7.3.2 Inheritance of Two Traits by Dihybrid Cross

Introduction

In studying the inheritance of two traits at once, Mendel performed a dihybrid cross, which involves crossing individuals that differ in two traits. This topic explains Mendel's procedure, observations, and conclusions from his dihybrid cross experiments.

Definition

Dihybrid cross: A genetic cross between individuals that differ in two traits, such as seed shape and seed color.

Main Idea

In studying the inheritance of two traits at once, Mendel performed a dihybrid cross, which involves crossing individuals that differ in two traits.

Core Concept

Mendel's Dihybrid Cross

Phenotypic Ratio in F₂ Generation

Detailed Explanation

1. Mendel's Procedure and Observations

In one of his experiments, he examined seed shape (round or wrinkled) and seed color (yellow or green).

P Generation:

• True-breeding round yellow plant (RRYY)
• True-breeding wrinkled green plant (rryy)

F₁ Generation:

- When Mendel crossed a true breed round yellow plant (RRYY) with a true breed wrinkled green plant (rryy), all the F₁ offspring were round and yellow, showing the dominant traits.

F₂ Generation:

• When he self-fertilized these F₁ plants to produce the F₂ generation, he expected a 3:1 ratio like in his monohybrid cross.
• Instead, he observed four combinations: round yellow, round green, wrinkled yellow, and wrinkled green in a 9:3:3:1 ratio.

Simple explanation: When Mendel crossed a round yellow plant with a wrinkled green plant, all the offspring in the first generation were round and yellow. When these offspring self-fertilized, the second generation had four combinations: round yellow, round green, wrinkled yellow, and wrinkled green in a 9:3:3:1 ratio.

2. Conclusion/Interpretation of the Results

This result, which included new combinations (round green and wrinkled yellow), led Mendel to conclude that alleles of one gene pair (R and r) assort independently to the alleles of other gene pair (Y and y) during gamete formation (Meiosis).

Therefore, the F₁ individuals produced four kinds of gamete (RY, Ry, rY, ry) in equal proportions.

Simple explanation: Mendel concluded that the alleles for seed shape and seed color assort independently during gamete formation. This means that the inheritance of one trait does not affect the inheritance of another trait.

3. Law of Independent Assortment

Mendel's observation in F₂ generation of dihybrid cross became known as the law of independent assortment, which states that the alleles for one trait are inherited independently of the alleles for another trait.

In other words, the alleles of one gene pair have equal probability (chance) to assort with the alleles of other gene pair during gamete formation (Meiosis).

However, in case of linked genes (whose loci are present on the same chromosome), the independent assortment of alleles is conditional to the occurrence of crossing over between those two linked genes during meiosis.

Simple explanation: The Law of Independent Assortment states that alleles of different genes are inherited independently of each other. This means that the inheritance of one trait does not affect the inheritance of another trait.

Summary Table

Intermediate Level

A dihybrid cross involves parents differing in two traits.

Mendel's procedure:

• Crossed true-breeding round yellow (RRYY) with true-breeding wrinkled green (rryy).
• F₁ generation: All round yellow (RrYy).
• F₂ generation: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

Mendel's conclusions:

• Alleles of one gene pair assort independently to the alleles of other gene pair during gamete formation.
• F₁ individuals produce four kinds of gametes (RY, Ry, rY, ry) in equal proportions.

Law of Independent Assortment: The alleles for one trait are inherited independently of the alleles for another trait.

Important Points

• - In studying the inheritance of two traits at once, Mendel performed a dihybrid cross.
• - A dihybrid cross involves crossing individuals that differ in two traits.
• - In one of his experiments, he examined seed shape (round or wrinkled) and seed color (yellow or green).
• - When Mendel crossed a true breed round yellow plant (RRYY) with a true breed wrinkled green plant (rryy), all the F₁ offspring were round and yellow, showing the dominant traits.
• - When he self-fertilized these F₁ plants to produce the F₂ generation, he expected a 3:1 ratio like in his monohybrid cross.
• - Instead, he observed four combinations: round yellow, round green, wrinkled yellow, and wrinkled green in a 9:3:3:1 ratio.
• - This result, which included new combinations (round green and wrinkled yellow), led Mendel to conclude that alleles of one gene pair (R and r) assort independently to the alleles of other gene pair (Y and y) during gamete formation (Meiosis).
• - Therefore, the F₁ individuals produced four kinds of gamete (RY, Ry, rY, ry) in equal proportions.
• - Law of Independent Assortment states that the alleles for one trait are inherited independently of the alleles for another trait.
• - In other words, the alleles of one gene pair have equal probability (chance) to assort with the alleles of other gene pair during gamete formation (Meiosis).
• - However, in case of linked genes, the independent assortment of alleles is conditional to the occurrence of crossing over between those two linked genes during meiosis.

Remember

• - Dihybrid cross = two traits
• - P generation = true-breeding round yellow × true-breeding wrinkled green
• - F₁ generation = all round yellow
• - F₂ generation = 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
• - Phenotypic ratio = 9:3:3:1
• - Law of Independent Assortment = alleles of different genes assort independently
• - F₁ individuals produce four kinds of gametes (RY, Ry, rY, ry)
• - Linked genes = independent assortment conditional to crossing over

Common Mistakes

• 1. Confusing monohybrid and dihybrid crosses.
• - Monohybrid = one trait.
• - Dihybrid = two traits.
• 2. Mixing up the F₂ phenotypic ratio.
• - Monohybrid = 3:1.
• - Dihybrid = 9:3:3:1.
• 3. Forgetting the new combinations in F₂.
• - New combinations: round green and wrinkled yellow.
• 4. Confusing the Law of Segregation with the Law of Independent Assortment.
• - Law of Segregation = alleles separate during gamete formation.
• - Law of Independent Assortment = alleles of different genes assort independently.
• 5. Thinking linked genes assort independently.
• - Linked genes (on the same chromosome) do not assort independently unless crossing over occurs.
• 6. Believing F₁ individuals produce only two types of gametes.
• - F₁ individuals (RrYy) produce four types of gametes: RY, Ry, rY, ry.
• 7. Thinking the dihybrid cross ratio is 3:1.
• - The dihybrid cross ratio is 9:3:3:1.

Quick Revision

60-Second Revision

• Dihybrid cross = two traits
• P generation = true-breeding round yellow × true-breeding wrinkled green
• F₁ generation = all round yellow
• F₂ generation = 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
• **Phenotypic ratio** = 9:3:3:1
• Law of Independent Assortment = alleles of different genes assort independently
• F₁ individuals produce four kinds of gametes (RY, Ry, rY, ry)
• Linked genes = independent assortment conditional to crossing over
• Warning: Do not confuse monohybrid (3:1) with dihybrid (9:3:3:1) crosses. New combinations (round green and wrinkled yellow) appear in F₂ generation.