The
X chromosome is one of the two
sex-determining chromosomes (
allosomes) 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.