Why You Share 98% of Your DNA With a Chimp, but Only Half With Your Mom

The old wildebeest shoved into the river was helping himself, not his herd. That single-minded drive to pass on your own genes is powerful, but it runs into a puzzle the moment you ask a simple question: what counts as "your" genes when a relative is involved?

What Mendel Actually Figured Out

Long before anyone knew what a gene physically was, a monk named Gregor Mendel worked out two things just by breeding pea plants and counting the results.

• Why we resemble our relatives.
• Why, unless you are an identical twin, we are never exactly like them either.

The answer is that you get about half your DNA from each parent, so you are one half related to each parent. That halving rule keeps going outward:

• Full sibling: you share about half your genes with them.
• Half sibling: you share about a quarter.
• First cousin, second cousin, and beyond: the shared percentage keeps halving each step further out.

The closer the relative, the higher the percentage of genes you share with them.

The Chimp Question

Here is where a strange fact usually causes confusion. You share half your genes with each parent, yet it is often said that humans share 98% of their DNA with chimpanzees. Does that mean you are more related to a chimp than to your own mother?

No, these are two completely different comparisons. The 98% figure is about whether a gene for a trait exists at all. Humans and chimps both have genes for growing a nose, growing eyebrows, having two ears, and having four limbs. Neither humans nor chimps have genes for growing antlers, the way a deer does.

So the 98% number just means: out of all the genes that build a body plan like ours, chimps have almost the same set that we do. It says nothing about which specific version of each gene you have.

That's the piece that matters when comparing you to your parents. Everyone in your family already has the gene for eye color. What differs is which flavor of that gene each person got, blue, brown, green.

You share the broad category of traits, having eyes at all, with a chimp. You share the specific flavor of each trait, this exact eye color, with your parents, at a rate of about 50% per parent. One number is about which traits exist. The other is about which version of each trait you got, and only the second one is what "relatedness" means in kin selection.

With that cleared up, the next question becomes obvious: if relatedness is about shared genes, does it actually change how animals, including humans, treat each other?

Further Reading

• Book: Behave: The Biology of Humans at Our Best and Worst by Robert Sapolsky, which covers Mendelian genetics and kinship in more depth.
• Wikipedia: Gregor Mendel · Coefficient of relationship · Human evolutionary genetics