Question

A genetically dwarf plant starts growing and becomes a tall plant after application of plant growth hormones. This genetically dwarf plant was crossed with a pure line tall plant and the resulting $F_1$ hybrid was self-pollinated. What will be the phenotype of the $F_2$ generation?

• A. All will be dwarf plants
• B. All will be tall plants
• C. $50\%$ will be dwarf and $50\%$ will be tall
• D. $25\%$ will be dwarf and $75\%$ will be tall

Answer 1

The correct option is D $25\%$ will be dwarf and $75\%$ will be tall

The genetically dwarf plant (tt) grew tall after its treatment with growth hormones.
However, genetically, it is a dwarf plant.
Hence, the gametes produced by this plant carried ' t 'allele for the character, height.
This dwarf plant was crossed with a pure line tall plant. Hence, the gametes produced by the pure line tall plant carried T allele for height.

When these plants are crossed, all the $F_1$ hybrids are tall (Tt). When the $F_1$ hybrids are self-pollinated, then the tall and dwarf plants are produced in the ratio of 3 : 1 (tall : dwarf).



$\Rightarrow$ Out of 4 plants, no. of dwarf plants = 1
$\Rightarrow$ Out of 100 plants, no. of dwarf plants $=\left(\frac{1}{4}\right)\times 100=25$
Hence, $25\%$ of plants in the $F_2$ generation will be dwarf.

$\Rightarrow$ Out of 4 plants, no. of tall plants = 3
$\Rightarrow$ Out of 100 plants, no. of tall plants $=\left(\frac{3}{4}\right)\times 100=75$
Hence, $75\%$ of plants in the $F_2$ generation will be tall.