Male chromosomes. Computer illustration of the complete set (karyotype) of human male chromo- somes. Humans have 46 chromosomes in each cell, divided into 23 pairs containing one from each parent. Males differ from females only in the last (23rd) pair. Here, an X and a Y-shaped chromosome are seen as the 23rd pair (lower right). In a female karyotype, there would be two Xs. Chromo- somes contain deoxyribonucleic acid (DNA) which is responsible for passing genetic information from parents to their offspring. The DNA is divided into sections called genes, which each have a specific purpose. Genes on the Y chromosome are responsible for the development of male features.