||Fergus(s)on Y-DNA Project
- Time = Generations
- The left side of the scale on the left is the number of
generations since the founder and the right side since the present. Generations
is the natural measure of time in a family tree and in a mutation analysis. The
time scale can be converted to years but you have to assume an average number
of years between generations. This introduces additional uncertainty because
the choice of a number is equivocal, typical values used are 25 or 30 years per
- Pairwise TMRCA
- The Time To Most Recent Ancestor (TMRCA) is a measure of
relatedness and well defined in the case of a family tree wherein one simply
counts the number of generations back to the most recent common ancestor (MRCA)
shared with someone else in the tree. The simplified diagram illustrates shows
a couple of examples.
- Clade TMRCA
- Often called the age of the clade it is simply the time to the
MRCA of everyone in the clade. In the diagram below the ages of clades A and B
are 4 and 2 generations respectively. The age of a clade can be
estimated from yDNA data using the variance method of
Nordtvedt and his spreadsheet Generations111T.xlsx.
- Interclade TMRCA
- This is the the time to the MRCA of everyone in one clade with
respect to everyone in another clade.
- Founder Age
- Between A+ and "z" is a period where only one son is
propagating the line. It should be clear a clade age used as an estimate of the
founder's age would be an under estimate.
- Daughtered Out
- Son "g" had only daughters and he is said to have daughtered
out. It is counter intuitive but over time most lines if left to chance
daughter out. Suppose son "g" instead had a male line that survived. The age of
Clade A would then be two generations older as would the interclade age of A
and B. The age of Clade B would remain the same.
- Sampling Errors
- Now suppose "g" instead had a male line that survived but none
of his descendants DNA tested. The MRCA of clade A would then be "m" and the
TMRCA is equal to 6. However, then the MRCA of the people who actually tested
is still "z" and the clade age would be underestimated by 2 generations.
- If "c" and "d" had not DNA tested then the MRCA for clades A
& B would be "k" and "h". The clade ages would be equal to 3 and 1 whereas
"z" remains the MRCA of Clades A & B. It is for this reason interclade ages
can be computed more accurately than clade ages which will be underestimated.
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