[Campbell Biology P.354] Why is the observed recombination frequency between two distant gen... | Practice Question

Why is the observed recombination frequency between two distant genes like b and vg at 17% often slightly less than the sum of the frequencies between them and an intermediate gene b-cn at 9% + cn-vg at 9.5% = 18.5% ?

  • A: The experimental data must be incorrect.
  • B: The presence of the intermediate gene (cn) physically blocks some crossovers.
  • C: Double crossovers between the two distant genes can occur, which restores the parental combination of alleles.
  • D: The distance in map units is not directly proportional to recombination frequency.

Explanation

The text explains that the b-vg frequency 17% is less than the sum 18.5% because of double crossovers. A double crossover event one between b and cn, and another between cn and vg would swap the middle segment, but leave the alleles for b and vg on the same chromatid as they started. This event produces a parental-type chromosome and is therefore not counted as a recombinant for the b-vg pair, leading to an underestimation of the true genetic distance.