Question

A pea plant with yellow seeds was crossed with a plant with green seeds. The F1 generation produced plants with yellow seeds. Explain why green seeds reappeared in theF2 generation.

Answer 1

You could initially cross a homozygous and heterozygous and get an F1 generation of 100% dominant phenotype (yellow).. If you took the two heterozygousindividuals and crossed them, you would get three dominant and one recessive phenotype in the F2 generation. Draw out some punnet squares to see thisprogression from the F1 to F2 generation. Cross Yy x YY for F1 and Yy x Yy for the F2.. Y= yellow y=green

Answer 2

Gregor Mendel performed his experiments by crossing two pure races. This means that each parent is homozygous.
In this case the yellow is dominant.
Let Y represent the dominant allele responsible for yellow seeds, let g represent the recessive allele responsible for green seeds.

Yellow seed genotype: YY (homozygous)
Green seed genotype: gg (homozygous)

When crossed, in F1, you obtain 100% Yg (Mendel's 1st law of uniqueness of hybrid F1 generation when crossing pure races)

In F2, you are performing F1xF1

This is the cross between Yg and Yg
When drawing the punnet square, you obtain the following percentages/probabilities of the F2 offsprings:
GENOTYPES:
25% YY
50% Yg
25% gg

PHENOTYPES:
75% yellow seeds
25% green seeds
thus the green seeds reappeared in F2 generation where we obtain genotypes being gg => a recessive allele may only appear when homozygous.