Maths: 50% of my grandchildren, or not?

[FlamedToACrisp]

I found a photo of my son (=A) and his cousin (=B) as children.

My DS showed it to his son, who thought it was a photo of himself (=C) and his brother (=D).

I said, "Well, he's 50% correct!"
DS says, "No, he's only 25% correct."

Who is right, and why?

Ummm... Neither of you. It’s not a picture of him or his brother.

He's zero % correct because it's not a pic of him and his brother?

I think the question is about what percentage related the subjects are but I'm struggling with it!

I think I understand what you mean. If I am understanding correctly, he is correct.

Assuming each person in the original photo represents 50% - your grandson is then 50% of one of them (your son)

Which makes him 25% over all.

Or have I misunderstood completely

I make it 31.25%

Hmm, if I understand what you are asking,

C has 50% in common with A.
D has way less than that in common with B. 25% between cousins, I think, so an extra generation makes that 12.5% maybe???
So, overall 25% of picture contribution from father/son and 6.25% from cousins once removed means 31.25% in in common???
Someone check my maths, please!

If this is really homework I think it is a rubbish question.

It's not homework, just a discussion with my DS, but I couldn't think where to put it!

Well done digbee and Redcat - I hadn't considered the percentage of the cousin!

I think I completely misunderstood what was being asked

He’s 0% right. It’s not about genes.
There were two people in the photo, he didn’t identify either of them correctly so was 0% correct.