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

A dihybrid cross involves parents that differ in two traits, controlled by two different genes that assort independently (Mendel's Law of Independent Assortment). Let the two genes be A/a and B/b, with A and B being dominant alleles. If the parental cross is, for example, AABB (dominant for both traits) \(\times\) aabb (recessive for both traits):

• P generation: AABB \(\times\) aabb
• Gametes from P: AB from first parent, ab from second parent.
• F\(_1\) generation: All offspring are AaBb (dihybrid, heterozygous for both genes). They will all express the dominant phenotype for both traits.

Now, consider the F\(_2\) generation obtained by selfing or intercrossing the F\(_1\) generation: AaBb \(\times\) AaBb. The gametes produced by an AaBb individual are AB, Ab, aB, ab (each with probability 1/4, due to independent assortment). Using a Punnett square (4x4 grid) for the F\(_2\) generation, we get 16 possible genotypic combinations. These combinations result in four distinct phenotypes in the following ratio, assuming complete dominance for both genes:

• 9/16 will have the dominant phenotype for both traits (e.g., A\_B\_: AABB, AABb, AaBB, AaBb)
• 3/16 will have the dominant phenotype for the first trait and recessive for the second (e.g., A\_bb: AAbb, Aabb)
• 3/16 will have the recessive phenotype for the first trait and dominant for the second (e.g., aaB\_: aaBB, aaBb)
• 1/16 will have the recessive phenotype for both traits (e.g., aabb)

Thus, the classic F\(_2\) phenotypic ratio for a dihybrid cross with independent assortment and complete dominance is 9:3:3:1. Options (b) and (d) have typographical errors in the ratio order. Option (c) correctly states 9:3:3:1. \[ \boxed{\text{9:3:3:1}} \]