Re: frequency of crossovers at meiosis
- From: Ron O <rokimoto@xxxxxxx>
- Date: Fri, 22 Feb 2008 13:49:17 -0500 (EST)
On Feb 19, 1:19 pm, psl...@xxxxxxxxx wrote:
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
.
- References:
- frequency of crossovers at meiosis
- From: pslant
- frequency of crossovers at meiosis
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