Dihybrid cross
- ICSE Class 10, Unit 4 Genetics
A dihybrid cross follows two pairs of contrasting traits at the same time, such as plant height and fruit colour, across two generations.
The ICSE syllabus asks for the phenotype only. Cross a tall plant with red fruit (TTRR) with a dwarf plant with yellow fruit (ttrr). Every F1 plant is TtRr. All show both dominant traits: tall, with red fruit. Cross two F1 plants, and the F2 shows four kinds:
- 9 tall with red fruit.
- 3 tall with yellow fruit.
- 3 dwarf with red fruit.
- 1 dwarf with yellow fruit.
Two of the four kinds are new mixes not seen in either parent. This follows Mendel's law of independent assortment. The alleles for one trait go into gametes independently of the alleles for the other trait.
A small example
The ratio 9 : 3 : 3 : 1 adds up to 16. Out of 16 F2 plants, you expect 9 to show both dominant traits and 1 to show both recessive ones.
Where marks go
The 2025 ICSE Biology paper used this tomato cross for 3 marks. CISCE's report on the 2025 ICSE Biology paper names three errors in the F2 ratio. A wrong ratio was given, the order was reversed, or the monohybrid ratio written instead. For the F1, "tall red" was written without showing it came from a hybrid.
For the law, the report says answers gave the law of segregation or dominance instead. It adds that writing the law "in their own words" could not capture its essence. Learn its exact wording. Answer all three parts separately; step marking explains how each part is checked.
Sources
Facts last checked against these sources on 30 September 2026.
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