Help! My Segments Are So Sticky! Back in the day, it used to be popular to refer to certain segments as “sticky” when they appeared to be passed down from generation to generation untouched. That sort of name-calling has reduced greatly now that we have a clearer understanding of the statistical rules that our chromosomes follow as a result of random recombination. It turns out that the smaller a segment, the more likely it is to escape the chopping block of recombination in each generation and instead either be passed to the child in full or not at all. Let’s take a look at some numbers and see how this plays out. As our starting point, we’re going to go back to our definition of centiMorgan, as explained in my blog from a few weeks ago about the statistical impossibility of two full siblings not sharing any DNA segments. If you missed that one, that’s OK, here’s the way I like to think about a cM: A cM is a unit that denotes a span of a chromosome that has