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We got this task:

There are 3 genes (acb) on one chromosome (linked inheritance). Distance a-c = 12cM, a-b=16cM. Probability for double crossing-over (CO) is 0,6%. Find ratio of genotypes of descendants (AcB/acb X acb/acb).

I don't know whether the 0,6% for double CO is included in the 16% (16cM) probability.

I think it is not, so I got these ratios: No CO and double CO: 42% AcB/acb, 42% acb/acb, 1x CO: 8% Acb/acb, 8% acB/acb

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Centi-Morgan (cM) is based on observation not precise measurements. Now since double crossing-overs (and actually any arbitrary even number of crossing-overs) revert gene combination to the parental type, resulting lower recombinant frequency. So in your case since the chance (or the frequency if you wish) for double COs is 0.6%, you have to subtract this from parental genotypes and add it to the recombinants.

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