Chromosome which is found in both males and females. XY = male. XX = female. Sperm can be either an X or Y chromosome. During fertilization, if the Y chromosome sperm reaches the egg, then the baby is a boy. If the X chromosome sperm reaches the egg, then the baby is a girl.
Think of it as females having two copies of the same CD music album in every cell in their body, while males have one copy of that album in every cell in their body(x chromosome) and an extra single CD (the y chromosome) also in every cell in their body.
Even if the y chromosome was suffering from genetic degredation(which it is not) the effect could easily be reversed by genetic engineering.