Sharing Remember that recombination is the reason that a maternal nephew and uncle are likely to share some of their X chromosome.In humans, there seem to be between 20-40 recombination events every generation, with 26 being published recently as the average (ref 1,2,3).This is especially true since recombination events do not occur randomly along a chromosome.Recombination tends to happen in what are called “hot spots” or regions that seem more susceptible to breaking and swapping DNA (ref 5).This means that it is the dad that determines whether a baby will be a boy or a girl. And why a paternal uncle and nephew would only do so in very special situations. Paternal Let’s examine the case where the uncle is on the mother’s side.Here is what this family tree might look like: We are just going to focus on the sex chromosomes.
Getting a Y chromosome and the SRY gene region that comes with it tells the embryo to begin making a man as opposed to a woman. This also means that guys always get their X from their mom. It is this last point that explains why a maternal uncle and nephew could share large chunks of their X chromosome.
Notice that grandma and mom have two X chromosomes and that all the males have an X and a Y.
I have also color coded the chromosomes so we can follow who got what from which relative.
OK so that explains how an uncle and a nephew might share some of their X chromosome.
But a valid question may be how likely it is that any given maternal uncle and nephew will share large segments of the X chromosome.