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NEET Biology · Genetics and Evolution

Chromosome theory, linkage and crossing over

Taught in NCERT Class 12 Biology, Chapter 4: Principles of Inheritance and Variation.

2 verified questions: Easy 0 · Medium 2 · NEET level 0

Sample questions

Sample 1Medium

A diagram of two dihybrid crosses in Drosophila shows the F2 results. In cross 1 (yellow body, white eyes with wild type), parental combinations make up 98.7% and recombinants 1.3%. In cross 2 (white eyes, miniature wings with wild type), parental combinations make up 62.8% and recombinants 37.2%. What can be concluded?

  1. A

    Eye colour and wing size genes are more tightly linked because they give more recombinants

  2. B

    Both gene pairs assort independently, since recombinants appear in both crosses

  3. C

    The genes for body colour and eye colour are more tightly linked than those for eye colour and wing size

  4. D

    Body colour and eye colour genes are farther apart than eye colour and wing size genes

Show the answer
The answer is C.

Morgan found that when genes are grouped on one chromosome, some are very tightly linked and show very low recombination, while others are loosely linked and show higher recombination. The 1.3% versus 37.2% recombination shows the y and w genes are more tightly linked.

  • A.

    More recombinants mean weaker linkage, not tighter.

  • B.

    Independent assortment would give about 50% recombinants; both pairs deviate from that.

  • D.

    Fewer recombinants show the body and eye colour genes lie closer together.

NCERT: Class 12 Biology, Chapter 4, 4.3

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