if the father and mother have no mutation the first son should have muscular dystrophy (the probability is 10% if they are both carriers). Duchenne Muscular dystrophy is passed on via X-inactivation in female carriers which means that the chance of getting muscular dystrophy if the mother is a carrier is 25%. Women only have one X chromosome (monosomy) so when they reproduce they will always pass on their X chromosome. Since males are XY and females are XX males can transmit an X chromosome to their sons; however mothers can only transmit their X chromosome to their daughters. making the probability of the second son be 25%
