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In cucumbers, dull fruit (D) is dominant over glossy fruit (d), orange fruit (R) is dominant over cream fruit (r), and bitter cotyledons (B) are dominant over non-bitter cotyledons (b). The three characters are encoded by genes located on different pairs of chromosomes. A plant homozygous for dull, orange fruit and bitter cotyledons is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons. The F1 are intercrossed to produce the F2 .

a. Give the phenotypes and their expected proportions in the F2 .
b. An F1 plant is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons. Give the phenotypes and expected proportions among the progeny of this cross.

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Answer:

a)

  • 27/64 Dull, Orange, Bitter
  • 9/64 Dull, Orange, non-bitter
  • 9/64 Dull, cream, bitter
  • 9/64 glossy, orange, bitter
  • 3/64 dull, cream, non bitter
  • 3/64 glossy, orange, non-bitter
  • 3/64 glossy, cream, bitter
  • 1/64 glossy, cream, non bitter

b)

  • 1/8 Dull, Orange, Bitter
  • 1/8 Dull, Orange, non-bitter
  • 1/8 Dull, cream, bitter
  • 1/8 glossy, orange, bitter
  • 1/8 dull, cream, non bitter
  • 1/8 glossy, orange, non-bitter
  • 1/8 glossy, cream, bitter
  • 1/8 glossy, cream, non bitter

Explanation:

  • Dull fruit (D) is dominant over glossy fruit (d)
  • Orange fruit (R) is dominant over cream fruit (r)
  • Bitter cotyledons (B) are dominant over non-bitter cotyledons (b)

The three genes are located on different chromosomes.

a) A plant homozygous for dull, orange fruit and bitter cotyledons (DDRRBB) is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons (ddrrbb).

The F1  (DdRrBb) are intercrossed (DdRrBb X DdRrBb) to produce the F2.

The genes are on different chromosomes so they assort independently during meiosis. If you do a Punnett Square of this cross, you will see that the phenotypic frequencies are:

  • 27/64 DRB (Dull, Orange, Bitter)
  • 9/64 DRb (Dull, Orange, non-bitter)
  • 9/64 DrB (Dull, cream, bitter)
  • 9/64 dRB (glossy, orange, bitter)
  • 3/64 Drb (dull, cream, non bitter)
  • 3/64 dRb (glossy, orange, non-bitter)
  • 3/64 drB (glossy, cream, bitter)
  • 1/64 drb (glossy, cream, non bitter)

 

b) An F1 plant  (DdRrBb) is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons (ddrrbb).

In this case, the homozygous recessive plant can only produce 1 type of gamete (drb). The phenotype of the offspring will depend entirely on the type of gametes the F1 plant produced, of which there are 8 types:

DRB, DRb, DrB, Drb, dRB, dRb, drB, drb.

As the genes are independent, all of these gametes have the same probability of being produced: 1/8.

The phenotypes and their proportions will be:

  • 1/8 DRB (Dull, Orange, Bitter)
  • 1/8 DRb (Dull, Orange, non-bitter)
  • 1/8 DrB (Dull, cream, bitter)
  • 1/8 dRB (glossy, orange, bitter)
  • 1/8 Drb (dull, cream, non bitter)
  • 1/8 dRb (glossy, orange, non-bitter)
  • 1/8 drB (glossy, cream, bitter)
  • 1/8 drb (glossy, cream, non bitter)

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