Re: Size of Y Chromosome

On Jun 27, 1:53 pm, Dr Umesh Bilagi <umeshbil...@xxxxxxxxx> wrote:
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

Your parameter, "chance to recombine," is misleading partly
because it is irrelevant to natural selection. What you really need is
to compare the chances of survival of a particular reproducing entity.
I think the reproducing entity that has to be examined is the gene
that does the deletion of other genes. I will give an illustrative
example, for imaginary and perhaps overly simple conditions.

Suppose we are looking at two genes on a sex chromosome, at two
different locations that I designate A and B. Site A holds genes that
deletes genes at other sites that have mutated. The gene at site A is
the reproducing entity that we will examine. The gene at site B does
something vital for the survival of the organism. I won't discuss what
it does, but it doesn't delete anything. For simplicity, I assume that
a gene at a particular site can vary, it can mutate, but it can't
change its function. I am only going to consider mutations on gene B,
and consider their effect on the natural selection of gene A.

I will further oversimplify by saying that gene A has some means
of monitoring how much gene B has mutated. Gene A has a threshold of
change beyond which it deletes genes at other sites. The gene at site
A is a policeman, a judge, and an executioner. The question is how
merciful the gene at site A can be to a mutation at B, and still
survive.

Initially, gene B is perfect. If a individual has the
unmutated gene, it will live forever. However, let us say that the
organism that inherits the mutated gene at B will die after 100
generations. What ever lineage inherits the mutated B gene is cursed.
After 100 generations, the organism that inherits the mutated gene
dies. Zero chance of survival after 100 generations. However, the
chances that the organism with the mutated B gene surviving 99
generations is excellant. Even 100% survival. This is overly
simplified, but you can fix the idea later.

Okay, suppose the A and B sites are on the section of the Y
chromosome which does not have crossover. The mutation occurs. If
there is no deletion, what are the chances for survival, on the 101th
generation, of gene A? The answer is zero. Gene A and gene B are
locked together for life, for every generation forever. For the gene
on A, the mutation at B is certain death.

So what type of gene gets selected at site A on the Y
chromosome? One with no mercy. The gene at site A has to have no
tolerance for change, otherwise it can not survive. The threshold
standards of the gene at A has to be set so high that any deviation
at B results in deletion. However, eventually gene B will mutate. So
with standards so high, the shortening of the Y chromosome is
inevitable. Gene A will keep on chopping of other genes, preferably it
is the only gene on the Y chromosome. Because on the Y chromosome, any
change means death for A.

Okay, suppose the A and B sites are on an X chromosome. Again, the
initial condition is that gene B is in perfect condition. The X
chromosome can have crossover with other X chromosomes. Let us say
(totally hypothetical), the chances strongly favor the genes at sites
A and B being separated after 10 generations. After 10 generations,
the gene at site B can look forward to a new partner at site B. Then
the same mutation occurs to the gene at site B. What are the chances
that the gene at site A will live to the 101th generation without
deletion? If there is no deletion, what are the chances for survival,
on the 101th generation, of gene A? The answer is close to 100%.
Because even if there is no deletion, and this mutated gene is a
ticking time bomb, the gene will be replaced after 10 generations. The
number of mutated copies of gene copies in other individuals is close
to zero, because it kills after 100 generations. So when crossover
occurs, after 10 generations, the X chromosome will get a perfect copy
of gene B.

So what type of gene gets selected at site A on the X chromosome?
The gene at site A can have a lot of mercy and still survive. If the
gene at site A doesn't delete the gene at B, it gets to survive
anyway. And by being tolerant,
it avoids certain types of risk. For example, an overzealous gene A
could end up accidently killing a nonmutated gene, one that is
necessary for survival. Genes at site A will probably survive best if
they are slightly tolerant. However, this also means the chromosome
isn't going to get shorter. The reason is that gene A isn't deleting
genes most of the time. So the chromosome can be the same size.

So I think that the simplified answer is that the Y chromosome is
full of overzealous and intolerant DNA repair genes. The genes on the
Y chromosome will delete mutations at the drop of a hat. That is what
has shortened the Y chromosome. The X chromosome has fewer DNA repair
chromosomes, and these are mostly tolerant of change in other genes.
Mutated genes have a much better chance of not being deleted. That is
why the X chromosome to some extent stays the same length.


.

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