It has been a very long time since I studied genetics, and the state of the field is now light-years beyond the things I learned back then. I presume (for no good reason) that some of the rudimentary components remain fairly intact, and the following is based on my recollection of those basics. If I am in error, I welcome gentle correction from the more genetics-savvy.
I spent a lot of time with family this weekend, which got me thinking about how we’re related. X chromosomes popped to mind, partly because it was my mom’s side of the family and partly because you can actually tell, because of their gender and relationship, that certain people have the same X chromosome.
After thinking about it for a bit, though, I realized that the scope of this is fairly limited. It wasn’t possible to tell at all which of us at the gathering this weekend shared an X chromosome from that side of the family. My mother and her brother might have gotten the same X chromosome from their mother, but you can’t tell by looking. It’s possible that my mom in turn passed that same X chromosome to me or one of my two sisters, but there’s no way to tell just by looking.
Because there are three of us, at least two share an X chromosome from Mom, but there’s no outward way of determining who does or doesn’t. In the event that I share that X chromosome with my sister who has a child, it’s also possible that either or both of us passed that same X chromosome along to our children, but again it’s impossible to tell by looking.
Things are a little simpler on the other side of the family, but not by much. It is certain that my sisters and I all share an X chromosome from our father, which he in turn got from his mother. We may or may not share that same chromosome with our aunt or uncles – his siblings – or our only female first cousin. It’s certain we share that X chromosome with other members of our paternal grandmother’s family, but there’s no way to determine that by looking. It is, though, a fact that we don’t share any X chromosomes with members of our paternal grandfather’s family.
I briefly started to calculate the probabilities of these various potential chromosomal overlaps. But then I remembered the two most important things I learned about probability in school: it is always rather more complicated than it first appears, and I wasn’t very good at it. If any math nerds out there would like to tackle these calculations for their amusement, I promise to be inordinately impressed with the results.