So, just putting this out there: I'm not sure if this is already how it works, but if it isn't, just in case, I'm throwing this idea out there.

So, the suggestion:

Roll the extra ten slots SEPARATELY when breeding from the base 10. i.e. if you bred a lioness to a 20-marked stud, you'd have the SAME chances of getting the base 10 markings as you would before, and then an extra markings roll to determine if you get any of the extra markings or not- sort of like an extra mutation roll, but with normal marking probability.

That way, by having extra slots, you wouldn't have any LESS of a chance of passing on the first 10 by virtue of having more options.

So... if someone were to use those extra 10 slots for mane markings, they still have the same chance to pass on the first ten as they would if they did NOT have those extra slots- so it wouldn't make extra slots less desireable on a stud.