In humans, biological sex is determined by the chromosomes X or Y. People having a Y chromosome are typically male, as this small chromosome –which evolution seems to be doing away with– contains a gene which determines the formation of male sexual organs in the embryo.
However, despite its small size, the Y chromosome is an important one. So much so, that losing it might result in increased likelihood of suffering certain kind of cancers, such as bladder cancer or Alzheimer’s disease. Worse still is that, with aging, loss of the Y chromosome is relatively common, which increases the odds of things going wrong for males with increasing age.
To learn more, check the whole article at MappingIgnorance.org