In a monohybrid cross Tt × Tt — draw the Punnett square and find ratios

Question

In a monohybrid cross between two heterozygous tall plants ($Tt \times Tt$), draw the Punnett square and determine the phenotypic and genotypic ratios of the offspring.

Solution — Step by Step

Why This Works

This is Mendel’s Law of Segregation in action. Each parent has two alleles for height ($Tt$), and these alleles separate during gamete formation (meiosis). Each gamete receives exactly one allele. At fertilization, alleles come together randomly — giving the 1:2:1 genotypic ratio.

The 3:1 phenotypic ratio arises because $T$ is dominant — it masks the expression of $t$. Whether a plant has one or two copies of $T$, it looks tall. Only plants with two copies of $t$ (homozygous recessive) appear dwarf.

This 3:1 ratio is what Mendel observed in his pea plant experiments, and it was the experimental foundation of genetics.

Alternative Method — Probability Approach

Instead of a Punnett square, use probability:

• $P(T \text{ from parent 1}) = \frac{1}{2}$, $P(t \text{ from parent 1}) = \frac{1}{2}$
• Same for parent 2

$P(TT) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

$P(Tt) = P(Tf) + P(tT) = \frac{1}{4} + \frac{1}{4} = \frac{1}{2}$

$P(tt) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

Genotypic ratio: $\frac{1}{4} : \frac{1}{2} : \frac{1}{4} = 1:2:1$

Phenotypic ratio: $\frac{3}{4}$ tall : $\frac{1}{4}$ dwarf = 3:1

Common Mistake