Cue Column | Note Column
Question | Answer
F1 genotype of TT × tt? | All Tt (heterozygous). All phenotypically tall (dominant).
F2 from Tt × Tt? | Phenotypic ratio: 3 Tall : 1 Dwarf. Genotypic ratio: 1 TT : 2 Tt : 1 tt.
Why 3:1 phenotypic but 1:2:1 genotypic? | TT and Tt are phenotypically indistinguishable (both tall). Genotypically distinct. In incomplete dominance, all three genotypes differ phenotypically → ratio is 1:2:1 for both.
Punnett Square (Tt × Tt):
| T | t | |
|---|---|---|
| T | TT (Tall) | Tt (Tall) |
| t | Tt (Tall) | tt (Dwarf) |
How does test cross work? | TT × tt → all Tt (all tall). Tt × tt → 1 Tt : 1 tt (1 tall : 1 dwarf). The 1:1 ratio reveals heterozygosity.
Summary
The monohybrid cross demonstrates both Law of Dominance (F1 all dominant) and Law of Segregation (F2 returns 3:1). The distinction between phenotypic (3:1) and genotypic (1:2:1) ratios depends on whether the heterozygote is phenotypically distinguishable. Test cross (with tt) is the diagnostic for unknown dominant genotypes.