Monohybrid Cross Explained: Definition, Diagram & Example | Vedantu

Understanding genetics can be fun and straightforward! In this guide, we will explore the monohybrid cross, learn to define a monohybrid cross, see a clear monohybrid cross diagram, and work through a monohybrid cross example. We will also explain how to explain the law of dominance using a monohybrid cross and distinguish between monohybrid and dihybrid cross. Keep Learning!

Introduction to the Monohybrid Cross

Gregor Mendel, often called the Father of Genetics, understands heredity with his experiments on pea plants. By crossing plants with one contrasting trait at a time, he discovered that traits are inherited as discrete units (later known as genes). In simple terms, a monohybrid cross involves one gene with two alleles. If you’re wondering, what is a monohybrid cross? – it is a genetic cross between two organisms that differ in a single trait, with each parent contributing one allele.

Also Read: Mendel's Law of Inheritance

What is a Monohybrid Cross?

To define a monohybrid cross, think of it as a breeding experiment between two homozygous parents for a single trait. For example, if one pea plant is tall (TT) and the other is dwarf (tt), their offspring (Tt) will all display the dominant tall trait. This simple experiment shows that when you define a monohybrid cross, you are examining how one pair of contrasting alleles is passed on to the next generation. This concept is also central when we explain the law of dominance using a monohybrid cross.

Also Check: Mendelian Genetics

Monohybrid Cross Diagram and Example

Read More: Law of Dominance and Law of Segregation

Let’s look at a monohybrid cross example using pea plants:

• Parent 1: Tall (TT)

• Parent 2: Dwarf (tt)

Punnett Square (Monohybrid Cross Diagram):

All offspring are Tt (tall). Later, when these heterozygous plants are crossed, you get a Punnett Square that gives you a 3:1 ratio of tall to dwarf plants. This monohybrid cross example clearly shows how a single gene can be inherited.

Read More: Mendelian Ratio

Explaining the Law of Dominance Using a Monohybrid Cross

One of Mendel’s key observations was that one trait can mask the other. In our tall and dwarf example, the tall allele (T) is dominant, and the dwarf allele (t) is recessive. When you cross TT with tt, all F1 offspring are heterozygous (Tt) and show the dominant tall trait. This is how you explain the law of dominance using a monohybrid cross. Remember, whenever you conduct a monohybrid cross, you are also reinforcing that dominant alleles will mask recessive ones in heterozygotes.

Also Read: Principle of Inheritance and Variation

Step-by-Step Guide to Carry Out a Monohybrid Cross

Carrying out a monohybrid cross is simple and follows these steps:

1. Identify the Alleles: Write the dominant allele with an uppercase letter (e.g. T) and the recessive with a lowercase letter (e.g. t).

2. Determine the Parent Genotypes: For instance, cross a homozygous dominant (TT) with a homozygous recessive (tt).

3. List the Gametes: The pure gametes from the parents are T and t respectively.

4. Draw a Punnett Square (Monohybrid Cross Diagram): Place the gametes along the rows and columns to visualise the combinations. This monohybrid cross diagram will show that all offspring (Tt) exhibit the dominant trait.

5. Analyse the Outcome: The F1 generation will show a 100% dominant phenotype. When F1 individuals are self-crossed, you get a 3:1 ratio in the F2 generation (three dominant: one recessive).

Distinguishing Between Monohybrid and Dihybrid Cross

It’s important to distinguish between monohybrid and dihybrid cross. While a monohybrid cross looks at the inheritance of one gene (one pair of alleles), a monohybrid and dihybrid cross-comparison is made by examining two traits simultaneously in a dihybrid cross. For example:

• Monohybrid Cross: Tall (Tt) × Dwarf (tt)

• Dihybrid Cross: Round-yellow seed (RrYy) × Wrinkled-green seed (rryy)

By comparing these, you can distinguish between monohybrid and dihybrid cross clearly. In a dihybrid cross, you observe a 9:3:3:1 ratio in the F2 generation, which is more complex than the simple 3:1 ratio of the monohybrid cross.

Unique Insights and Applications

Beyond the classroom, the principles learned from a monohybrid cross have far-reaching applications:

• Test Crosses: When the genotype of an organism with a dominant phenotype is uncertain, a test cross with a homozygous recessive individual can reveal if the genotype is heterozygous or homozygous dominant.

• Modern Genetics: The basics of a monohybrid cross continue to underpin modern genetic research and breeding experiments. Understanding these crosses helps researchers trace gene inheritance, predict genetic disorders, and even explore new gene editing techniques.