Sex chromosome , either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of human beings and other mammals are designated by scientists as X and Y. In humans the sex chromosomes consist of one pair of the total of 23 pairs of chromosomes. The other 22 pairs of chromosomes are called autosomes. The X chromosome resembles a large autosomal chromosome with a long and a short arm. The Y chromosome has one long arm and a very short second arm.
The evolution of the sex chromosomes: Step by step
These chromosomes would come to be known as the sex chromosomes, X and Y. The answer begins in the late s, when insect gonad cells, whose large chromosomes are easy to view through a microscope, were the specimen of choice for investigating the cellular basis of heredity. In , German biologist Hermann Henking counted 11 chromosomes in firebug Pyrrhocoris apterus sperm nuclei. McClung soon hypothesized that the accessory chromosome determined male sex.
The Y chromosome is one of two sex chromosomes allosomes in mammals , including humans , and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species , since it is the presence or absence of Y that typically determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the gene SRY , which triggers male development. The DNA in the human Y chromosome is composed of about 59 million base pairs.