Inheritance Patterns » Monohybrid Crosses

What you'll learn this session

Study time: 30 minutes

The principles of monohybrid inheritance
How to use Punnett squares to predict offspring genotypes and phenotypes
The difference between dominant and recessive alleles
How to calculate genetic ratios from crosses
Real-world examples of monohybrid inheritance in humans and other organisms

Introduction to Monohybrid Crosses

Monohybrid crosses are one of the fundamental concepts in genetics, first explored by Gregor Mendel through his famous pea plant experiments. A monohybrid cross tracks the inheritance of a single characteristic that is controlled by a single gene with two different alleles. Understanding these crosses helps us predict the likelihood of specific traits appearing in offspring.

Key Definitions:

Monohybrid cross: A genetic cross that tracks the inheritance of a single characteristic controlled by one gene.
Allele: Different versions of the same gene.
Dominant allele: An allele that always expresses itself when present (usually represented by a capital letter).
Recessive allele: An allele that only expresses itself when two copies are present (usually represented by a lowercase letter).
Genotype: The genetic makeup of an organism (the alleles it possesses).
Phenotype: The physical expression or appearance of a genetic trait.
Homozygous: Having two identical alleles for a particular gene (either both dominant or both recessive).
Heterozygous: Having two different alleles for a particular gene (one dominant and one recessive).

🌱 Mendel's Pea Experiments

Gregor Mendel, often called the "Father of Genetics," conducted experiments with pea plants in the 1860s. He chose peas because they:

Were easy to grow and had a short life cycle
Had clear, distinct traits (tall/short, yellow/green seeds, etc.)
Could self-pollinate or be cross-pollinated

By carefully controlling which plants were bred together, Mendel discovered patterns of inheritance that formed the foundation of modern genetics.

📈 Mendel's First Law

Mendel's First Law (Law of Segregation) states that:

Each individual has two alleles for each gene
These alleles separate during gamete formation
Each gamete receives only one allele
Fertilization randomly brings two gametes together

This explains why offspring inherit one allele from each parent for every gene.

How to Perform a Monohybrid Cross

A monohybrid cross can be represented using a Punnett square, which is a simple grid that helps predict the possible genotypes of offspring. Let's walk through the process step by step:

Creating a Punnett Square

Imagine we're tracking the inheritance of pea plant height. The tall allele (T) is dominant over the short allele (t).

Step 1: Identify the genotypes of both parents.
Step 2: Determine the possible gametes each parent can produce.
Step 3: Create a grid with one parent's gametes along the top and the other's along the side.
Step 4: Fill in the grid by combining the gametes.
Step 5: Analyze the resulting genotypes and phenotypes.

For example, if we cross a heterozygous tall plant (Tt) with another heterozygous tall plant (Tt):

	T	t
T	TT	Tt
t	Tt	tt

Results:

Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 tall plants : 1 short plant

This 3:1 phenotype ratio is a classic sign of a monohybrid cross between two heterozygous individuals.

Different Types of Monohybrid Crosses

Depending on the genotypes of the parents, monohybrid crosses can produce different ratios of offspring:

📊 Homozygous Dominant × Homozygous Recessive

Cross: TT × tt

Genotype ratio: All Tt

Phenotype ratio: All dominant trait (tall)

This is often called the P (parental) generation in Mendel's experiments.

📊 Heterozygous × Heterozygous

Cross: Tt × Tt

Genotype ratio: 1 TT : 2 Tt : 1 tt

Phenotype ratio: 3 dominant (tall) : 1 recessive (short)

This is the classic monohybrid cross that produces the 3:1 ratio.

📊 Heterozygous × Homozygous Recessive

Cross: Tt × tt

Genotype ratio: 1 Tt : 1 tt

Phenotype ratio: 1 dominant (tall) : 1 recessive (short)

This is called a test cross, used to determine if an organism with the dominant phenotype is homozygous or heterozygous.

Real-World Examples of Monohybrid Inheritance

Human Genetic Traits

Many human traits follow simple monohybrid inheritance patterns:

Earlobe attachment: Free earlobes (dominant) vs attached earlobes (recessive)
Tongue rolling: Ability to roll tongue (dominant) vs inability to roll tongue (recessive)
Widow's peak: Pointed hairline (dominant) vs straight hairline (recessive)
Dimples: Presence of dimples (dominant) vs absence of dimples (recessive)

However, it's important to note that many human traits are actually controlled by multiple genes or are influenced by environmental factors, making them more complex than simple monohybrid inheritance.

Case Study Focus: Sickle Cell Anaemia

Sickle cell anaemia is a genetic disorder that follows monohybrid inheritance patterns. It's caused by a recessive allele (s) that affects haemoglobin production.

Genotypes:
- HH: Normal haemoglobin (healthy)
- Hs: Carrier of sickle cell trait (generally healthy)
- ss: Sickle cell anaemia (medical condition)
Interesting fact: The sickle cell allele provides resistance to malaria, which is why it's more common in regions where malaria is prevalent. This is an example of heterozygote advantage, where having one copy of the allele can be beneficial.

Genetic Probability and Ratios

When predicting the outcomes of genetic crosses, we use probability. Each fertilization event is independent, meaning the outcome of one birth doesn't affect the probability of the next.

For example, if two heterozygous parents (Tt × Tt) have children:

Each child has a 75% chance of being tall (TT or Tt)
Each child has a 25% chance of being short (tt)

This doesn't mean that exactly 3 out of every 4 children will be tall. Rather, each individual child has a 3 in 4 chance of being tall. With a large sample size, the ratio will approach 3:1.

💡 Common Misconceptions

Students often make these mistakes when working with monohybrid crosses:

Confusing genotype with phenotype
Forgetting that dominant doesn't mean "more common" in a population
Assuming that a 3:1 ratio means exactly 3 dominant and 1 recessive in every 4 offspring
Mixing up the letters used to represent alleles (remember: dominant = capital, recessive = lowercase)

✅ Exam Tips

To succeed in IGCSE Biology questions on monohybrid inheritance:

Always clearly identify the genotypes of the parents first
Draw out the Punnett square neatly and completely
State both the genotype and phenotype ratios in your answer
Use consistent notation (e.g., if the question uses "R" for red and "r" for white, stick with those symbols)
Remember to explain the relationship between genotype and phenotype

Summary of Monohybrid Inheritance

Monohybrid inheritance is the simplest form of genetic inheritance, involving just one gene with two alleles. By understanding how to track these alleles through generations using Punnett squares, we can predict the likelihood of specific traits appearing in offspring.

Key points to remember:

Dominant alleles (capital letters) express themselves when at least one copy is present
Recessive alleles (lowercase letters) only express themselves when two copies are present
A classic cross between two heterozygous individuals produces a 3:1 phenotype ratio
Genetic probability applies to each individual offspring, not to the family as a whole
Punnett squares are a visual tool to help calculate genetic probabilities

Understanding monohybrid inheritance is essential for more complex genetic concepts, including dihybrid crosses (involving two genes) and linked genes, which you'll study in future lessons.

🧠 Test Your Knowledge!