Cousin DNA help

Question

My grandmother has a 3rd to 4th cousin match on Ancestry, I have already confirmed which family/surname we connect in, but would like help on how far he would be to try and find out more, I am in communication with this match (he was adopted and knows nothing about his parents)

His match is 3rd to 4th cousin 66 cm < 1% shared DNA

How far would that be? What great grandparent? He was born in 1977 and my grandmother was born in the 1956

Comments

If your grandmother and her match are 4th cousins, that would mean that they share 3rd great grandparents. If they were 3rd cousins, it would be 2nd great grandparents that they share.

Answer 1

I am far from being fluent in DNA but I would say that since this match is 'due to the age difference' young enough to be in your own (child)  generation. Since not one gets the "exactly half" of each generations dna both going up the generations or down the generations (from each parent 3400 for both or 1700 for each parent  and  dividing in half for each generation) I would look at all of the folks in the  generations from the 3nd great-grandparents on down. Reason I say this is perhaps this 3 or 4th cousin has in 'his' genetic family history may be like mine where there was a recent pedigree collapse that may look like he should be in a closer generation than he actually is that your mother did not have in her tree. Or "he" may have received more dna from on the in common great-great-great-granny/granddads than any of the other cousins. The 3xgreat-grandparent level you are dealing with 32 individuals or 16 ancestral couples. In theory each of these individual from this generation only contribute statistically a very small amount of dna to someone of your mother's generation and almost none to the generation that you are in.

Comments

Do you mean 3rd or 2nd, sorry

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I donʻt think you can use the ʻdue to age differenceʻ as a real clue.

My great grandson has an Auntie who is only 18 months older than he.

My great grandmother produced children for at least 21 years. Her husband started before he married her and therefore likely had at least 25 years of kids in the same generation.

DNA is confusing! With it, I found a cousin in my age bracket who is the descendant of that grandfather, but the DNA relationship does not explain much.

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I agree, my Dad was 20 years younger than his eldest sister and had a niece only 2 years younger than himself. There is a 30 year age difference between me and my eldest first cousins, so we could look like different generations without the paper trail

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Age difference or not, though, over several generations, the timing differences can add up.  I mean, for example, that if one descendant is the oldest child of an oldest child etc. and another is the youngest child of a youngest child etc., then two living people could appear to be in the same generation when they're not, or appear to be in different generations (a "removed" relationship) even though they're actually in the same generation.

I wish I could explain that in a less confusing way!

Answer 2

The tool at DNA painter is very useful for this.

The answer for 66 cMs is:

Assuming no pedigree collapse or endogamy, and that you're related in just one way, the furthest back you might need to go to find common ancestors for a match of 66cM is 4th-Great-Grandparent level or generation 7 on your pedigree chart.The connection may be closer.

Comments

The same tool gives the probabilities:

• 37%3C Half 2C1R 2C2R Half 1C3R
• 35%Half 2C 2C1R Half 1C2R 1C3R
• 16%Half 3C 3C1R Half 2C2R 2C3R
• 5%Half GG-Niece / Nephew † Half GG-Aunt / Uncle † 2C Half 1C1R 1C2R
• 4%4C Half 3C1R 3C2R
• 0.95%4C1R Half 3C2R 3C3R
• 0.50%5C 4C2R
• 0.50%* Great-Great-Grandchild Great-Great-Grandparent Great-Great-Aunt / Uncle Great-Great-Niece / Nephew Half Great-Aunt / Uncle Half Great-Niece / Nephew 1C1R Half 1C
• 0.45%4C3R † 5C2R † 5C3R † 6C1R † 6C2R † 7C † 7C1R † 8C † 6C 5C1R

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Here's the thing i go thru in my head to keep straight which level of grandparents go with which level of cousins:

First cousins share grandparents

Second cousins share great-grandparents

Third cousins share gg grandparents

Answer 3

At Ancestry:

I have an 8th cousin 1x removed I share 72cM (1%,  3 segments), definite endogamy.

At 23andMe:

I have a 3rd cousin 1x removed I share 78cM (1.04%, 4 segments) with, less chance of endogamy.

I have a 5th cousin I share 95cM (1.27%,  4 segments) with, and there is probable endogamy.

Answer 4

Given the age difference, probably the adoptee is in the generation after grandma. 66cM likely means that the adoptee is the son of one of grandma's 2nd cousins (in other words, a 2C1R). It's a bit high for a 3C1R, but not impossible. There are other possibilities, but it really just sounds like a 2C1R.

If grandma has some relatives on that side that she matches, you need to ask the adoptee how well he matches them too, and that will help sort it out.

Comments

By way of example, I have a bunch of 2C1Rs with roughly that number of cM: ..., 74cM, 69cM, 69cM, 64cM, 60cM, 60cM, 58cM, ...