Matching Cousins
Tests like those from 23andMe, Ancestry.com, MyHeritage.com and FamilyTreeDNA.com are really good at seeing relationships between people. Even distant people like the 5th cousins have a pretty good shot at seeing that shared DNA that ultimately came from their great-great-great-great grandparents.This is a great table that breaks out the odds that two relatives will share DNA in one of these tests (along with how many of these relatives you have on average):
As you can see, these tests are incredibly powerful even up to third cousins. Theoretically, you should see shared DNA in every first and second cousin and 98% of third cousins. It goes down after this but you even have a pretty good chance to see seventh cousins!
What these tests are less precise about is the actual relationship. So sometimes the test will call someone as a third cousin when they may actually be a fifth cousin and so on. The more distant the relationship, the less precise the relationship assignment. See my article about using the "green chart" and DNA Painter to look at all possible relationships.
The number of "expected" cousins is pretty imprecise. It assumes each member of each generation will have 2 - 3 children each. Personally, my mother had 37 - yes 37 - first cousins just on her mother's side! So, each family is different.
What these tests are less precise about is the actual relationship. So sometimes the test will call someone as a third cousin when they may actually be a fifth cousin and so on. The more distant the relationship, the less precise the relationship assignment. See my article about using the "green chart" and DNA Painter to look at all possible relationships.
The number of "expected" cousins is pretty imprecise. It assumes each member of each generation will have 2 - 3 children each. Personally, my mother had 37 - yes 37 - first cousins just on her mother's side! So, each family is different.