In a di-hybrid cross, when one pair of alleles show incomplete dominance, genotypic ratio comes to

A) 3: 6: 3: 1: 2: 1

B) 1: 2: 2: 4: 1: 2: 1: 2: 1

C) 9: 3: 3: 1

D) 1: 2: 1

In a dihybrid cross involving incomplete dominance, the genotypic ratio can be quite different from what you might expect in a typical Mendelian inheritance scenario. To clarify this, let's break down the concepts involved and analyze the options provided.

Understanding Dihybrid Crosses

A dihybrid cross examines the inheritance of two different traits, each controlled by two alleles. For example, let's consider two traits: flower color (with alleles R for red and r for white) and plant height (with alleles T for tall and t for short). In incomplete dominance, the heterozygous condition (Rr or Tt) results in a phenotype that is a blend of the two homozygous phenotypes. For instance, Rr might produce pink flowers instead of red or white.

Setting Up the Cross

When we perform a dihybrid cross, we typically start with two parents that are homozygous for both traits. For example, if we cross RR TT (red tall) with rr tt (white short), the F1 generation will all be Rr Tt (pink tall). However, in the case of incomplete dominance, we need to consider how the alleles interact.

F1 Generation and Gametes

The F1 generation (Rr Tt) can produce gametes with the following combinations: RT, Rt, rT, and rt. When these gametes combine in a Punnett square, we can predict the genotypes of the F2 generation.

Analyzing the F2 Generation

In the F2 generation, we can expect a variety of combinations due to the incomplete dominance. The possible genotypes will include:

• RR (homozygous red)
• Rr (heterozygous pink)
• rr (homozygous white)
• TT (homozygous tall)
• Tt (heterozygous tall)
• tt (homozygous short)

Calculating the Genotypic Ratio

When we set up the Punnett square for the F2 generation, we find that the genotypic ratio for two traits showing incomplete dominance is not straightforward. Instead of the classic 9:3:3:1 ratio seen in complete dominance, we will see a more complex ratio. The expected genotypic ratio in this case will be:

• 1 RR TT
• 2 Rr TT
• 1 rr TT
• 2 RR Tt
• 4 Rr Tt
• 2 rr Tt
• 1 RR tt
• 2 Rr tt
• 1 rr tt

When we tally these up, we find that the genotypic ratio becomes 1:2:1 for each trait, leading to a total of 1:2:2:4:1:2:1:2:1. This matches option B from your question.

Final Thoughts

In summary, when dealing with a dihybrid cross where one pair of alleles exhibits incomplete dominance, the resulting genotypic ratio is indeed more complex than the classic Mendelian ratios. The correct answer to your question is option B: 1:2:2:4:1:2:1:2:1. Understanding these principles can greatly enhance your grasp of genetic inheritance patterns.