Question

In a cross between $$AABB\times aabb$$ the ratio of $$F_2$$ genotypes between $$AABB,AaBB,Aabb,$$ and $$aabb$$ would be
A. $$9:3:3:1$$
B. $$2:1:1:2$$
C. $$1:2:2:1$$
D. $$7:5:3:1$$

Answer 1

Hint: The genotype of the $$F_2$$ generation of a dihybrid cross with homozygous parents. In addition, from the Punnett square of the $$F_2$$ generation, the homozygous parents reappear again. One plant with $$AABB$$ genotype and one plant with $$aabb$$ genotype reappear in the Filial 2 generation.

Step by step answer:Consider two dihybrid homozygous parents. The dominant parent would be with the genotype $$AABB$$ and the recessive parent would have the genotype $$aabb$$. On performing the cross between these two parents the Filial 1 generation would be as follows –

The dihybrid parents segregate into the homozygous gametes. These gametes that fuse together to form the heterozygous offspring. Hence, all the offspring produced are Heterozygous$$AaBb$$. Now in the Filial 2 generation, a cross between any two plants of the Filial 1 generation is carried out.
The segregation of gametes will be as follows –

Performing a cross like this is confusing and hectic. Hence, this cross is performed using a Punnett square. The gametes are arranged on two sides of the box and then the cross is carried out as follows –

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

There are 16 possible combinations of offspring. From this Punnett square, the following conclusions can be drawn –
There are 1 offspring of the genotype$$AABB$$.
There are 2 offspring of the genotype $$AaBB$$.
There are 2 offspring of the genotype $$Aabb$$.
Lastly, there are 1 offspring of the genotype $$aabb$$.
So, the ratio of $$F_2$$ genotypes $$AABB:AaBB:Aabb:aabb$$ is $$1:2:2:1$$.
Hence, the correct answer is option C

Note: Genotype means the composition of genes or the genetic constituent of any character. The genotypic ratio is the ratio obtained from the various kinds of genotypes obtained from all the offspring of a certain generation. The genotypic ratio of a dihybrid cross is $$1:2:2:4:1:2:1:2:1$$. The number 1 in the ratio represents all the genotypes that have only homozygous alleles. The number 2 in the ratio represents all the genotypes that have one homozygous and one heterozygous allele. The number 4 in the ratio represents all the genotypes that have only Heterozygous alleles.