Hattix t1_j1tsye0 wrote
Reply to comment by crixx93 in TIL in the movie Kate and Leopold (spoiler alert!) References to Kate being Stuart's great great grandmother had to be removed because critics were horrified by their incestuous relationship by mankls3
You work out how much of your genome would be identical to the relation to calculate the Relationship Coefficient. As the relative is direct-line, then it's simply how much of your genome is contributed by that ancestor and this simplifies much of the math.
It'd be 6.25% for a great great grandparent.
In terms of centiMorgans, it'd be around 200. Full first cousins share the same as great grandparents (around 850 cM), first cousins once removed would be the same as great great grandparents.
To avoid inbreeding related malady, you usually want shared DNA to be below 10%, ideally below 5%.
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