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Science Forum Index » Bio Evolution Forum » frequency of crossovers at meiosis
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| Guest |
 Posted: Tue Feb 19, 2008 9:19 am |
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I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
Am I thinking about this wrong ? |
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| r norman |
| Posted: Fri Feb 22, 2008 8:49 am |
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Guest
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On Tue, 19 Feb 2008 14:19:50 -0500 (EST), pslant@yahoo.com wrote:
Quote: I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
Yes it is true.
The frequency of crossover is high enough so that, for all practical
purposes, traits far apart are essentially unlinked. Distance between
genes is measured in centimorgans, where 1 cM represents a 1%
frequency of crossover. Chromosomes are several hundred centimorgans
in length. Of course, at large distances, multiple crossovers are
likely. When distances are rather large, the probability of an even
number of crossovers is very close to the probability of an odd number
so that the genes are essentially unlinked. |
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| Perplexed in Peoria |
| Posted: Fri Feb 22, 2008 8:49 am |
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Guest
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<pslant@yahoo.com> wrote in message news:fpfa4m$2e5n$1@darwin.ediacara.org...
Quote: I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
Am I thinking about this wrong ?
I can't tell. Genes on different chromosomes have a linkage (% recombination)
of exactly 0.5. Pairs of genes on the same chromosome have a linkage
which approaches 0.5 asymptotically as the distance between the genes on the
chromosome increases.
So if you were pointing out that 'asymptotically approaches' is not quite
the same thing as 'exactly equals' then you are thinking right (but a bit
too pedantically, IMHO). Otherwise, you may be thinking wrong. |
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| Lorentz |
| Posted: Fri Feb 22, 2008 8:49 am |
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Guest
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On Feb 19, 2:19 pm, psl...@yahoo.com wrote:
Quote: I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
Am I thinking about this wrong ?
Number 2 is true only if the total probability per generation of
cross over, for the entire chromosome including every gene on the
chromosome, is high. Maybe the article you are reading is assuming
this. It doesn't sound implausible to me, but that is the implicit
assumption of number 2.
Note that the total rate of crossover can be quite high, and even
be 1, while the probability of two nearby genes being split by a cross
over is quite low. When two genes in adjacent loci are split by a
cross over, all the genes on each side of this pair are split. The
probabilities of adjacent pairs splitting up add up for all the genes
intermediate between two far away chromosomes. If there are a lot of
genes between the two chromosomes, the total probability can get quite
high.
So I think the implicit assumption is simply that each chromosome
is split by crossover about once every generation. Many of the genes
on the same chromosome will not be inherited independently, but a lot
will be inherited independently. I don't know if this is true for each
and every eukaryote, but it may be. |
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| Ron O |
| Posted: Fri Feb 22, 2008 8:49 am |
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Guest
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On Feb 19, 1:19 pm, psl...@yahoo.com wrote:
Quote: I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
Am I thinking about this wrong ?
The phenomena that you are thinking about is real. Genetics has the
centiMorgan as a measure of recombination distance between two genes
or markers. A centiMorgan (cM) is equal to 1% recombination. In
theory a distance of 50 cM should mean that two genes are
independently segregating, but due to double recombination events two
genes can look like they are less than 50 cM apart out to over 60 cM.
So with enough progeny you can detect linkage out past 50 cM. You
figure this out by mapping markers or genes between the outer two and
adding up the shorter distances. The effect is less than you might
expect because there is interferance where one recombination event
will inhibit another in close proximity, so there are fewer double
recombination events than expected by chance.
Some chromosomes are very large and have quite a few recombination
events on them every meiosis. Chicken chromosome 1 may be over 500 cM
in recombination length.
Recombination length does not correlate very well with actual number
of base-pairs. Humans average about one cM every megabase-pair. On
the macrochromosomes in chickens one cM is only around 0.3 megabase-
pairs, and on the chicken microchromosomes one cM is only around 50
kb.
Chromosomes are usually around 100 cM in recombination length. Mice
have shorter ones, usually less than 100 cM for some reason.
There was some idea that recombination was necessary for accurate
meiosis. Chiasmata (probable recombination points) are observed on
all chromosomes during meiosis.
Ron Okimoto |
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| Tim Tyler |
| Posted: Fri Feb 22, 2008 8:49 am |
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Guest
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pslant@yahoo.com wrote:
Quote: I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
What - because small even numbers are more common than
small odd numbers?
"Far" is intended to mean that you don't need to bother much
about that.
--
__________
|im |yler http://timtyler.org/ tim@tt1lock.org Remove lock to reply. |
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| dhoyt |
| Posted: Sun Feb 24, 2008 9:09 am |
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On Feb 19, 2:19 pm, psl...@yahoo.com wrote:
Quote: I read that mendels second law (saying that traits are inherited
independently) if either
1) the genes regultating the traits reside on different chromosones
2) the genes are far from each other on the same chromosone
Can this really be true for 2) ?
If even nr of crossovers (including 0) is as common as odd, the traits
are more likely to follow each other than if the genes are at separate
chromosones.
Am I thinking about this wrong ?
Maybe this will help clear up your confusion.
Consider two loci (genes) that are on the same chromosome but far
enough apart so that there is ALWAYS at least a single crossover
between them. Since a crossover involves only a pair of chromatids,
there will be a pair of non-recombinant chromatids for every pair of
recombinant chromatids. (This just says that AB, ab, Ab, and aB
gametes will be equally frequent.) Therefore the %recombination
between the A and B loci will be 50% when there is always one
crossover between them.
But what if there are some meioses with a double crossover? These will
also produce the 4 gametic types in equal frequencies. Why? Because
there are three types of double crossovers: two-strand doubles, three-
strand doubles, and four-strand doubles. If you can find an older
genetic text it will show you the genetic consequences of each type of
double exchange. It turns out that the 4 gametic types, AB, ab, Ab,
and aB are produced in equal frequencies, just like in the single
exchange.
So, if two loci are far enough apart that there is a least one
crossover between them in 100% of the meioses they will show 50%
recombination, even if there are also some meioses that have a double
exchange. |
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