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 matchingDNA >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.