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I share a DNA match with a male person of 152cM coming up as a 3rd cousin. I also share a match with his half brother but much Lower at 45cM. These two share the same father but different mothers. I’ve since learnt that I am the Great Grand son of their Grand father. ( Grand father fathered my Grandmother). These two males are of the same generation , share same father and share same Grand father ( my Great Grand father )
Why is one so far removed from than the other ?
I see two possible reasons.
The Shared cM Tool at DNA Painter says that a 3rd cousin on average shares 73 cM, but the range can be between 0 and 234 cM, so as a first check that shows that both values are possible.
Now people get 50% of each parents' DNA. But they get a different random 50%. Let's say that your 3rd cousin's and his half brother's father, who is your 2C1R shares 225 cM with you. That's on the high side (average 122 cM) but is within the range of 14 cM to 353 cM of the Shared cM Tool.
Now just as an illustration, let's break up the father's matching DNA with you into 15 cM segments. Then there are 15 of them. 15 x 15 = 225.
Now why don't you flip a coin 15 times twice. The first time, you get 10 heads. 10 x 15 = 150 cM. The second time, you only get 3 heads. 3 x 15 = 45 cM.
That's the random way it can happen.
The second possibility is if you happen to be related to your 3rd cousin in an unknown way through his mother. In that case, you'll have extra cM from that relationship on your mother's side but you will not have any more with his half brother.
What you can do to check this is look at your common matches with your 3rd cousin and compare them to your common matches with his half brother. Do you know how you're related to any of the common matches that you only have with your 3rd cousin? Are those relations through the grandfather who fathered your grandmother? Or are they through some other line on your side. If they are through some other line, that may be the line through which you have your unknown relationship with your 3rd cousin's mother.
Alternatively, if your DNA test was taken at a site which shows you your segments matches, then you can look at the individual segments where you match your 3rd cousin and those where you match his half brother. Use the tools at the site to find other people who triangulate with each of your segment matches with your 3rd cousin and his half brother. See if you can determine the common ancestor on each segment. DNA Painting is a technique to help you do this, which you can do at DNA Painter. The goal is the same as in comparing the shared matches: to find some other line that you are related to your 3rd cousin through his mothers' side.
