DNA INHERITANCE

When helping people with family searches, I'm often asked things like, "How come my ethnicity chart looks different than my sibling's chart?"  Or, "Why are the centimorgan numbers so different for people I know have the same relationship to me?"  Both are very good questions.  The answer has to do with the randomness of DNA you inherit from your ancestors.  You should also be aware that ethnicity data from these genealogy sites is only an estimate.  Determining ethnicity is complex.  There is a video about it on Ancestry.com if you really want to get into it.  I'll try to explain DNA inheritance the best I can without getting too complex.  Let's begin by looking at a color coded family tree I made.

I'll start with ethnicity.  In the family tree below, in addition to people's names, I'm showing colors to represent each person ethnicity.  To keep things simple, notice the eight great-grandparents in the tree are all a solid color.  We'll say they each have 100% of a single ethnicity.  You can call each color whatever ethnicity you'd like.  Maybe red is Russian, green is Irish, or whatever.  It really doesn't matter for this example.  Just know that each color is a different ethnicity.

Here's an important thing to remember, you inherit 50% of your DNA from each parent.  So, you'd think we'd inherit 25% from each grandparent, and 12.5% from each great-grandparent, right?  Well, that's not the way it works.  If it did, everyone's chart would look similar the image above.  The amount of each ethnicity would be reduced by half with each generation.  The image above is unrealistic.

Notice the purple and blue great-grandparents have a child, Edward Smith, and he's half purple and half blue.  That one makes sense.  He inherited half from each parent and those parents have 100% purple and blue.  What about the next generation?  Notice the next child, James Smith, is 25% purple, 50% blue and 25% green.  That would mean he inherited exactly 25% from each grandparent.  It's possible to get perfect percentages like this, but it's more likely you'll end up with something different.

Now look at the first person in the tree above, Thomas Smith.  Each of his five ethnicities are half as much as the previous generation.  Again, this isn't realistic because his grandparents have several different ethnicities.  Instead of inheriting 25% from each, what if he inherited something like 17%, 33%, 21% and 29%.  It adds up to 100% and something like that is more likely.  You might see large amounts of one ethnicity and small amounts of another.  Let's take a look at the tree again, but this time with random amounts of each ethnicity.

Notice the amount of each color has changed for Thomas, James and Katerina.  Some people's ethnicities didn't change at all however.  Do you understand why?  For example, Ivan Petrov is still 100% red.  His parents are 100% red, so he will be as well.  There's no other ethnicities for him to inherit.  What about Maria Kuzmin?  Remember, you inherit 50% from each parent.  Her parents are 100% red and 100% yellow.  So, she will still be 50% red and 50% yellow.  Now look at James Smith.  Again, you don't necessarily inherit 25% from each grandparent.  Since his grandparents are multiple ethnicities, you can end up with lopsided percentages.  He still inherited 50% of his DNA from each parent, but he inherited more blue than you might expect.  Instead of being 25% purple, 25% green, and 50% blue, he's more like 15% purple, 10% green, and 75% blue.

Finally, we get to Thomas Smith.  Look what happened.  He inherited no green at all, or such a small amount it's not noticeable.  If Thomas has siblings, they could show a totally different result.  Maybe they inherited an unusually large about of green and no purple, or something like that.  It's random and can give you unexpected results.

What about centimorgan numbers?  The same concept can be applied here as well.  When you compare yourself to a DNA match, you might be related to the same people, but you both inherited different DNA segments from those people.  For example, if two of your first cousins submitted DNA, you might expect similar centimorgan numbers to appear.  You all have the same grandparents.  Maybe one of your cousins matches you at 700cM and the other at 1000cM.  This is completely normal.  You're all still first cousins, but one of them just happened to inherit more of the same DNA segments as you from those grandparents.  Again, it's all random.

I hope this little introduction into the randomness of DNA was helpful.  An important thing to remember here is...  Don't take ethnicity too seriously on genealogy sites.  It's an estimate.  In some cases, it's very accurate and clear, especially when dealing with parents and grandparents with high percentages.  Also remember, centimorgans can be really tricky because if random inheritance.  Two closely related people might just happen to inherit very different parts of their ancestor's DNA.  Distant relatives could inherit an unusually large amount of similar DNA.  You never know what you'll get, and the numbers can vary quite a bit.

You should also be aware that centimorgans can only vary so much.  If you see a full first cousin with only 350cM in common with you, that's not because of DNA randomness.  Something is wrong.  You or your cousin probably don't know who one of your grandparents really is.  Centimorgan charts like the ones shown in my previous posts can help you determine what's in range for each relationship.   Determining what's random and what isn't can sometimes make genetic genealogy quite challenging.

Next, find out about an unexpected discovery after finding a birth parent.