Question

In Mendel's dihybrid cross study mating that involve parents that differ in two genes (two independent traits) the F2 generation phenotype ratio would be

• A. 1:2:1
• B. 9:3:1:3
• C. 9:3:3:1
• D. 9:1:3:3

Hint:

• Dihybrid cross: Involves two different genes controlling two different traits.
• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation (if genes are on different chromosomes or far apart on the same chromosome).
• F\(_2\) phenotypic ratio for a dihybrid cross (with complete dominance and independent assortment): 9 (Dominant-Dominant) : 3 (Dominant-Recessive) : 3 (Recessive-Dominant) : 1 (Recessive-Recessive).

Answer 1

The Correct Option is C

In Gregor Mendel's experiments with dihybrid crosses, he examined the inheritance patterns of two different traits simultaneously. When two heterozygous parents (each with two traits) are crossed, the traits segregate and assort independently according to Mendel's laws. Here is how the process works and the phenotypic ratio is determined in the F2 generation:

1. Parents' Genotype: Typically, a dihybrid cross involves parents with genotypes such as AaBb, where 'A' and 'a' represent alleles for one trait, and 'B' and 'b' represent alleles for another trait.

2. Gametes Formation: Each parent can produce four types of gametes due to independent assortment:

Parent	Possible Gametes
AaBb	AB, Ab, aB, ab

3. Fertilization: The gametes combine randomly during fertilization, resulting in the F2 generation.

4. Punnett Square Calculation: A 4x4 Punnett square is used to illustrate all possible combinations of offspring genotypes:

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

5. Phenotypic Ratio: The resulting phenotypes and their ratios are as follows:

• 9 with both dominant traits (e.g., A_B_): These are combinations where at least one dominant allele of each gene is present.
• 3 with the first dominant and second recessive trait (e.g., A_bb): There is at least one dominant allele for the first trait and both recessive for the second.
• 3 with the first recessive and second dominant trait (e.g., aaB_): Both recessive for the first trait and at least one dominant for the second.
• 1 with both recessive traits (e.g., aabb): All recessive alleles.

The standard phenotypic ratio for a dihybrid F2 generation is 9:3:3:1.