantation biology, denoting individuals or tissues from individuals of different species and hence of disparate cell type, called also heterologous. Xenograft: A graft of tissue transplanted animal of different species. Xeroderma pigmentosa: A rare hereditary disease in which there is extreme skin photosensitivity to ultraviolet light as a result of deficient DNA repair enzyme. It begins in childhood. Zinc: An essential trace element being an essential component of the active site of a variety of enzymes. Zygote: Cells formed by the union of two mature sex cells, ovum and the sperm. A human zygote has 23 pairs of chromosome.