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map units, crossing-over percentages? - (Mar/16/2006 )

Can someone explain why recombination frequencies cannot exceed 50% even for genes on opposite ends of a chromosome? What about distances greater than 50 map units? Is it true that anything over 50 map units would just be a 50% frequency?



When two genes are unlinked (on different chromosomes or very far apart on the same chromosome) their recombination frequency is 50%.

Why isn't it 100%? Well, you have to first ask the question: What would happen if the homozygous parent was homozygous dominant? (it actually explains your inquiry).

So: parental types = (a +)/(a B ) and (+ B )/(a B ). Recombinant types = (a b/a B ) and(+ +/a B ) .

Using crosses like this, you can measure recombination frequency.
Recombination Frequency is defined as (no. of recombinants)/(total of progeny) and is usually expressed as a percent recombination or map units measured in centimorgans
(cM) = (r/(P+r)x100.

Recombination frequency between genes increases with increasing distance between genes and can be used to create genetic maps of chromosomes. This is why recombinantion frequency is 50%. When a and b are on different chromosomes - all 4 types of gametes are produced with equal frequency. No matter which are the parental or recombinant types, each type represents 50% of the total.

0% recombinantion frequency is when the genes are very tightly linked. and 50% recombinaiton frequency means the genes are entirely linked.
The recombination frequency can never be exactly 0%.



thanks - ga'day mate!