@BenHoare more common than you might think. Half and half mosaic must happen in this instance at the 2 cell embryo (or a double fertilization and the two zygotes fuse). It is very rare in humans for a number of reasons, including circulating hormones. But in humans often, there is actually a mosaic of maternal cells and fetal cells. Nature is messy! Thanks for posting this and keep us posted on the book.