Re: frequency of crossovers at meiosis

On Feb 19, 2:19 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 ?

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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