Double Cousins And Half-Siblings

Stupid Question ™
Nov. 30, 2000
By John Ruch
© 2000

Q: I have “double cousins”: our mothers were sisters and our fathers were brothers. We also share all four grandparents. Are we more closely related than half-siblings are?
—Paul Koebel


A: While awaiting a retroactive 1600 on my SATs for figuring this out, I’ll say that this is (unintentionally, I’m sure) a trick question. Between double cousins and half-siblings, it’s a draw.

To define the terms: a cousin is the child of one’s aunt and uncle. And a half-sibling is someone with whom you share one (and only one) biological parent.

The only definitive way to judge “relation” is genetically. In sexual reproduction, which we humans engage in, two individuals each contribute half the genes that will make up the child. The genes are segments of DNA attached to structures called chromosomes. Humans have 46 chromosomes—23 pairs from each parent.

To put it another way, each parent contributes 50 percent of the child’s genes, and the child is therefore 50 percent related to each parent.

Be careful in reading the percentages I’m about to give: remember that they are not actual numbers, but only descriptions of an average ratio of shared genes. For example, it’s also true that all humans are about 99.99 percent genetically similar to each other. What we’re talking about is shared genetic material gathered from a specific, individual source.

Two regular siblings—say, two brothers—are about 50 percent related. Why not 100 percent related, since each parent gave them both half their genes? Because the chromosomes are not directly copied in order, but randomly sampled every time. Therefore, on average, siblings will share 50 percent of the random results. (Also, these values are slightly skewed by the sex chromosomes: males are always slightly more related to males, and females to females, in terms of sharing genetic material. If you’re the same sex as your cousins, you get an extra edge in this game.)

Thus half-siblings are, ironically, only one-quarter related. They each got 50 percent of their genetic material from only one parent; on average, they’ll share half of that, or 25 percent.

Regular first cousins are more distant. Let’s say your father and your uncle were brothers, and your aunt has married into the family. Your father and uncle are 50 percent related. You’re 50 percent similar to your father, and therefore 25 percent similar to your uncle. The child your uncle has with someone to whom you’re not related is, therefore, 12.5 percent related to you.

But in the case of double cousins, where both parents are siblings of your parents, the relationship is obviously doubled to 25 percent—the same degree as with a half-sibling.

Or simply put: whether you’re halving a sibling or doubling a cousin, the mathematical result is the same, and either way you’re 25 percent related.