To understand how SNPs help scientist locate disease genes, you first need to understand how genes are inherited. When you inherit a trait or disease, you don't just inherit the DNA for that trait. Instead you get a long chunk of DNA that may affect many characteristics. So maybe the piece of DNA from your dad that gave you his big blue eyes, also gave you his big feet. In this hypothetical example, big-blue-eyes DNA and big-shoe-size DNA make up a block of DNA that is always inherited together. You inherited this genetic chunk from your father, he from his father, and so on, all the way back to the original ancestor who first developed this particular trait. So, even in a large mixed population, anyone with this specific chunk of DNA would be genetically related to each other, because they share a common ancestor - the first big-blue-eyed big foot.