I have seen various numbers for % of population that are bedwetters. One standard that I see is 6% (I know this may be off, but please bear with me). According to Wikipedia, if one parent was a bedwetter, then the child has 44% chance of being one as well, if both parents, 77%. Wikipedia also mentions a 15% chance otherswise. Thus in a population of 100:

6 out of 100 are bedwetters. If the 6 bedwetters intermarry, then thats 3 couples of bedwetters and 47 non-bedwetters.
After one generation, assuming each family had enough kids for replacement level (2), that would create:
6 x .77=4.62 + .15 x 94 = 15.94 = 18.72 bedwetters. Round up to 19.

G0=6
G1=19
G2

9 couples of bedwetters, 40 couples of non-bedwtters, and 1 couple where one parent was a bedwetter.

G2=28
G3=32
G4=35
G5=37
G6=38
G7=39
G8=39
… after this it remains at 39.

A couple of questions:

Assuming these numbers (or the more accurate numbers), why don’t we see an increase in bedwetting along these lines over the past several generations?

What would be the numbers needed to get a majority of bedwetters?

There’s also an assumption you’re making that the proportion of “bedwetters” (however that term is defined in the statistics you’re providing) are equally likely as non-bedwetters to marry and have children, which might not be true. Given the vague or nonexistent definition of bedwetting relevant to these numbers, it’s hard to tell what the percentages mean anyway. Additionally, you’re assuming for the purposes of your question that the numbers are accurate, but then asking an ultimate question that depends on the validity of the numbers to be answered properly. In other words, my answer for why this phenomenon doesn’t occur in real life might well be that I think the percentage chances you’ve quoted are disproportionately high, but you’ve already asked that we assume the numbers are correct.

It’s also possible that this phenomenon IS actually occurring, but given the private and socially embarrassing nature of the problem, the general population isn’t aware of it. It’s not like you can walk through a grocery store and identify all the kids who are bedwetters by sight. The few you might identify by the presence of GoodNights or some other signature purchase are likely to be a small subsegment of the overall bedwetting population.

Kita: I think your concern about a 77% chance not meaning for sure is misplaced here. It’s pretty basic statistics, I think, to take the percentage chance of a trait occurring in a population and multiply it by the number of individuals in the population to get an average number of people with that trait. For any particular child, yes, a 77% chance of bedwetting is not a guarantee, but if the percentage is accurate, if we gather 100 children who have bedwetting parents, then (on average) 77 of them should be bedwetters and the remaining 23 will not. The smaller the population, though, the less accurate those sorts of extrapolations become.

The proportion of male bedwetters is also higher than female, meaning that intermarriage is impossible and it’s your intermarriage assumption that’s doing all the work.

If 6% of the population has a history of bedwetting then there’s only a 0.36% chance of any given couple both having been bedwetters. Beyond that it can get pretty comlicated