Just for the heck of it, I went ahead and calculated some percentages for those who are curious. This is based off simple statistics calculations + data I've personally collected out of my pride, so it's not 100% accurate, but the sample size seemed sufficiently accurate after further observation.

First Gens
Litter Size 1	Litter Size 2	Litter Size 3	Litter Size 4
10%	19%	27%	33%
*Natural Chance of at least one hybrid in a litter: 14.8%
Buff Scrotum Chance: 27% - 33%
Second Gen+
Litter Size 1	Litter Size 2	Litter Size 3	Litter Size 4
5%	10%	14%	18%
*Natural Chance of at least one hybrid in a litter: 7.6%
Buff Scrotum Chance: 14% - 18%

* Natural Chance means without the use of any breeding items that affect litter size.