The chromosomes that dictate sexual development are mixed and matched in predictable combinations: A female inherits one X chromosome from each parent; a male inherits an X chromosome from his mother and a Y chromosome from his father.

Illuminating this complex palette was the work of an international team of 250 scientists led by geneticist Mark Ross at the Wellcome Trust Sanger Institute in Hinxton, England. The team produced the first complete sequence of the X chromosome about two years after the decoding of the male Y chromosome.

The researchers found that most of the genes on the X chromosome reside on chromosome 1 and chromosome 4 of chickens.