Eggs are expensive, sperm are cheap. It’s a fundamental fact of biology . . . for now.Currently, embryo selection can improve any heritable trait, but the degree of improvement is limited by the number of embryos from which to select. This, in turn, is because eggs are rare.But what if we could select on sperm instead? We could choose the best sperm from tens or even hundreds of millions, and use that to make an embryo. However, any method that relies on DNA sequencing must destroy the sperm. Su...