Y-chromosome

the small chromosome that carries a dominant gene for maleness. All normal males have 22 matched pairs of chromosomes and one unmatched pair, one large X-chromosome and one small Y-chromosome. The X-chromosome, which is inherited from the mother, carries many more genes than the Y-chromosome, which is inherited from the father. During sexual reproduction, the mother must contribute one X-chromosome, but the father can contribute either an X or Y-chromosome. The effect of the Y-chromosome is that a testis develops in the embryo instead of an ovary. Thus the sex of the resulting offspring is dependent on the father's contribution -- female (X,X) or male (X,Y).

Taken from Dictionary of Science