Re: Universal Ancestors

"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:gni0l.7318$W06.6011@xxxxxxxxxxxxxxxxxxxxxxx
Perplexed in Peoria wrote:
Harshman's 'coalescence' is actually much more relevant to
evolution than the kind of thing that the OP was talking about;
because having descendents does not necessarily mean that
any of your genes have been passed on. The naive calculation
shows that one has 2^10 = 1024 ancestors 10 generations back
(assuming no inbreeding).

Which is a bad assumption, that far back, by the way. Though marrying a
10th cousin isn't exactly inbreeding.

But it is probably the case that fewer
than 400 of those ancestors left you any of their genes. The
rest were unlucky in the chromosome crossover and segregation
lottery which takes place during reproduction.

I would be interested in your calculations of this number.

Ok. First let me make a correction. I wrote "fewer than 400".
I should have written "fewer than 700". I learned this calculation
from Joe Felsenstein over on sci.bio.evolution. But he made the
mistake of figuring the number of ancestors for a single genome's
worth of genes (23 chromosomes), whereas we actually each have
two genome's worth (46 chromosomes). I repeated Joe's mistake
when I wrote "fewer than 400".

The key fact is that crossing-over in meiosis takes place roughly
every 10^8 bp. That makes roughly 70 crossover points among
the 46 chromosomes. So, while it can be said that we received
46 'pieces' of genome from our parents (23 from each), we
got 46 + 70 = 116 pieces from our grandparents (an average
of 29 from each. Each 'piece' was inherited in toto from a single
ancestor. Those pieces have not been broken up by recombination
in the course of being passed to you.

So, we can create a table, giving for each generation the number
of ancestors and the number of pieces.

Generation # of ancestors # of chromosome pieces
01 2 46
02 4 116
03 8 186
04 16 256
05 32 326
06 64 396
07 128 466
08 256 536
09 512 606
10 1024 676


In generation #10 the ancestor count finally passes up the piece
count, so some of the ancestors do not get to contribute a piece.
And it only gets worse as you increase the number of generations.

You will have to think about it to see the reason why the piece
count only goes up by 70 with each generation, rather than
doubling like the ancestor count.

.

Relevant Pages

  • Re: Universal Ancestors
    ... than 400 of those ancestors left you any of their genes. ... I learned this calculation ... worth of genes (23 chromosomes), whereas we actually each have two genome's worth. ...
    (talk.origins)
  • Re: Evolution Stinks, or... The Nose KNOWS!
    ... What is interesting about the ofactory system in humans is that it is ... defective compared to our ancestors. ... lost or mutated so as to have lost their function in humans. ... like a fossil graveyard of genes. ...
    (talk.origins)
  • Re: How many YECs are there?
    ... One can be the descendant of a Neanderthal and ... inherit none of his or her genes. ... The mitochondrial work shows that all mitochondria in humans ... the number of our ancestors doubles with each generation. ...
    (talk.origins)
  • Re: How many YECs are there?
    ... inherit none of his or her genes. ... descended from a single mitochondria in a female long time ago. ... Actually, these studies, as far as I know, don't conclude there were any bottlenecks. ... the number of our ancestors doubles with each generation. ...
    (talk.origins)
  • the usefulness of evolution
    ... delusional creationists often assert evolution is useless. ... for drugs that can stop those genes from working. ... In our distant, amoeba-like ancestors, clusters ... known as homology? ...
    (talk.origins)