Question

In Mendelian inheritance, the phenotypic ratio of a dihybrid cross is:

• A. 1 : 2 : 1
• B. 3 : 1
• C. 9 : 3 : 3 : 1
• D. 15 : 1

Hint:

A 9 : 3 : 3 : 1 ratio is always associated with a Mendelian dihybrid cross showing independent assortment.

Answer 1

The Correct Option is C

To understand the phenotypic ratio of a dihybrid cross, we first need to recall Mendel's laws of inheritance, specifically with regard to dihybrid crosses. A dihybrid cross considers two different traits simultaneously. Let's break down the process and reasoning: 

Concept of Dihybrid Cross:

A dihybrid cross involves individuals that have different alleles for two separate traits. For example, consider a cross between plants that are heterozygous for two traits such as seed color and seed shape.

The possible genotypes in the F2 generation are obtained by using the forked-line method or a Punnett Square for two traits, which predicts the phenotypes of the offspring.

Mendelian Inheritance:

In a typical dihybrid experiment, Mendel crossed peas with two pairs of contrasting traits, such as Yellow round (YyRr), and Green wrinkled (yyrr). The offspring in the F2 generation showed the phenotypic ratio.

Phenotypic Ratio in Dihybrid Cross:

The phenotypic ratio in a dihybrid cross is 9:3:3:1. This can be understood as follows:

• 9: Offspring exhibit both dominant traits.
• 3: Offspring exhibit one dominant trait and one recessive trait (first dominant, second recessive).
• 3: Offspring exhibit one recessive trait and one dominant trait (first recessive, second dominant).
• 1: Offspring exhibit both recessive traits.

Therefore, for a classical Mendelian dihybrid cross of two heterozygous parents, you observe the 9:3:3:1 phenotypic ratio.

This corresponds exactly with option 9 : 3 : 3 : 1. Other options do not fit the expected results of a dihybrid cross and are characteristic of other genetic crosses.

Phenotype	Ratio
Both dominant traits	9
One dominant trait, one recessive trait	3
One recessive trait, one dominant trait	3
Both recessive traits	1

In conclusion, the correct answer for the phenotypic ratio of a dihybrid cross in Mendelian inheritance is 9:3:3:1.

Answer 2

The question refers to Mendelian inheritance, specifically the phenotypic ratio that results from a dihybrid cross. Let's explore this step-by-step:

Definition and Background:

A dihybrid cross is a cross between two different lines/genes that differ in two observed traits. Gregor Mendel discovered the principles of inheritance, and through his experiments with peas, he established what we now call Mendel's Laws of Inheritance.

Understanding Mendel's Dihybrid Cross:

In a dihybrid cross, we're considering two traits, each with two alleles. For instance, if we assume the traits are seed shape (round vs. wrinkled) and seed color (yellow vs. green), the alleles could be:

• Round (R) is dominant over wrinkled (r).
• Yellow (Y) is dominant over green (y).

Expected Genotypic and Phenotypic Ratio:

The genotypic representation of the parent plants in a dihybrid cross is typically heterozygous for both traits (RrYy). When these are crossed (RrYy x RrYy), according to Mendel's Law of Independent Assortment, we expect the alleles to segregate independently, resulting in the following phenotypic ratio:

1. 9 — Both traits are dominant (Round Yellow).
2. 3 — First trait is dominant, second is recessive (Round Green).
3. 3 — First trait is recessive, second is dominant (Wrinkled Yellow).
4. 1 — Both traits are recessive (Wrinkled Green).

Therefore, the phenotypic ratio of a dihybrid cross is 9:3:3:1, making the third option correct.

Conclusion:

In Mendelian inheritance, the phenotypic ratio of a dihybrid cross is 9:3:3:1.