Midwives have been told about the benefits of “close relative marriage” in training documents that minimise the risks to couples’ children.
The documents claim “85 to 90 per cent of cousin couples do not have affected children” and warn staff that “close relative marriage is often stigmatised in England”, adding claims that “the associated genetic risks have been exaggerated”.
If you have one person with recessive genes and one person with dominant genes, then the baby will have the dominant gene. So if the grandparents were cousins both with recessive genes it wouldn’t matter, as far as I know.
The thing is, with subsequent incestual generations, the likelihood of the recessive gene manifesting increases a lot. So, the problem is not a single generation of incest, it’s the normalisation of incest that might lead to multiple generations doing it.
Oh I see what you’re saying. I did some reading earlier that said that in a lot of places 20% - 40% of all marriages are to first cousins.