7.3.1 Inheritance of Single Trait by Monohybrid Cross

Introduction

Mendel's study of inheritance for a single trait, known as a monohybrid cross, involved crossing two plants that differed in one trait, such as seed shape. This topic explains Mendel's procedure, observations, and conclusions from his monohybrid cross experiments.

Definition

Monohybrid cross: A genetic cross between two individuals that differ in one trait, such as seed shape (round vs. wrinkled).

Main Idea

Mendel's study of inheritance for a single trait, known as a monohybrid cross, involved crossing two plants that differed in one trait, such as seed shape.

Core Concept

Mendel's Monohybrid Cross

Genotypic and Phenotypic Ratios

Detailed Explanation

1. Mendel's Procedure and Observations

He crossed a true-breeding round-seed plant with a true-breeding wrinkled-seed plant.

P Generation:

• True-breeding round-seed plant (RR)
• True-breeding wrinkled-seed plant (rr)

F₁ Generation:

• In the first filial generation (F₁), all offspring displayed the round seed trait.
• Mendel designated this phenotype as dominant and the other as recessive.

F₂ Generation:

• When these F₁ plants self-fertilized, their offspring (F₂) exhibited both round and wrinkled seeds in a 3:1 ratio.
• Further investigation revealed that 25% of F₂ generation were true breed round, 50% were non-true breed round, and 25% were true breed wrinkled.
• So the genotypic ratio in F₂ generation was 1:2:1.

Simple explanation: When Mendel crossed a round-seed plant with a wrinkled-seed plant, all the offspring in the first generation had round seeds. When these offspring self-fertilized, the second generation had both round and wrinkled seeds in a 3:1 ratio.

2. Conclusion/Interpretation of the Results

Mendel concluded that:

(i) The traits are controlled by factors or elements (now called genes) passed from parents to offspring through gametes.

(ii) Each plant carries two factors (alleles) for each trait — one from each parent.

(iii) Dominant alleles, like "R" for round seeds, are expressed, while recessive alleles, like "r" for wrinkled seeds, are not expressed in F₁ but reappear in F₂ generation.

(iv) During gamete formation both alleles of a gene pair segregate from each other and pass into different gamete.

(v) The gene pair is restored when gametes are fertilized to make the next generation.

Simple explanation: Mendel concluded that:

• Traits are controlled by genes.
• Each plant has two alleles for each trait.
• Dominant alleles are expressed; recessive alleles are hidden in F₁ but reappear in F₂.
• Alleles separate during gamete formation.
• Alleles recombine during fertilization.

3. Law of Segregation

Mendel's analysis in monohybrid cross while studying the inheritance of single trait led to the formulation of the law of segregation which states that co-existing alleles (occupy the same locus) segregate (separate) during gamete formation (Meiosis) and recombine randomly during fertilization.

Simple explanation: The Law of Segregation states that the two alleles for a trait separate during gamete formation, so each gamete carries only one allele. When gametes fuse during fertilization, the allele pair is restored.

Summary Table

Intermediate Level

A monohybrid cross involves parents differing in one trait.

Mendel's procedure:

• Crossed true-breeding round-seed plant (RR) with true-breeding wrinkled-seed plant (rr).
• F₁ generation: All round (Rr).
• F₂ generation: 3 round : 1 wrinkled; genotypic ratio 1 RR : 2 Rr : 1 rr.

Mendel's conclusions:

• Traits are controlled by genes.
• Each plant carries two alleles for each trait.
• Dominant alleles are expressed; recessive alleles are hidden in F₁ but reappear in F₂.
• Alleles segregate during gamete formation.
• Alleles recombine during fertilization.

Law of Segregation: Co-existing alleles segregate during gamete formation and recombine randomly during fertilization.

Important Points

• - Mendel's study of inheritance for a single trait is known as a monohybrid cross.
• - He crossed a true-breeding round-seed plant with a true-breeding wrinkled-seed plant.
• - In the first filial generation (F₁), all offspring displayed the round seed trait.
• - He designated this phenotype as dominant and the other as recessive.
• - When these F₁ plants self-fertilized, their offspring (F₂) exhibited both round and wrinkled seeds in a 3:1 ratio.
• - Further investigation revealed that 25% of F₂ generation were true breed round, 50% were non-true breed round and 25% were true breed wrinkled.
• - So the genotypic ratio in F₂ generation was 1:2:1.
• - Mendel concluded that traits are controlled by factors or elements (now called genes) passed from parents to offspring through gametes.
• - Each plant carries two factors (alleles) for each trait — one from each parent.
• - Dominant alleles, like "R" for round seeds, are expressed, while recessive alleles, like "r" for wrinkled seeds, are not expressed in F₁ but reappear in F₂ generation.
• - During gamete formation both alleles of a gene pair segregate from each other and pass into different gamete.
• - The gene pair is restored when gametes are fertilized to make the next generation.
• - Law of segregation states that co-existing alleles segregate during gamete formation and recombine randomly during fertilization.

Remember

• - Monohybrid cross = one trait
• - P generation = true-breeding round × true-breeding wrinkled
• - F₁ generation = all round
• - F₂ generation = 3 round : 1 wrinkled
• - Genotypic ratio = 1 RR : 2 Rr : 1 rr
• - Phenotypic ratio = 3 round : 1 wrinkled
• - Law of Segregation = alleles separate during gamete formation
• - Alleles recombine during fertilization

Common Mistakes

• 1. Confusing monohybrid and dihybrid crosses.
• - Monohybrid = one trait.
• - Dihybrid = two traits.
• 2. Mixing up F₁ and F₂ generations.
• - F₁ = first filial generation (all round).
• - F₂ = second filial generation (3 round : 1 wrinkled).
• 3. Forgetting the genotypic ratio.
• - Genotypic ratio in F₂ = 1 RR : 2 Rr : 1 rr.
• 4. Thinking recessive traits disappear in F₁.
• - Recessive traits are hidden in F₁ but reappear in F₂.
• 5. 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.
• 6. Believing all F₂ round seeds are true-breeding.
• - 25% of F₂ are true-breeding round (RR), 50% are non-true-breeding round (Rr).
• 7. Thinking Mendel knew about DNA.
• - Mendel called genes "factors" or "elements".

Quick Revision

60-Second Revision

• Monohybrid cross = one trait
• P generation = true-breeding round × true-breeding wrinkled
• **F₁ generation** = all round
• F₂ generation = 3 round : 1 wrinkled
• **Genotypic ratio** = 1 RR : 2 Rr : 1 rr
• Phenotypic ratio = 3 round : 1 wrinkled
• Law of Segregation = alleles separate during gamete formation
• Alleles recombine during fertilization
• Warning: Do not confuse monohybrid (one trait) with dihybrid (two traits) crosses. Recessive traits are hidden in F₁ but reappear in F₂.