Explain why maximum recombination frequency between two genes is 0.5?

Question

Explain why maximum recombination frequency between two genes is 0.5?

Answer 1

http://www.fao.org/DOCREP/003/X3910E/X39...

Linkage mapping is done by estimating the recombination fraction between loci, from the genotypes of offspring of particular matings. The further apart two loci are on a chromosome, the greater will be the frequency of recombination between them up to a maximum of 50%, the situation observed when they are sufficiently far apart on a chromosome that recombinant gametes are as frequent as non-recombinant gametes, or when they are on different chromosomes.

Also 50% recombination (0.5) is when there are 100% meioses with  SINGLE crossovers. Any more crossovers (DOUBLE) would result in less % recombination as the linked loci became unlinked and linked again.

Answer 2

Recombination occurs between genes on different chromosomes.  So, 0% recombination means the genes are very close together (or are the same gene):

----------------YZ--------------

crossover:  X

----------------yz--------------

So, the genes are close together and there are no recombinants (0%), so no crossing over occurred, so the genes are closely linked.

In the case where the genes are very far apart and there is maximum amount of crossing over occurring:

(pardon my poor drawing):

----------Y----------Z----------

crossover:  X

----------y-----------z---------

The possible single crossover products are:

Yz and yZ

The non-crossover products are:

YZ and yz

So, since the likelihood of a single crossover event occurring is the same as the likelihood of no crossover event occurring, then the two possibilities have the same likelihood:

crossover product:  Yz and yZ, 50% probability

non crossover product:  YZ and yz, 50% probability

Another way of thinking of it is to ignore chromosomes and linkage and pretend we are doing a basic Punnett Square and finding out what the possible gametes are (note that this implicitly assumes the genes are unlinked, they just didn't confuse the issue when you were first starting to learn to do the Punnett Squares in the beginning of the class).  So, the possible gametes are:

parental:  YyZz

possible gametes:

YZ  25%

yz 25%

yZ 25%

Yz 25%

So, the parental gametes (YZ and yz) are 50% of the total possible gametes and the non-parental gametes (yZ and Yz) (recombinant) are 50% of the total.  So, the maximum possible recombination frequency is 50%.