The male parent does. Sex is determined by two chromosomes, X and Y. A female is XX, a male is XY. Since women completely lack the Y choromosome, they always contribute an X chromosome to the baby.The sex is determined by whether the sperm that fertilizes the egg is carrying another X or a Y.es | No Thank you! The male parent does. Sex is determined by two chromosomes, X and Y. A female is XX, a male is XY. Since women completely lack the Y choromosome, they always contribute an X chromosome to the baby. The sex is determined by whether the sperm that fertilizes the egg is carrying another X or a Y.