My textbook says that the maximum propotion of MII gametes in the ordered tetrad analysis is 2/3, and if we put it into the formula RF(recombination frequency)=MII/2(MI+MII), we can find out that the maximum RF is 1/3, instead of 1/2. The textbook persumes that each of the 4 gametes(actually 8) are randomly dominant or recessive, and gives a perfect provement, but I wonder if the persumption really exists.
I've found out that four-strand double crossing over provides 4 recombined gametes, while it's MI in this model, which may make the calculated RF smaller than real RF, but I can't figure out why it is 1/3 from the aspect of exchange and recombination.

My textbook:Genetics:Analysis of Genes and Genomes
Maybe my English isn't very good:(



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