Multiple choice

In Drosophila melanogaster, cherub wings (ch), black body (b) and cinnabar eyes (cn) are recessive to their corresponding alleles (represented as ch+, b+ and cn+, respectively) and are all located on chromosome 2. Homozygous wild type flies were mated with cherub, black and cinnabar flies and the resulting F1 females were test crossed with cherub, black and cinnabar males. The following progenies were produced from the test cross: ch b+ cn 110 ch+ b+ cn+ 780 ch+ b cn 70 ch+ b+ cn 6 ch b cn 769 ch b+ cn+ 60 ch+ b cn+ 111 ch b cn+ 9 Total 1915 Of these three genes, which one is in the middle?

  1. The locus that determines cherub wings

  2. The locus that determines cinnabar eyes

  3. The locus that determines black body

  4. Cannot be determined

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In a three-point cross, the double crossover classes have the lowest frequency. Here, ch b+ cn+ (60) and ch+ b cn+ (111) are the double crossovers. Comparing these to the parental types (ch+ b+ cn+ at 780 and ch b cn at 769), we see that cn+ stays with ch+ (parental) and with ch (double crossover), meaning cn is in the middle. The gene order is ch-cn-b.

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