Diana sussed it all out.
 
No doubt the powers-that-be had thoroughly checked her family's gene pool to ensure that there was nothing 'embarrassing' that might express itself in her progeny (as per Katherine Bowes-Lyon et al) before allowing her to become Charles's baby-making factory.
 
Whether she realised this king/queen maker role was to be her sole purpose (plus of course the occasional outing as the token 'model' wifey) before she accepted big ears' proposal of marriage or later we shall never know. But it's pretty obvious that the penny finally dropped somewhere along the line, after which she proceeded to mimic her husband's infidelity and extra-marital bedroom activities.
 
It's pretty clear to see that the inherent outcome of inbreeding amongst European royal families over the centuries has led to the modern Windsors increasingly adopting a selective breeding program where family members are now matched with suitable 'commoners' instead of their own cousins and other close relations. It means the bloodline isn't quite so 'blue' anymore, but it is a lot healthier.
 
Ta-da............. royal genetics in 3.1825 paragraphs.
 
And, for something completely not different:
 

 
 

Jonbouy
I think an American monarchy would be cool.
 
I'd recommend Donald Trump being the first King as he'd be able to provide a bunch of tasteful grand palaces at short notice. 
 
King Don the 1st of the UKoA sounds cool doesn't it? 
 
See how the first 10 generations of Trumps get on then perhaps re-appraise it after say 250 years so it can get properly established then take it from there.

Yes, we need another war.