Re: Chromosome number mismatch - Problem?

On Dec 15, 4:13 pm, Iain <monkeyboy.i...@xxxxxxxxxxxxxx> wrote:

Thanks for the link. It seems to me from the citations within that
centric fusions within the same species (or between related subspecies
in the case of mice) does not present any barrier to producing viable
offspring. I was just wondering whether the reason is because the
chromosome arms remain sufficiently homologous so that recombination
can occur, or is there some other compensatory mechanism that accounts
for continued viability?
I think it is merely because the number of genes and the percentage
of each gene in the genome have not been changed by the fusion. In
addition, the fusion doesn't even change the local environment of the
spindle. The genes see the same proteins near them as before. So there
is no epigenetic effect. So far as the genes are concerned, nothing
has changed.
As long as the fused chromosome keeps both centromeres, the
other chromosomes continue to see the fusion as two separate
chromosomes. The cell during meiosis sees only the centromeres. Two
centromeres line up with two centromeres of the unfused genes.
I read in another thread how the fused chromosome in question has
two centromeres. If so, the original mutation must have had very
little phenotypic effect. Neither the individual nor any of the
relatives knew there was something different about him.
A trisomy mutation changes both the number of genes in the cell,
and the balance of switch-on to switch-off enzymes. So trisomies tend
to be lethal or crippling. A fusion would hardly be noticed in a
single generation.
A chromosome fusion type of mutation would reduce meiosis
variability in the long run. A fusion would reduce Mendelian
segregation. Two genes which had been previously inherited separately
would now be highly correlated in occurrance. That could be important
to the grandchildren and great grandchildren of the original mutant.
However, that wouldn't have produced a hybridization barrier.
By reducing Mendelian segregation, the chances of encountering a
lethal dominant gene is reduced. However, the phenotype still wouldn't
be much different. So the reduction in genetic variability may have
been the original fitness advantage of the Robertson fusion. I don't
know what conditions would favor reduced variability in meiosis.
Mutations that don't change the relative number of each gene have
a much greater chance of surviving. If the relative fraction of each
gene in the genome is unchanged, then the phenotypic changes are
small. Some mutations that don't change relative number are:
1) Chromosome fusions (keeping both centromeres)
2) Chromosome splitting.
-the centromere has to be duplicated for this mutation to be
viable.
3) Genome doubling (all genes are duplicated at the same time.
-Can happen during meiosis. This has been repeatedly induced
in plants. Experiments clearly show that this type of mutation has
minor phenotypic effects. This is one of the few cases where the
mutation is fully documented.
4) Single gene doubling.
5) Single gene deletion.
Numbers 4 and 5 create small changes when some genes have multiple
copies.

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