Re: Size of Y Chromosome

In article <f5u870$2eva$1@xxxxxxxxxxxxxxxxxxx>,
Dr Umesh Bilagi <umeshbilagi@xxxxxxxxx> wrote:
Regarding Y chromsome I will give better expanation I think my
posting
was less comprehensive
.....
I read that size of Y chromosome has reduced because it did not have
partner at all. but X chromosome has not reduced, they say that it is
because, it has partner in female sex to recombine.
If Chromosome don't get partner to recombine than there is tendency
to
delete genes which may be harmful produced by mutation. which intern
will reduce the size of chromosome is the logic.
Now if X chromosome gets only 50% chances to recombine then I think
size of X chromosome should also reduce to a size in between Y and
its
initial size in reptiles
If gene deletion by natural selection if it is correct why there is
preservation of X chromosome size when it has only 50% Chance to
exchange with its real partner. It should also have 50% disadvantage.
How is this?
This is my guess. Please comment

The phenomenon you are thinking of is Muller's Ratchet. Random
associations between genes on the Y will reduce the effectiveness of
natural selection. For example, if two loci on the Y have deleterious
mutants and these become negatively associated so that each chromosome
has one or the other of the two deleterious alleles, then natural
selection cannot eliminate one deleterious allele withoug causing the
other to fix. However this does not happen easily on the X because even
a modest amount of recombination will produce some chromosomes that
have both favored alleles, and natural selection at one locus will be much
less likely to be opposed by natural selection at the other.

The technical paper on this explanation of Y chromosome degeneration
is
Charlesworth, B. 1978. Model for the evolution of Y chromosomes and
dosage compensation. Proceedings of the National Academy of Sciences,
U.S.A. 75: 5618-5622.

----
Joe Felsenstein joe@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Department of Genome Sciences and Department of Biology,
University of Washington, Box 355065, Seattle, WA 98195-5065 USA



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