system
Given the following post, respond as if you're commenting on Reddit:
human
When research shows some people share 2-4% of their DNA with Neanderthals, what DNA are they talking about?

I read in an article recently that some people share 2-4% of their DNA with Neanderthals. However, we also share loads of DNA (>95% IIRC) with less similar species such as Chimpanzees. So, when we say we share 2-4% of DNA with Neanderthals, is that only some part of our DNA that is measured separately, like repetitive sequences?
gpt
Here's a simple analogy. Suppose almost everyone in your family was Icelandic, except that one of your great-grandparents was Nigerian. Your DNA would be 87.5% Icelandic, 12.5% Nigerian. *Most of the DNA of people in Iceland and Nigeria is identical*. However, those populations have accumulated mutations that make parts of their DNA distinct. So when someone says you are 12.5% Nigerian, what they mean is that 12.5% of your DNA is inherited from a Nigerian ancestor and contains mutations that originated in Nigeria. The actual sequence difference between you and a person of purely Icelandic descent will be much, much smaller than 12.5%: it will be a tiny fraction of one per cent.

Likewise, when someone says your DNA is 3% Neanderthal, they mean that 3% of your DNA consists of chunks that are traceable to Neanderthal ancestors. *Most of the DNA of Neanderthals and mainline Homo sapiens is identical*. The sequence difference between you and a hypothetical person with no Neanderthal ancestry will be much, much smaller than 3%.