F2 Genotypic and Phenotypic Ratios — NEET Biology
✅ Asked in NEET 2016From the Punnett square it is easily seen that 1/4th of the random fertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt. Though the F1 has genotype of Tt, but the phenotypic character seen is 'tall'. At F2, 3/4th of the plants are tall, where some of them are TT while others are Tt. Externally it is not possible to distinguish between the plants with the genotypes TT and Tt. Hence, within the genotypic pair Tt only one character 'T' tall is expressed. Hence the character T or 'tall' is said to dominate over the other allele t or 'dwarf' character. It is thus due to this dominance of one character over the other that all the F1 are tall (though the genotype is Tt) and in the F2 3/4th of the plants are tall (though genotypically 1/2 are Tt and only 1/4th are TT). This leads to a phenotypic ratio of 3/4th tall: (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1.
NTA tests whether students understand that in a monohybrid cross, F2 generation shows a 1:2:1 genotypic ratio (1 TT : 2 Tt : 1 tt) but a 3:1 phenotypic ratio (3 tall : 1 dwarf) due to dominance. The common mistake is confusing the two ratios or assuming equal expression of all alleles. Remember: genotypic ratio depends on allele combinations, while phenotypic ratio depends on which alleles are dominant. Only one dominant allele needed to show the dominant trait, so both TT and Tt appear tall. This concept is fundamental to understanding inheritance patterns and frequently appears in MCQ and genetics problems.
A tall true-breeding garden pea plant is crossed with a dwarf true-breeding garden pea plant. When the F₁ plants were selfed, the resulting genotypes in the ratio of: (NEET 2016)
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