Gender is determined by the presence or absence of the Y chromosome. The Y chromosome carries the genes that are needed to override the body's biological default, which is female development, resulting in the development of the male reproductive system.
The human genome consists of 23 pairs of chromosomes, with genetic information from each parent carried by one chromosome within each pair. The X and Y chromosomes, also known as sex chromosomes, determine the biological gender of an individual. Chromosomes consists of DNA that have been tightly packed into thread-like structures. While women are able to pass on only the X chromosome to their offspring, men are able to pass on either the X or the Y needed to begin male development.
While each pair of chromosomes are superficially indistinguishable, the X and Y chromosomes are different. The Y chromosome is much smaller than the X, and while these chromosomes do make up a pair, their ability to recombine genetic material is limited. The majority of the genetic information stored in the Y chromosome is passed from father to son in a manner not unlike asexual reproduction. The limited recombination of genetic material between the X an Y chromosome means that deleterious mutations cannot be independently selected against.