#### especially long matching DNA segment

Ellen Caplan

Can someone please explain if there is any significance to a DNA match
that is relatively small, but that has one very long matching segment.

I have one instance where the total amount of matching DNA is only 59.9
centiMorgans, but there is one shared segment of 45.5 cMs.
I have a 2nd instance where the total amount of matching DNA is only 85
centiMorgans, but there is one shared segment of 57.6 cMs.

Thank you.

Ellen Caplan

Researching: EISENBERG, NAGLER, GINIGER, KLINGER (Mielnica & Ustye
Biscupie, Galicia)
BREGER, LIEBMAN (Belarus)
SOLOMON, PARADISGARTEN (Tukums & Mitau, Latvia)

Sarah L Meyer

Ellen,
I am in the same boat. I have one match with 4 segments (My Heritage) but
the longest is 66.6 cMs and my match has only three surnames in his tree.
And common matches >from both sides of my family. Any and all suggestions
welcomed.
Sarah L Meyer
Georgetown TX
https://sarahsgenies.com

^^^
Can someone please explain if there is any significance to a DNA match that
is relatively small, but that has one very long matching segment.

I have one instance where the total amount of matching DNA is only 59.9
centiMorgans, but there is one shared segment of 45.5 cMs.
I have a 2nd instance where the total amount of matching DNA is only 85
centiMorgans, but there is one shared segment of 57.6 cMs.

Thank you.

Ellen Caplan

Arline and Sidney Sachs

Marilyn Gelber asked what the chances of getting a matching segment of
105.5 cM with a second cousin. I estimated it is about 1% of the time.
My logic which is not simple:

Only half of the parent DNA is pass to each of the children. Between
sisters, they is a 75% chance that they get the matching DNA >from one or
both of their common parents. Then >from each grandparent, we will get on
average 25% of their DNA. Therefore second cousins should have around
4.7% (.75*.25*.25) on average common DNA. That means that second cousins
on average should share 159 cM (3400 * .047) of DNA. That DNA is not
random over the 22 chromosomes that are passed to the next generation. On
each chromosomes, segment of DNA are copy >from one parent for awhile and
then switch to the other parent. The point that this occurred is call a
crossover point. On average with each passing >from parent to child, there
are on average around 34 crossovers randomly over all of the DNA.
Therefore in three generations there are around 102 (34 * 3) crossovers.
Adding the 22 chromosomes, >from a parent we will have on average 124
segments >from the great-grandparents. This average about 15.5 (124 / 8)
segments for each with an average length about 27 cM (3400/124). This is
about one quarter the length of Marilyn Gelber longest segment with her
second cousin. Using Poisson, the chances of getting a segment four time
the average is about 1.5% of the time. Times this percentage by the
numbers of segments (124) times the percentage of common DNA (.047), on
average second cousins will have a common segments length of this length
about 1% of the time.

Sidney Sachs
Lorton, VA

Arline and Sidney Sachs

Re: especially long matching DNA segment

I was asked “why the likelihood of matching DNA would be 75% between
sisters and not 50%?”. The reason is on each chromosome pair, everyone
get one of the pair >from their mother and the other >from their father.
There is only a 50% chance of the same DNA to be pass to both children
from one parent. If they get differ DNA >from one, they could get matching
DNA >from the other parent. Therefore it another 25% match (half of 50%).
This does produce an error for some people when using DNA matches to find
their most recent common ancestor (MRCA) >from their trees and then
thinking that their matching DNA came >from the person listed on both
trees. This is only right about half the time because it have an equal
chance that the DNA came >from the partner of the MRCA. It will take at
least three people to find >from which partner the common DNA came from,
but it is usually many more.

Another common error which I made using DNA results was the numbers of
years per generation. When working with Y-DNA, I was using the average
number for everyone and not by males alone. Since males are on average
about 10 years older then their mates, using the wrong number cause an
underestimated for when a mutations on the Y-DNA took place.

Sidney Sachs
Lorton, VA

Michael Goldsmith <michaelg32@...>

In my own family, these situations seem to occur once in a while. They
present contradictory conclusions. Segments as long as 45cM or longer seem
to indicate an ancestor recent enough that recombinations haven't broken
down this large block. However, the absence of very much additional DNA
leads to a conclusion that there was plenty of time for the recombinations
to break down any measurable blocks. Where my relative was known to me, this
result has belonged to someone who was a 2nd cousin once removed or even
more distant. Why an unusually long block survives in a distant relative is
most likely due to statistical chance in a process generally thought to be
largely random. However, I wonder (without any evidence) whether some groups
of genes are harder to recombine than others. If some one has some knowledge
on this particular point, please share it.

Michael Goldsmith
Bedford, NH

Jules Feldman

My wife and her 3rd cousin have a segment of 46 on chromosome 7 in
common. We contacted him, traced the connection and last summer met
him in Warsaw where he lives.
So far this is the only relative we have discovered via DNA but it
justified taking the test.
Jules Feldman
Kibbutz Yizreel

Alan Ehrlich

I've likewise on occasion been notified of DNA matches where the
cumulative cM value wasn't particularly high, but with one very long
matching segment... plus no apparent mutual ancestor identifiable in
their/my records-based tree.

In these instances, my "guess" was that this could be attributed to
endogamy; that is cousins or uncle-niece marriages in the family
history resulting in a composite segment issuing >from intermarriage
between more than one intermediary ancestor, but originally having
its source in the still earlier mutual ancestor.

In other words, the original ancestral segment was re-created >from
partial components of the mutual ancestor's DNA which transited down
the generations separately, and then were recombined again.

Alan Ehrlich
Geneva, Switzerland

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