Definition for X chromosome
From Biology Forums Dictionary
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals (the other is the Y chromosome) and is found in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in the alphabet, after it was discovered later.