Thanks to the research of one of my friends, I could extract and rebuild my scientific genealogical tree: one can go from me to my PhD advisors, from them to their own PhD advisors, etc.
In a classical tree, one has two parents; in a scientific tree, the usual is one advisor, but sometimes two, and consanguinity is much less frown upon. In France, to be a PhD advisor, one needs to get a special diploma (HDR) (définition). This diploma is much easier to obtain if you already were an advisor for a PhD. To solve this egg-and-chicken problem, the solution is co-advisoring a PhD (sometimes with an official PhD advisor and a real PhD advisor). In my personal case, however, my two advisors really coached me (this also happens).