I once hitched a ride with a guy, picking up a pony in Exmoor, involved in horse-breeding. He was telling me in the equine-pedegree world its ok for fathers to mate with daughters, but NOT mothers with sons because the offspring go doolally. No full-blood siblings either, but, apparently, half-siblings can if they don't stem from same mother(?)...
The gist i got was direct womb-link regards in-breeding is a defo no no. It got me thinking about the Old Testament and Lot and his daughters continuing the progeny, and Abraham and his half-sister Sarah among others. Although post-Law bans all incest, it would seem mother-son incest is considered the worst of all such examples, though such a sinful relationship never explicitly exampled pre-Law (though I'm sure there were).
Well, that got me to thinking. I've heard similar things, too. And since we do occasionally breed our "purebred" dogs, I've had to do a little research on who goes with who. Usually, the kennel clubs keep track of that and if you are unsure, you can ask.
But, back to the mother-son thing; aside from evincing a strong "yuck" in the Western psyche, there may actually be a medical/genetic reason for it and somebody may have noticed a consequence way back when. What I have read is that dna from the child a woman is carrying finds its way into her system and persists there. It's probably not so bad if one is carrying a female child, but what about a male child? Some of that male child's dna is floating around in her body somewhere. If there are sex-linked mutations that are harmful, the mother carrying a child by that same son might have problems. Of course, I'm just speculating, but you never know.