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Biology ratio

Mendel's Monohybrid Ratio ratio:
story, analogy and memory trick

The Mendel's Monohybrid Ratio ratio is Phenotype 3 : 1 and genotype 1 : 2 : 1 in F2. When two heterozygous (Tt) parents are crossed, the F2 offspring show a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

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BiologyClass 12NEETCUET

The Mendel's Monohybrid Ratio ratio

Phenotype 3 : 1 and genotype 1 : 2 : 1 in F2
  • TThomozygous dominant (tall)
  • Ttheterozygous (tall)
  • tthomozygous recessive (dwarf)

The story

Mendel crossed tall and dwarf pea plants, and every F1 plant was tall (Tt). When those plants were crossed with each other, the dwarf trait came back like a hidden guest: one plant in four was dwarf. Three tall to one dwarf, a ratio he found again and again.

Real-life analogy

Think of tossing two coins: HH, HT, TH, TT. If H is the dominant allele, three of the four outcomes have at least one H, which gives the 3:1 ratio.

Memory trick

Draw the Punnett square with four boxes: TT, Tt, Tt, tt. Three look tall and one looks dwarf, so the phenotype is 3:1, and the genotypes read TT : Tt : tt = 1 : 2 : 1.

Exam tip

The phenotypic ratio is 3:1 but the genotypic ratio is 1:2:1, so do not mix them up. A test cross of a Tt plant with tt gives 1:1.

FAQ

Questions about Mendel's Monohybrid Ratio

What is the Mendel's Monohybrid Ratio ratio?
Phenotype 3 : 1 and genotype 1 : 2 : 1 in F2. When two heterozygous (Tt) parents are crossed, the F2 offspring show a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
How do I remember the Mendel's Monohybrid Ratio ratio?
Draw the Punnett square with four boxes: TT, Tt, Tt, tt. Three look tall and one looks dwarf, so the phenotype is 3:1, and the genotypes read TT : Tt : tt = 1 : 2 : 1.
What should I watch out for in exams with Mendel's Monohybrid Ratio?
The phenotypic ratio is 3:1 but the genotypic ratio is 1:2:1, so do not mix them up. A test cross of a Tt plant with tt gives 1:1.

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