noun, plural: X chromosomes
A mammalian sex chromosome which typically occurs in two in females and one in males
A sex chromosome is a type of chromosome responsible for the chromosomal determination of the sex of an individual. In most mammals (including humans), there are two forms of sex chromosomes: the X chromosome and the Y chromosome. The sex chromosomes in males are X chromosome and Y chromosome; females have two X chromosomes. In this XX/XY sex-determination system, the sex of an organism is determined by the sex chromosomes (especially Y chromosome) since these chromosomes bear the genes that control the development of reproductive organs and other sexual characteristics of an individual.
The X chromosome was first described by the cytologist, Hermann Henking, in 1890. Henking named the chromosome initially as X for showing an unusual behavior during cell division compared to the other chromosomes. While the other chromosomes took part in the process, the X chromosome stayed on one side of the cell and did not take part in the process.1 He was correct to suggest that the “X” is indeed a chromosome. However, he was wrong when he theorized that it was the chromosome controlling the sex of an individual.
1 Popp, J. (2007). Evolution’s first philosopher: John Dewey and the continuity of nature. Albany: State University of New York Press. p.28