The Problem of Small Families – A Segment of DNA


Have you had problems finding any
matches with DNA?
Do all of your close matches only share about fifty centimorgans or less with you?
If so, you may suffer from the problem of small
families Howdy, I’m Andy Lee with Family History Fanatics and this is a segment of DNA Be sure to subscribe to our channel and click on the bell if you’d like to be
notified about upcoming episodes family sizes have veried a lot over history but in the last 200 years
the trend has been to smaller and smaller family sizes but when it comes to DNA and genealogy how small is too small? The concept of using
DNA in genealogy is that we share DNA with our relatives but like I’ve said
before we don’t share DNA with all of our relatives Once you get out to third cousins and beyond
you share fifty centimorgans or less with these relatives
but not only that you share DNA with less and less relatives the further out
you go I did a video about this and you can check out a link in the description
below to watch that video The other difficulty in using DNA as a record is
it takes two to tango Your DNA by itself is worthless I is only when others test
and we can match your DNA with their DNA that DNA becomes valuable as a
genealogical record Now, as of 2019 only about 5% or less of the US population has had
their DNA tested Many people get discouraged with their DNA test because they’re not finding matches that they can identify a common ancestor with This has caused some people to doubt the validity of using DNA as a genealogical
tool DNA is a record and just like any other record it has its advantages and
its disadvantages mainly DNA is a record of our
relationships but it doesn’t contain information about every relationship Now, just like with a birth certificate you don’t get upset because it doesn’t
tell you what your relationship with your Aunt Margie is You wouldn’t expect it to That birth certificate doesn’t serve that purpose Same way with DNA
Once we understand what relationships we can find with DNA
and once we understand how those relationships occurred Then we can better use DNA to find relationships that we can identify a common ancestor with Autosomal DNA can reliably match
you with people out about five to seven generations Now this doesn’t mean that
you’re not going to find 8th, 9th or 10th cousins It just means that most of those
finds are really going to be by pure luck For instance I’ve actually matched
with a ninth cousin on 23andme But the reason why we were able to determine
that we were ninth cousins is because both of us had extensive trees that we
were able to see how we matched up and I was clued in on the match because there
was a last name in there that was common with some of the surnames that I already
knew were in my family Small families means that there are
less people that you can match with But how much less are we talking about Let’s make a table and show you Down the side you can see that we have the number of generations so
this corresponds to siblings first cousins second cousins third cousins and
so on now the row across the top represents
the average family size over those generations now not every generation is
going to have the same family size but this is an average to give you an idea
of how many people that are going to be that you possibly could match with I am only looking at between two and seven children on average now many families
had much more than seven children however what we’re really concerned with
is the number of children that reached adulthood and so they had the
possibility of passing on their DNA to somebody else that you may match with Let’s see how much of a difference just a couple of children really matter over
seven generations Starting with the family size of two that would mean that there are two siblings now each one of these siblings had two children each and
so there were four first cousins each of those cousins also had two
children apiece and that means that there were eight second cousins this
continues on until you have 128 sixth cousins That may sound like a lot of
people but we need to compare it to another family size so let’s look at a
family size of three Wow more than two thousand sixth cousins
just by going from a family size of two to an average family size of three That is 17 times the number of relatives which means it’s going to be 17 times
easier to identify somebody that you share a common ancestor with All because the average family size increased by just one So what about with four to
seven children Ss you can see the numbers begin to explode into the tens of thousands
and even hundreds of thousands Which means if you have a very
large family size over the last five to seven generations Your ability to identify cousins using DNA is hundreds and thousands of times easier than
somebody who only had a family size of two over the last seven generations Now let’s assume that you are down here
at the seventh generation and you want to find a cousin
that also is at the seventh generation Well your common ancestor would actually be
at generation zero They would be the parents of those
siblings from generation one Now we don’t all share DNA with all of our
relatives In fact, the further we go the less DNA we share and so at the fifth, sixth and seventh generation we only share about 15% about 5% and about 1% of
DNA So really only the last three generations so generations five, six, and seven of this could have possibly tested because DNA testing wasn’t around before
that So the first thing we need to do is multiply each one of those generations
by that 15%, 5%, 1% But then remember only about 5% of adults in the United States
have actually had their DNA tested so we need to multiply each one
of those numbers by another 5% That gives us this kind of numbers You can see that for an average family size of two you actually will probably have zero
people that have had their DNA test so that you could match with at that sixth
or seventh generation level At a family size of three it’s much better because
you can have up to five people that might have tested at each of those
levels Unfortunately if you look at all of your matches you might have thousands
or tens of thousands of potential matches at that sixth and seventh
generation level and you’re trying to find five specific ones in that That’s still a really difficult task and this exemplifies the problem with small
families There’s just not a lot of DNA all around to be able to match to So finding new relationships through DNA is going to be difficult
but not always impossible so how can you figure out whether you’re closer to two or whether
you’re closer to seven on the average family size scale Most people don’t know
all of their relationships out to seven generations so let me show you a way
that I estimate it just using three generations We’ll use my dad as a starting point and
use this chart to help us out On my dad’s family there were eight siblings
so we’re gonna put an 8 in the children box and in the next
box for the number of families that we’re looking at there was only one
family so we put the number 1 that averages out to eight children per
family which is much greater than seven so right now it’s looking like there’s
gonna be hundreds of thousands of people that I could potentially match with but
then we go down to the next generation I want to look at each of those eight
siblings and see how many kids were in those families So there’s actually 28 of us cousins at that level So that means in the first
box we’re going to put 28 plus the 8 that we already found that would be 36 And in the second box we’re going to put 8 because we’ve added 8 new families
that one family that we’ve already calculated and that’s going to give us
nine And you can see that the average family size is now only four just in one
generation we’ve cut that family size in half But four is still quite high and
it’s in the tens of thousands six cousins that we would be able to find Now the last line is going to be all of my kids plus all of my cousin’s kids
so all those cousins families we’re gonna be adding in Now I’ve added up all
the cousin’s children that I know of and if I add it to the 36 that we already
have what I get is 90 Now that gives us an average of two point four three which is a lot lower than that original eight that we started with One thing to keep in mind is I have about six to ten cousins who are still at that age where
they’re having kids or they’re just barely married and they haven’t had any
kids yet so that ninety number may increase if we increase that number to
one hundred and twenty then the average family size on my dad’s family will be
about three point two four and I ran this same calculation on my mother’s
side of the family and she actually starts out instead of with eight
siblings like my dad’s family there’s only two siblings in her family her and
her sister But even though it starts out small because their kids and their
grandkids had so many they also have a family size just over three 3.09 Then I get to my wife’s and my wife’s father was an only child and his father was an
only child so it’s looking like their family may actually be a small family
size When I calculated her father’s three generations of family size it was
2.75 so it’s still close to what these other family sizes are but you know
somewhat less Then when I calculated her mom’s family size it was only 2.6 so it
was the lowest of all of them Now again this is just an estimate based on the most recent three generations If you happen to know more information about
older generations Then you can always include that to get a better estimate of what the average family size is over seven generations So let’s take a look at our table again except instead of going from two to seven let’s actually
look at one point five to two point seven five and we’ll do that in point
two five increments Now you can see as we decrease that number that the numbers of course go down but what I want to point out is that even a 1/4 increment
will multiply the number of six cousins by 2.5 So if you’re comparing my wife’s family
which is about 2.75 for an average family size
to my family which is just over 3.0 for an average family size
Most likely I’m going to find 2.5 times as many relatives through DNA as she is And honestly that’s exactly what happened when we first tested She actually found nobody for the first couple of years there was really no
matches that we could link up to find a common ancestor whereas I was able to
probably find about a dozen ancestors in the first couple of years Now in the last couple of years with the explosion of DNA testing it has been much better
for everybody My wife has been able to link up with several people as well as
myself and I think going forward that’s just going to increase as more and more
people are tested So if you’re not finding a lot of matches take a look at
the pedigree chart that you do have and see if you suffer from the problem of
small families Unfortunately there’s no way around this The best you can do is get as many of your relatives to test So if you have any questions about small
family sizes and how it’s affecting your ability to find DNA relatives put it in the comments below and we’ll try to answer it for you and if you like this video make sure you give it a thumbs up and be sure to tell all your friends

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