Question

A pure tall plant (TT) is crossed with a pure dwarf plant (tt).

(a) Draw Punnet squares to show:

(1) F₁ generation (2) F₂ generation.

(b) Give the phenotype of the F₂ generation.

(c) Give the phenotypic and genotypic ratio of the F₁ and F₂ generation.

Answer 1

Punnet square:

1. It is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment.
2. It is named after Reginald C.Punnett.

(a)

Below has been given a Punnett square to show F₁ and F₂ generation between a pure tall plant (TT) and a pure dwarf plant (tt).

(b)

Phenotype:

1. It is the physical expression of a trait (character).
2. The phenotype of the F₂ generation is that all will be 75% tall plants and 25% dwarf plants.

(c)

Phenotypic ratio:

It is defined as the quantitative relation between phenotypes showing the number of times frequency of one phenotype correlates with another.

Genotypic ratio:

It is the ratio depicting the different genotypes of the offspring from a test cross.

For F₁ generation:

1. Phenotypic ratio - all tall plants.
2. Genotypic ratio - all hybrids

For F₂ generation:-

1. Phenotypic ratio - 3:1 (3 tall plants: 1 dwarf plant)
2. Genotypic ratio - 1:2:1 (1 pure homozygous tall : 2 heterozygous tall : 1 homozygous dwarf)