Most of you are probably aware that I have a piece that was just published in Slate today, Which Grandparent Are You Most Related to? I’m not used to writing articles for the general interest audience, so I really appreciate Laura Helmuth’s feedback. All errors and confusions in the piece are mine, and there remains one major issue which I did not correct or refine to my satisfaction in the final draft. Michael Eisen pointed out on Twitter that my use of the law of segregation to compute the probability of inheritance from a single grandparent (50% of the autosome vs. 25% as expected) is misleading. I did include a section on recombination in earlier drafts, but couldn’t find a way to elegantly write about crossing over at length without become turgid. In the end I thought my reference to recombination was clear enough to allow one to infer that the true probability was much lower (close to zero as Michael notes), all the while maintaining the illustrative power in relation to the segregation in meiosis of homologous chromosomes. I was wrong, and hopefully this is will serve as a lesson in terms of how to hone my skill at balancing between opaque oversimplification and excessive technical detail. The empirical result of variation across grandparents was probably sufficient in this case.