Summary of Role of Sex Chromosome

The X chromosome carries gene essential for normal human development of both sexes. (otherwise YO would not be fatal and XO would be completely normal)

SRY gene on Y chromosome determines maleness, only a single Y is required to produce a male (XXXY are still male)

Fertility genes on both X and Y and females need nothing X’s to be fertile (Turner Syndrome, XO , causes infertility)

Additional copies of the X chromosome may upset normal development (XXXX females likely intellectually disabled)