Monohybrid cross
- ICSE Class 10, Unit 4 Genetics
A monohybrid cross is a cross between two parents that differ in one pair of contrasting traits, such as tall and dwarf pea plants.
Cross a pure tall plant (TT) with a pure dwarf plant (tt). Every plant in the first generation, F1, is Tt. All of them are tall. Now cross two F1 plants. Each passes on T or t, so the second generation, F2, can be:
- TT, Tt, Tt and tt.
- Phenotype ratio: 3 tall to 1 dwarf.
- Genotype ratio: 1 TT to 2 Tt to 1 tt.
Two of Mendel's laws explain it. Law of dominance: of a pair of contrasting alleles, only one shows. Law of segregation: the two alleles of a pair separate when gametes form, so each gamete gets one.
A small example
Round seed (R) is dominant over wrinkled (r). Cross two Rr plants. You expect 3 round to 1 wrinkled.
Where marks go
The 2024 ICSE Biology paper showed this seed-shape cross as a diagram. It asked which allele is dominant and which recessive. CISCE's report on the 2024 ICSE Biology paper says answers copied the diagram and did not state the seed shapes in words. For what the 3 : 1 ratio shows, the report names "genotype" written where "phenotype" was meant.
In the ICSE 2027 specimen paper, stating the law of segregation earns 2 marks. The draft marking scheme's answer: pairs of alleles separate during gamete formation.
Sources
Facts last checked against these sources on 30 September 2026.
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