Can the DNA of the son and mother be used to determine whether the (now deceased) father of the mother can be the father of her son?

Answer

Hi Trees,

The DNA of the mother’s father (I’ll call it the grandfather for clarity) has some resemblance to that of the mother (if the grandfather is also the biological father of the mother, that is). When a ‘strange’ father makes a child with the mother, there is more variation in the DNA than when the grandfather makes a child with the mother (his daughter). If DNA profiles show that there is more similarity (or: less variation) between mother and son than average (i.e. when the father is not genetically related to the mother), one might suspect that the grandfather fathered the child with his daughter.

Without a DNA profile of the grandfather, this cannot be said with certainty, but one can find clues.

This answer is based on my theoretical knowledge of these things. I do DNA analyses, but not in that context. So forgive me if my answer is not completely 100% correct, but I doubt the amount of certainty that one can have about the father when there is no DNA profile of the grandfather.

I hope you are into this answer anyway,

Regards,
Christine