Question

If a heterozygous tall plant (Tt) is crossed with a dwarf plant (tt), what will be the phenotypic ratio of the offspring?

• A. 1 Tall : 3 Dwarf
• B. 1 Tall : 1 Dwarf
• C. All Tall
• D. All Dwarf

Hint:

A Punnett square is a powerful tool to visualize genetic crosses and determine the probability of offspring genotypes and phenotypes. In cases where you have dominant and recessive traits, always check the genotype of the parents before drawing the Punnett square!

Answer 1

The Correct Option is B

In this genetic cross, we are dealing with a heterozygous tall plant (Tt) crossed with a dwarf plant (tt). The trait for tallness (T) is dominant over the dwarf trait (t), which is recessive.
The first step is to set up the Punnett square to determine the possible offspring genotypes. Let's analyze the parental genotypes:

• The first parent is heterozygous tall, Tt.
• The second parent is homozygous dwarf, tt.

We will now cross these genotypes:
\[ \text{Tt} \, \times \, \text{tt} \]
The Punnett square looks like this:
\[ \begin{array}{c|c|c} & T & t \\ \hline t & \text{Tt} & \text{tt} \\ t & \text{Tt} & \text{tt} \\ \end{array} \]
Now, let's interpret the results:

• Half of the offspring will inherit the T allele (tall) from the Tt parent and the t allele (dwarf) from the tt parent, resulting in a Tt genotype (which expresses the tall phenotype because T is dominant).
• The other half will inherit the t allele from both parents, resulting in a tt genotype (which expresses the dwarf phenotype because both alleles must be recessive to show this trait).

Thus, the genotypic ratio is 1 Tt : 1 tt, and the phenotypic ratio is 1 Tall : 1 Dwarf.
This means there will be an equal number of tall and dwarf plants among the offspring.
In summary:

• Tt (tall) will be dominant and appear as tall plants.
• tt (dwarf) will be recessive and appear as dwarf plants.