Moderate

In Morgan's linkage experiment, when males and females of the F2 generation mate with each other and produce offspring, the percentage of recombinants was:

Correct answer: A. 37

  • A. 37
  • B. 22
  • C. 2
  • D. 70

Explanation

Morgan and his students have found that linked genes show varied recom­binations, some being more tightly linked than others. (i) In Drosophila, a crossing of yellow-bodied (Y) and white-eyed (W) females with brown-bodied (Y+) red-eyed (W+) males produced F1 to be brown-bodied red-eyed. On intercrossing of F1 progeny, Morgan observed that the two genes did not segregate independently of each other and, therefore, the F2 ratio deviated significantly from the expected 9: 3: 3: 1 ratio. He found 98.7% to be parental and only 1.3% recombinants. (ii) In a second cross in Drosophila between white-eyed and miniature-winged (mm) females with wild type or red-eyed normal winged (w+w+m+m+) males, all the F1 were found to be of wild type, i.e., red-eyed and normal winged. An F1 female fly was then test crossed with white-eyed and miniature winged male. 62.8% of the progeny was of parental types while 37.2% were recombinants.

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Mendel's laws explain segregation, independent assortment and the prediction of genetic ratios using alleles, genotypes and phenotypes. The chapter also covers linked genes, recombination and crossing over during meiosis, then applies pedigree reasoning to X-linked recessive traits, which show different inheritance patterns in males and females.

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