23andMe - Inheritance

By Roberta Estes

Today's question was whether three sisters would test the same. That's a great question.

First, take a little internet journey and watch a short clip about how autosomal inheritance works.    You can find it HERE and it shows you clearly how you inherit from your parents and they from their parents.    It's actually very easy to understand when you can see it graphically, much easier to understand than trying to understand it from a verbal description.    They explain how autosomal DNA can help you understand both our deep ancestry and our recent genetic past.    You can see in the video how each child actually inherits a random combination of the DNA from their 4 grandparents.    The key words here are random combination.    In each of the parents, they have DNA from their parents and they can only pass on half of that to each child, so it's randomly combined to form just enough to pass on to a child.    Each child gets a different random combination from their parents.    Think of 3 different card decks being shuffled.    None of the three will be in the exact same order, but they will indeed all have the same cards as members of the deck.    If you were to deal only half of each deck, each set cards would include different cards from the original deck.

Let's talk for a minute about random recombination.    We're learning that maybe random isn't quite as random as we had thought.    It has been discovered that some DNA is passed in groups typically and blocks of DNA are often not broken up and are passed together.    The closer you are related to someone, the more likely it will be that you will carry some of the same blocks of DNA that they do.    The more distantly related, the fewer blocks you'll have in common and the smaller those blocks will be.    This is one of the ways that 23andMe determines your genetic distance from someone and they classify you as "close relative", 3rd cousin, 4th, 5th, 6th, etc.    So to answer the original question, three sisters (or siblings) would not test the same because they would inherit different genes from their parents, which are actually a recombination of the DNA of their 4 grandparents.     Some portions of the test results you would expect to be more similar than others.

Health Traits    - some of these might be the same, but some might be completely different as well due to the inheritance patterns.

Haplogroup    - mitochondrial haplogroup would be identical assuming the same mother, Yline haplogroup would be identical for males assuming the same father.

Ethnicity    - this is the interesting area where sibling can actually differ depending on the genes they inherited.    One sibling could indeed have somewhat different results for their percent ethnicity due to the genes they did, or did not, inherit from their parents/grandparents.

Relative Finder    - this should be pretty much the same, but might differ on more distant relationships due to recombination.

In a nutshell, you can't extrapolate the health information of your sibling to be your own because you inherit differently from your parents.    You can extrapolate some of your ancestry information to be the same as your own, but not all of it.    I think it would be very interesting to compare the results of a number of parent/child and sibling tests to see how well 23andMe does in their Relative Finder predictions.