My kids were asking how related all of their relatives are to them.
I posed the question to gpt3o and asked for percentages in terms of genetic material. I thought I knew the answer, but was a little surprised.
Anyway, here’s the result again:
From a genetic perspective, relatedness is typically measured using the coefficient of relatedness, which represents the probability that a gene selected at random from one individual is shared with another due to common ancestry. Here’s a list of how genetically related various relatives are to you, expressed as percentages:
Genetic Relatedness to Relatives:
Parents – 50% (You inherit half your DNA from each parent.)
Children – 50% (They inherit half their DNA from you.)
Full Siblings – 50% (On average, share half of their DNA, but the specific half may vary.)
Grandparents – 25% (Each grandparent contributes 1/4 of your DNA on average.)
Grandchildren – 25% (You contribute 1/4 of their DNA on average.)
Aunt/Uncle (full sibling of a parent) – 25% (They share half of your parent’s DNA, and your parent shares 50% with you.)
Niece/Nephew (child of a full sibling) – 25% (Similar logic: your sibling shares 50% with you, and passes half to their child.)
First Cousins – 12.5% (You each inherit half of your DNA from siblings—your parents—who are 50% related.)
Summary Table
| Relative | % Shared DNA |
|---|---|
| Parents | 50% |
| Children | 50% |
| Full Siblings | 50% |
| Grandparents | 25% |
| Grandchildren | 25% |
| Aunts/Uncles | 25% |
| Nieces/Nephews | 25% |
| First Cousins | 12.5% |
The key takeaway is that you are 50% genetically related to your parents, children, and siblings.
I always thought that the most related person in the world to you were siblings, but it probably ties (on average) with parents and children. Ignoring identical twins of course.
It turns out siblings can range from 38% to 61% overlap, that’s a big variance. Crossover can cause big swings, so it could be higher or lower.