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Half-cousins and calculating years to most common ancestor with FF

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  • Half-cousins and calculating years to most common ancestor with FF

    In FF, it says there is a 4th cousin relationship.
    So, in a conventional family, where cousins share two grandparents, that would be two people who share g1g2g3g4gparents.
    However, if you only share one grandparent, when you have a 4th cousin show up in FF, would you then be sharing a single g1g2gparent?
    So if we assign 25 years to a generation, in the case of half-cousins, it is 100 years back, but in the case of full cousins, it would be 150 years or so?
    Can someone give me a more repcise understanding of this?
    I am trying to figure out about where in family trees I should be looking for the most recent common ancestor.
    Thanks!

  • #2
    We match a fourth cousin born in 1799 in FF.

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    • #3
      Originally posted by Kit-189387 View Post
      In FF, it says there is a 4th cousin relationship.
      So, in a conventional family, where cousins share two grandparents, that would be two people who share g1g2g3g4gparents.
      a 4th cousin shares common great great great grandparents.

      Originally posted by Kit-189387 View Post
      However, if you only share one grandparent, when you have a 4th cousin show up in FF, would you then be sharing a single g1g2gparent?
      No you share the same great great great grandparent, just one of them and not two is all.

      Originally posted by Kit-189387 View Post
      So if we assign 25 years to a generation, in the case of half-cousins, it is 100 years back, but in the case of full cousins, it would be 150 years or so?
      No.. It should be 125 years back but all lineages are different, if you have the family tree just count back to the 5th generation using the tree and not using 25 years per year.

      Originally posted by Kit-189387 View Post
      Can someone give me a more repcise understanding of this?
      I am trying to figure out about where in family trees I should be looking for the most recent common ancestor.
      Thanks!
      Look 4 - 6 generations back, 5 generations back being your starting point. The prediction will include a range of one generation above and below.

      All of the 4th generation children still received 1/2 of their DNA from one common great great great grandparent so if you match someone else with the same great great great grandfather then you know the DNA segment you share (the longest segment that is) is from him.

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      • #4
        Originally posted by darroll View Post
        We match a fourth cousin born in 1799 in FF.
        You mean you match a 4th cousin who believes he has traced his pedigree back to someone born in 1799?

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        • #5
          Originally posted by darroll View Post
          We match a fourth cousin born in 1799 in FF.
          My mother is still alive and well at age 94, and there are 4 living generations between her, and the youngest in her family tree.

          Judy

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          • #6
            Originally posted by mkdexter View Post
            All of the 4th generation children still received 1/2 of their DNA from one common great great great grandparent so if you match someone else with the same great great great grandfather then you know the DNA segment you share (the longest segment that is) is from him.
            I guess my question is, a cousin shares a set of grandparens (gfather+gmother).
            A half cousin shares only one grandparent.
            So, if FF says you are 4th cousins, that means it calculates, from the dna you share, that your closest ancestors were these 2 people.
            If you know that you share only a single grandparent, you are only getting half as much shared dna.
            So, you would have to be closer to that grandparent in order to share the same amount of dna as you would if you shared two grandparents.
            Ergo, a 4th cousin would be same as a 2nd half-cousin, so the shared ancestor would have to be a great-grandparent, not g-g-g-grandparents.
            At least that seems to be the way it is. So if I am looking for the ancestor I share with a "4th-cousin-equivalent" (2nd half-cousin) I would be looking for someone who lived 100 years ago. Generations are usually 25 years. I can't look for someone 4 generations ago because I have no family tree. But I need to know the rough time frame. For example, I found my grandfather, dob 1919. my father dob ca 1946. me 1970. So about 25 years per generation. So if someone is my 2nd half cousin, that common ancestor should be born ca 1895?

            I am finding records to be in 25-year bands. WWII seems to be the key event, resulting in many children by random soldiers, who of course were in their early 20s, and places our 4th cousin-equivalent ancestors being born ca 1895.
            Last edited by Kit-189387; 21 October 2010, 09:45 AM.

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            • #7
              Originally posted by Kit-189387 View Post
              I guess my question is, a cousin shares a set of grandparens (gfather+gmother).
              A half cousin shares only one grandparent.
              So, if FF says you are 4th cousins, that means it calculates, from the dna you share, that your closest ancestors were these 2 people.
              If you know that you share only a single grandparent, you are only getting half as much shared dna.
              So, you would have to be closer to that grandparent in order to share the same amount of dna as you would if you shared two grandparents.
              Ergo, a 4th cousin would be same as a 2nd half-cousin, so the shared ancestor would have to be a great-grandparent, not g-g-g-grandparents.
              At least that seems to be the way it is. So if I am looking for the ancestor I share with a "4th-cousin-equivalent" (2nd half-cousin) I would be looking for someone who lived 100 years ago. Generations are usually 25 years. I can't look for someone 4 generations ago because I have no family tree. But I need to know the rough time frame. For example, I found my grandfather, dob 1919. my father dob ca 1946. me 1970. So about 25 years per generation. So if someone is my 2nd half cousin, that common ancestor should be born ca 1895?

              I am finding records to be in 25-year bands. WWII seems to be the key event, resulting in many children by random soldiers, who of course were in their early 20s, and places our 4th cousin-equivalent ancestors being born ca 1895.
              Family Finder only reads half your DNA per match. It's called half identical by descent. That is why you have to use the same values and not try to re-calculate to compensate for a half relationship back in time. The only advantage to having both parents vs. one is when the children back then develop compound segments others end up matching to and that is rare because siblings will almost always have differing combinations of their parents DNA and when measured by half IBD testing you will see most of the combinations are different. You can match one person and not his/her full sibling due to things like that in present day. There's a lot more about this but it might be confusing so just remember not to re-calculate the generations and you should be good.
              Last edited by mkdexter; 21 October 2010, 11:32 AM.

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              • #8
                There is a difference between sum cM and block cM in the Family Finder matching process.

                You might be confusing sum cM matching. At 4th cousin you need to look at block cM matching, not sum. That is how FF works for 4th cousin.

                Full siblings share about 2600cM out of 3100cM in FF. This calculation is based on sum of blocks. Some of their blocks will be maternal and some paternal, when added all together they make a sum. When separated each block is from a single parent, not both parents. All blocks are either maternal or paternal not both. The sum is considering both but not the blocks.

                Half siblings will share about 1300 - 1500cM. They share less of a sum because they only share blocks from one parent.

                In sum cM with half siblings you do need to consider recalculation due to a differing sum from that expected with full siblings... but remember FF will detect that for you too down to 2nd cousins.

                With block cM you do not recalculate. Every block already considers the fact that it originates from one parent. As I did mention there is a rare case that a match is matching to both parents of an ancestor and that is called a compound block. It really only occurs if both matches have ancestry on both sides to each other and it is not the norm.

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                • #9
                  Originally posted by Kit-189387 View Post
                  You mean you match a 4th cousin who believes he has traced his pedigree back to someone born in 1799?
                  The family left us good records. We can only prove back to + - 1799. (per DNA)
                  The way the charts show the match is a fifth cousin. (per familytreemaker) I also figured back nine fathers. (this is when we crossed)
                  Maybe others have tested their DNA but I don't know and won't pay someone else to find out. It's not that I can't afford it, it's the fact that I don't trust their records or their record keeping. sorry,
                  Thank You..

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                  • #10
                    The # of years in a generation is just an estimate. Can

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                    • #11
                      Please retype your comment.

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                      • #12
                        I have asked ftdna this question twice by phone & got the same answer both times. If I am looking for a 1st cousin match in a family tree & some of the cousins are from a 2nd marriage of one grandparent, then these cousins (or half cousins) would be accorded the status(my word) of 2nd cousins as far as the dna is concerned, using (I emphasize), an average dna amount passed on after recombination). I was specific in wanting to use an average. This is what they told me two separate times.

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                        • #13
                          Originally posted by nolnacsj View Post
                          My mother is still alive and well at age 94, and there are 4 living generations between her, and the youngest in her family tree.

                          Judy
                          My mother is 85 and there are 5 living generations.

                          Do you mean that there are 6 living generations in total for your situation when you say that there are 4 living generations between her and the youngest?

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                          • #14
                            Originally posted by ragnar View Post
                            I have asked ftdna this question twice by phone & got the same answer both times. If I am looking for a 1st cousin match in a family tree & some of the cousins are from a 2nd marriage of one grandparent, then these cousins (or half cousins) would be accorded the status(my word) of 2nd cousins as far as the dna is concerned, using (I emphasize), an average dna amount passed on after recombination). I was specific in wanting to use an average. This is what they told me two separate times.
                            This goes back to the difference between sum and blocks in how FF makes predictions.

                            1st cousins with both grandparents in common will share about 700cM in sum. It will be a sum of grandfather's and grandmother's shared DNA between the two cousins. If there is only one grandparent there is less DNA shared, 300cM instead of 700cM for example (I am using FF numbers here not 23andMe which is different sums). FF will see the 300cM and predict the relation to be 2nd cousins even though they would be half first cousins.

                            Now that is not the same if FF sees a longest block of 12cM and a sum of 25cM. FF will predict that small amount of DNA to be a 4th cousin based on the longest block shared by two people that originated from one ggggrandparent.

                            Sums of cM DNA include paternal and maternal blocks combined (hence the need to fudge the sums for half relationships) and longest blocks just include one parent in most cases.

                            If the relation is 1st or 2nd you do need to look at the sums expected but not the case for most of the 3rd cousins and beyond since at that level FF does not use sums to make a prediction as much as it uses the longest block.
                            Last edited by mkdexter; 22 October 2010, 01:48 AM.

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                            • #15
                              Originally posted by ragnar View Post
                              I have asked ftdna this question twice by phone & got the same answer both times. If I am looking for a 1st cousin match in a family tree & some of the cousins are from a 2nd marriage of one grandparent, then these cousins (or half cousins) would be accorded the status(my word) of 2nd cousins as far as the dna is concerned, using (I emphasize), an average dna amount passed on after recombination). I was specific in wanting to use an average. This is what they told me two separate times.
                              This makes mathematical sense: share 2 grandparents=twice as much dna in common (on average) as when sharing only 1 grandparent, and half cousin should be accorded status of 2nd cousin.

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