I always like to get user comments and questions because many of them turn
into a good video that can be used to help you understand how you can evaluate your DNA better.
And today is one of those good questions. We're going to be talking about
half-siblings or double first cousins. I had my DNA done and discovered three,
not just one, three unknown half-siblings all having different biological mothers.
Okay. So we have a father then that has had
now four children from four different women. That's important to remember.
But then, this was the question that actually piqued my interest because...
How do I know if my half-siblings are actually half or double first cousins?
And the reason why this piqued my interest is because
the person must have done some other research and there's enough uncertainty that he doesn't know
whether or not they're just half-siblings or whether there's double first cousins.
So as far as the relationship between some of these mothers
might be sisters or cousins or something closer like that.
What we need to do is we need to figure out well how can we determine this.
Now, if you looked at DNA charts before, you know that first
cousins share half as much DNA as half-siblings. And so, that should be really easy to distinguish
whether they're first cousins or half-siblings. However, if they are double first cousins,
then they share twice as much DNA as first cousins which is the amount
of DNA that half-siblings shares. So they're right in the same range.
So, just by the amount of DNA that you share with somebody, yes, you could be double
first cousins or you could be half-siblings. There needs to be another way to distinguish
between double first cousins and half-siblings. And that actually is where a Chromosome
Browser comes in very handy. Because with the Chromosome Browser,
we can see more than just how much shared DNA between two people.
We can see how that DNA is distributed and what type of sharing it is.
So for instance, here we have double first cousin DNA.
And here we have half-sibling DNA. Now, you might be wondering
is there really any difference between these two? Because as I flip back and forth between them,
can you tell any difference. Well, let me start with just one difference,
and that's really the only one that is important. In this case, and it has to do with how
these relationships are created. With half-siblings, as we've already
talked about, you have one parent that is the same but the other parent is not the same.
And so, really only that half a DNA you got from that parent is what is matching.
With double first cousins, they happen to share two sets of grandparents.
And so, they actually got DNA from one side and they got DNA from the other side as well.
Now, this is similar to siblings who obviously share parents, and so, they got DNA from them.
But with double first cousins, what you end up getting is smaller portions
of DNA than with siblings. But you still also get some of
the fully matched regions. And you can see here there's this big
solid green bar which is over a longer blue bar. The blue bar indicates it's a half-match.
The solid green indicates it is a full match. So there's a segment that's fully-matched there.
There's another smaller segment right here. And you can see actually all throughout this
there's segments of fully-matched DNA, as well as longer segments of half-match DNA.
And that's because they've gotten that DNA from both sides of the family.
And so, they can match fully in some places. On the other hand, when we look
at half-sibling DNA, because they only share
DNA from one side of their family, there are no green bars indicating a fully-matched region.
Now, there might be some places like right here, where you see a little bit of green.
But that's not long enough to be called a full-segment.
And you might not be able to see it on your screen but there's actually a couple of yellow
lines in between there indicating that it is not fully matched all the way across.
And so, this is the distinguishing characteristic between double first cousins and half-siblings.
Double first cousins are going to have fully-matched
regions whereas half-siblings are not. So if you do come across the situation where
you think you might have either a half-sibling or a double first cousin, check out on a Chromosome
Browser. And if they have fully-matched regions they are double first cousins not half-siblings.
Now, Chromosome Browsers are not going to be used for everything.
And with a lot of relationships, as far as differences between relationships,
you can actually determine them simply by how much shared DNA.
As I said at the beginning, first cousins share half as much DNA as half-siblings.
The reason why this was tricky was because you had double first cousins
as a possibility would share the same amount of DNA as half-siblings.
But as you saw how that DNA sharing is distributed is different between those two relationships.
If you want to learn something more about using your DNA for genealogy,
then watch this video up here.
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