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”.
But it’s not a 15% risk. Unrelated couples have a 3% chance of having a child with a birth defect while cousins have a 5% chance of having a child with a birth defect.
Isn’t the problem being that the probability increases with each subsequent generations? That’s why having a child with a cousin should be discouraged, to prevent the accumulation of bad recessive genes.
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.