How Grandmas May Give Kids an Evolutionary Edge. Absolutely fascinating: XY v.s. XX chromosomes mean that paternal grandmothers have a 50% chance of sharing an X with their son’s daughters, but a 0% chance of sharing an X with their son’s sons. A study on survival rates of 43,000 children found a corresponding correlation with the proximity to a paternal or maternal grandmother. Men: Dad’s Mum is out to get you!