Question

Write the genotypes of the F1 and F2 plants?

I need an answer to part c. and I can't understand how it would expect the F2 genotypes.

Aren't the phenotypes given the F1? Therfore, aren't the F2 genotypes far too numerous because I would have to predict all possible genotypes from all possible F1 matings? I am very confused.

Answer 1

The answer is explained below by taking the specific example of pea plant in both monohybrid and dihybrid cross.

Explanation:

Let us take both monohybrid and dihybrid cross. This can be explained by taking a specific example of a plant (say pea plant).

In monohybrid cross one trait of plant is taken into consideration.

Let us take a cross between a homozygous tall and homozygous dwarf pea plants.

All the plants in F1 generation will be tall and will have same genotypes, i.e. all plants will be heterozygous tall.

Plants of F1 generation are allowed to interbreed freely among themselves to get F2 generation. The plants in F2 generation will be tall and dwarf in the ration of 3:1. But all tall plants are not similar genotypically. Homozygous tall and heterozygous tall will be in the ratio of 2:1.

Thus in monohybrid cross F1 plants have similar genotype. All are heterozygous tall (Tt).
In F2 generation genotypic ratio will be 1:2:1. , i.e. 1 homozygous tall: 2 heterozygous tall: 1 homozygous dwarf.

In dihybrid cross two traits are considered together.

Let us take a cross between Pure tall red plant and pure dwarf white pea plant.

Tall and dwarf are two alleles of height and red and white are two alleles of color of flower. Tall is dominant over dwarf and red flower color is dominant over white flower color.

All plants of F1 generation in this cross will have similar genotype, i.e. all are heterozygous tall and heterozygous red (TtRr ).

Plants of F1 generation are allowed to interbreed freely among themselves to get F2 generation. In F2 generation, tall red, tall white, dwarf red and dwarf white will be in the ratio of 9:3:3:1. Thus F2 phenotypic ratio is 9:3:3:1.

F2 tall red plants will have 4 genotypes, i.e. homozygous tall homozygous red (TTRR), homozygous tall heterozygous red (TTRr), heterozygous tall and homozygous red (TtRR), and heterozygous tall and heterozygous red (TtRr) will be in the ratio of 1:2:2:4.

Tall white will have 2 genotypes, i.e. homozygous tall homozygous white (TTrr) and heterozygous tall and homozygous white (Ttrr) in the ratio of 1:2.

Dwarf red will have 2 genotypes, i.e. homozygous dwarf and homozygous red (ttRR) and homozygous dwarf and heterozygous red in the ratio of 1:2.

Dwarf white plants will be have1 genotype, i.e. homozygous dwarf homozygous white (ttrr).

Thus overall genotypic ratio of F2 generation will be 1:2:2:4:1:2:1:2:1.