Crosses producing a genotypic ratio of 1:2:1 in F2

The genotypic ratio in F2 refers to the ratio of different genotypes that appear in the offspring resulting from a cross between two individuals. A genotypic ratio of 1:2:1 means that in F2, the offspring will have three different genotypes in the ratio of 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive.

There are different types of crosses that can produce a genotypic ratio of 1:2:1 in F2. However, the following three crosses are the most common:

1. Cross between homozygous dominant individuals (AA × AA)
2. Cross between heterozygous individuals (Aa × Aa)
3. Cross between homozygous recessive individuals (aa × aa)

Out of these three crosses, only the third one (aa × aa) is listed as an option in the question. This is because when two homozygous recessive individuals are crossed, all their offspring will be heterozygous for the trait. When these heterozygous offspring are crossed with each other, the genotypic ratio in F2 will be 1:2:1.

Explanation of the correct answer

The correct answer to the question is option 3 (Ab × Ab) because it represents a cross between two heterozygous individuals. When two heterozygous individuals are crossed, their offspring will have three different genotypes in the ratio of 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive. This is because the heterozygous individuals can produce two types of gametes in equal proportions: one with the dominant allele and the other with the recessive allele.

The Punnett square for the cross between two heterozygous individuals (Ab × Ab) would look like this:

A b
A AA Ab
b Ab bb

In this Punnett square, the offspring would have the following genotypic ratio: 1 AA: 2 Ab: 1 bb. This ratio is equivalent to the 1:2:1 genotypic ratio in F2 that was asked in the question.

In conclusion, the correct answer to the question is option 3 (Ab × Ab) because it represents a cross between two heterozygous individuals, which can produce a genotypic ratio of 1:2:1 in F2.