Answer: Y
In humans, the Y chromosome is responsible for maleness. Men have XY and women have XX sex-chromosomes. A woman's oocytes all carry 22 + X, while a man produces two kinds of sperms, 22 + X and 22 + Y. If a Y-bearing sperm fertilizes the ovum, the zygote is 44 + XY and develops into a boy. So the Y chromosome, which only men have, decides maleness.