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FTDNATiP™ & Generation length

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  • FTDNATiP™ & Generation length

    I think the new(?) FTDNATiP™ looks like a useful addition to the site. As a recent "convert" from the Genographic Project I await my 37 marker upgrade so I can use it.

    But. . . .

    I'm sure that FTDNA has its reasons, but I'm surprised that they are using the "25 years per generation" assumption. I thought the consensus was drifting more towards longer generations (my pedigree suggests something closer to 35 years).

  • #2
    vin the one thing i always stress is it doesnt have to be the 600 yrs my son is 33% at 100 lol an exact at 25

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    • #3
      Originally posted by vineviz
      I'm sure that FTDNA has its reasons, but I'm surprised that they are using the "25 years per generation" assumption. I thought the consensus was drifting more towards longer generations (my pedigree suggests something closer to 35 years).
      The best thing to do is forget the actual years and think of it in terms of generations. Each 100 years equals 4 generations, so when FTDNA says 200 years it is 8 generations, when they say 300 years it is 12 generations, etc. That way you can plug in whatever number of years per generation you like. You can see how I have set it up at http://blairgenealogy.com/dna/FTDNATiP.html

      John

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      • #4
        I have made a case in previous postings that the real average male generation length is more like 33 years. The fact that some scientists, like Spencer Wells, continue to use 20-25 years as an average generation length has caused him to suggest that the y-line left Africa as recently as 55,000 years ago, while mtDNA suggests 80,000 years ago. This can be easily reconciled if one shifts the average male generation length toward the 30-35 year range.

        Timothy Peterman

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        • #5
          My reasoning for the longer generation length goes as follows:

          1. Let's assume that for most of human history, men married in their late teens or early 20s.

          2. Let's also assume that, as I learned in school, life in the not too distant past was nasty, brutish, & short. Very few men lived to age 45. Lots of babies died in infancy.

          3. When I look at my family tree, I don't find very many direct ancestors that died in infancy

          4. Matter of fact, when I go back a few hundred years, on most sides of the family, I encounter ancestors who lived to be quite old -60, 70, even 80 or more. Most of these had huge families. They started young -early 20s. If the wife died in childbirth, the husband simply remarried & continued to sire children -usually well into his 40s or even 50s.

          5. The truth is that only a tiny percentage of the families a few hundred years ago were like the one described in #4.

          6. But thanks to good ole' natural selection, most people living today are descended from that tiny percentage. So when modern people find a spouse, chances are the spouse is also descended from ancestors in that tiny percentage.

          Because of all of the above, the average male generation length is more like to 30-35. Your chances of being descended from an older son or a younger son are about equal.

          Also, men with multiple wives (in societies where polygamy was allowed) tended to acquire them later in life -when they had clout & respect within their community. There was a vast disparity in the number of children that men sired -a lot had zero, while other like Genghis Khan had hundreds.

          Back in prehistoric days, I think it would be reasonable to surmise that women were married shortly after reaching puberty & probably to men that were somewhat older than they were. Women tended to have similar sized families of 5 to 10 children, unless they died in childbirth.

          Primate biologists attribute the moderate dimorphism in body size between men & women to the above stated reproductive rates. Large males with strength & cunning sire a lot more children than small males that are comparatively weak. Females tend to have the same number of children, regardless of whether they are large or small, so there is no selective pressure for size. Sexual dimorphism is the evidence for the disparity in male reproductive rates in prehistoric times.

          Based on the above, one could surmise that among reproductively successful women, children were born between ages 15 and 30 -each additional child was another chance for her to die in childbirth. So 22 or 23 was probably the average female generation length. One could surmise that among reproductively successful men, children were born between ages 20 and 45. So 33 was probably an average male generation length.

          Timothy Peterman

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          • #6
            My Pedigree

            For my own family, counting branches where I have accurate birth dates at least six generations back, the average generation length is about 31 years (this crosses back and forth between paternal and maternal lines since I looked at all 4th great grandparents).

            Interestingly, the shortest average generation length to 19 male ancestors was 27 years and the longest was 37 years. One man's family does not make a global average but still . . . .

            Your advice to adjust the numbers given by the FTDNATiP is well-taken and easy to do. I just that choosing to use the 25 year number was an odd choice.

            Comment


            • #7
              I think it would be interesting to see a lot of participants in this message board report the average male generation length: from paternal ancestor to his child (male or female) that one is descended from. Is there anyone that can really report an average of 20 to 25 years?

              Timothy Peterman

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              • #8
                Generation length

                Originally posted by T E Peterman
                I think it would be interesting to see a lot of participants in this message board report the average male generation length: from paternal ancestor to his child (male or female) that one is descended from. Is there anyone that can really report an average of 20 to 25 years?

                Timothy Peterman
                I have to agree with you. Just based on a general knowledge of how people marry and have children and elementary arithmetic, I've always wondered why the estimate for a generation is not 30 years. I've never "done the math" on my family tree, but here goes.

                My father was 35 when I (1st child) was born. My grandfather was 43 when my father (7th child) was born. My great-grandfather was about 30 when my grandfather (2nd child) was born. I can't say for the generations before that in this line, because my great-grandfather was abandoned at birth and at this point I don't know who his parents were. But the three generations I've cited give an average age of 36 and an average of 3.33 for the order in the family of the child born.

                Those numbers are probably higher than average. I can take my paternal grandmother's line for comparison. My grandmother was 35 when my father (7th child) was born. My great-grandfather was 25 when my grandmother (1st child) was born. My gg-grandfather was 33 when my great-grandfather (1st child) was born. (There may have been a previous marriage.) My ggg-grandfather was 29 when my gg-grandfather (3rd child) was born. The averages for this line are 30.5 for age of the parent and 3 for order of the child.

                It looks to me like 30 is a much better estimate for the average length of a generation.

                Mike Maddi

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                • #9
                  Originally posted by T E Peterman
                  I think it would be interesting to see a lot of participants in this message board report the average male generation length: from paternal ancestor to his child (male or female) that one is descended from. Is there anyone that can really report an average of 20 to 25 years?

                  Timothy Peterman
                  Going back 5 generations in my direct paternal Y-DNA line to my 3rd great grandfather, whose birthdate I know exactly, my average per generation is 38 years. My father was the youngest of 8 children, with his eldest sibling 26 years older. I'm from a long line of youngest children or sons. This leads to some interesting relationships. Most of the exact 3rd and 4th cousins in my tree are long since gone. Sometimes it looks like I'm living in the wrong century.

                  Bill Hurst

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                  • #10
                    MY DADS dad BORN 18 Sep 1868
                    MY DAD 11 FEB 1911
                    ME 25 JUL 1948
                    MY SON 24 DEC 1983

                    But then i know a friend at 45 was a ggranddad
                    all parents gave birth at 15
                    so go figure 25 makes it even in 100

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                    • #11
                      Jim,

                      For 25 to be a nice average because of 15 year old parents, one would have to maintain that 15 year old fathers are as common as 35 year old fathers. All the data submitted thus far suggests that 30 to 35 is a better average. The mean is probably about 22 or so on the low end to 40 or 45 on the high end, with half the results being below or above the mean.

                      Timothy Peterman

                      Comment


                      • #12
                        Yup, count generations NOT years. In my own family the spread between the men of one generation was 15 years. Then add that the elder got caught up in WWI thus having two families one before WWI and another after, whilst the younger had his family at one time.
                        We have a spread of 35 years in our 4th generation! Some are just beginning their families, while some are just beginning retirement!

                        Jeffrey Stewart

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                        • #13
                          Originally posted by T E Peterman
                          Jim,

                          For 25 to be a nice average because of 15 year old parents, one would have to maintain that 15 year old fathers are as common as 35 year old fathers. All the data submitted thus far suggests that 30 to 35 is a better average. The mean is probably about 22 or so on the low end to 40 or 45 on the high end, with half the results being below or above the mean.

                          Timothy Peterman

                          but when you go past paperwork and any history 35 is very old and 15 is mature like romeo
                          you ,Me and no one can win this debate

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                          • #14
                            Jim,

                            You are missing my point. True, a thousand or 10,000 years ago, without the rituals of school & other passings that lead to adulthood today, the average man probably married (or began to sire children) at a younger age. True, way back then, for the average man, 35 was old.

                            However, thanks to natural selection (which could be called the "law of differential reproduction"), none of us living today are descended from the average man. We are descended from that small minority that were exceptionally good reproducers. They may have begun siring children when they were young, but they continued until they were quite old. The average age of fatherhood for this small minority of men was probably 30 to 35 in agricultural times & may have been 25 to 30 in prehistoric times.

                            If geneticists like Spencer Wells would factor in the longer generation length for men, they would probably resolve the discrepancy between the suggested dates of departure from Africa for non-African men & women. Mtdna suggests 80,000 years ago. Y-dna, according to Wells, suggests 55,000 to 60,000 years ago.

                            Timothy Peterman

                            Comment


                            • #15
                              Timothy,

                              Thank you. You have resolved a problem that kept me awake nights. I always wondered what those Aboriginal women did in Australia for the 10,000 years they had to wait between the time they arrived there and the time their men showed up.

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