Thanks to everyone for this thread. I've learnt a lot and it gives me some
good info about whether to use DNA to search for ancestors.
Thanks again.
Foxy
----Original Message Follows----
OK, I was a bit rushed in writing and did not state the x and y chromosome
part quite correctly.
But you are absolutely wrong when you state that a man's x-sperm are all
alike and his y-sperm are all alike. You are leaving out the "shuffling"
that takes place between *all* the other chromosome pairs except for the x
and y. Only the x chromosomes in the "x-sperm" are alike, and the
y-chromosomes in the "y-sperm". But *all* the other 22 chromosomes are most
likely unique. Each pair of chromosomes in the sexual cells swaps bits and
pieces of the chromosomes *before* they separate and the sperm cells are
formed with one-each of the 22 chromosomes and either an x or a y
chromosome. Same goes for the 23 pairs that the female has - they swap
pieces before separating to form the egg cells. (I'm skipping a couple of
details here). So there is perhaps *slightly* more variability from the
female side than the male, but I'd hardly think that the ratio of 23 to 22
is terribly significant.
But to get somewhat back on-topic: a male's ancestry can be compared with
another male's, since the y chromosome, except for mutations, *is*
unchanged. So if you are a descendant in the male line from some ancestor,
you will have the exact same y chromosome as all other male-line descendants
of the same ancestor. Trouble is, this doesn't eliminate possibilities like
a *different* male descendant having been the father of a child somewhere
along the line. Like: was it Thomas Jefferson or his brother's son(s) who
fathered Sally Hemmings' children - no way to tell from the genes.
There are also some other things in human cells that get passed from
generation to generation. These are called mitochondria or mitrochondrial
DNA. They are handed down *only* in the egg cell from the mother, and also
don't change except for mutations. So men never pass them on. But the
mitrochondrial DNA can be compared with anyone else's who is also descended
in the *female* line (except for the last generation, which may be female >
male) from a common ancestor.
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