Chromosome is part of a cell that carries hereditary material inform of genes, it contains DNA molecules tightly packed around histone proteins. X and Y are the sex chromosomes which influences the sex of an individual. Chromosome 21 is the smallest autosome in the human genome and exists in two pairs one inherited from either parent. In this case, the answer is the Y chromosome; sex is determined by the SRY gene which is responsible of the fetus to male. Other genes on the Y chromosome are important for male fertility.