Dihybrid inheritance — CCEA A2 Biology Practice Questions

Question 1

In a species of plant, tall (T) is dominant to dwarf (t) and red flowers (R) are dominant to white (r). Two plants heterozygous for both genes were crossed. Using a genetic diagram, determine the expected phenotypic ratio of the offspring.

Accepted Answer

Parents: tall red (TtRr) × tall red (TtRr). Gametes from each parent: TR, Tr, tR, tr. A 4 × 4 Punnett square gives 16 offspring: 9 with at least one T and one R (tall red), 3 with at least one T and rr (tall white), 3 with tt and at least one R (dwarf red), and 1 ttrr (dwarf white). Expected phenotypic ratio = 9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white.

Question 2

A dihybrid cross produced offspring in a ratio of 9 : 3 : 4 rather than the expected 9 : 3 : 3 : 1. Suggest an explanation for this result.

Accepted Answer

The two genes are interacting, a phenomenon called epistasis. When the epistatic gene is homozygous recessive it masks the expression of the second gene, so the double recessive class and one of the single recessive classes have the same phenotype. Those two classes (3 and 1) are counted as one group of 4, giving 9 : 3 : 4 instead of 9 : 3 : 3 : 1.

Dihybrid inheritance: CCEA Biology practice questions

Sit the questions

In a species of plant, tall (T) is dominant to dwarf (t) and red flowers (R) are dominant to white (r). Two plants heterozygous for both genes were crossed. Using a genetic diagram, determine the expected phenotypic ratio of the offspring.

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