Inheritance Patterns » Homozygous and Heterozygous

What you'll learn this session

Study time: 30 minutes

The difference between homozygous and heterozygous genotypes
How to identify and represent genotypes using genetic notation
Dominant and recessive alleles and their expression
How to predict offspring genotypes using Punnett squares
Examples of inheritance patterns in humans and other organisms
How to solve genetic problems involving homozygous and heterozygous traits

Introduction to Homozygous and Heterozygous Inheritance

When we look at why you have blue eyes while your friend has brown, or why some peas are wrinkled while others are smooth, we're exploring inheritance patterns. These patterns are determined by the combination of alleles an organism has for a particular gene.

Key Definitions:

Gene: A section of DNA that codes for a specific protein or characteristic.
Allele: Different versions of the same gene.
Homozygous: When an organism has two identical alleles for a particular gene (e.g., AA or aa).
Heterozygous: When an organism has two different alleles for a particular gene (e.g., Aa).
Dominant allele: An allele that will always be expressed when present (usually represented by a capital letter).
Recessive allele: An allele that will only be expressed when two copies are present (usually represented by a lowercase letter).

🌱 Homozygous Genotypes

A homozygous organism has two identical alleles for a particular gene. These can be either:

Homozygous dominant (AA): Two dominant alleles
Homozygous recessive (aa): Two recessive alleles

Homozygous organisms are "pure breeding" for that trait, meaning they will always pass the same allele to their offspring.

🌾 Heterozygous Genotypes

A heterozygous organism has two different alleles for a particular gene (Aa). In this case:

The dominant allele (A) will be expressed in the phenotype
The recessive allele (a) is present but not expressed
The organism can pass either allele to its offspring

Heterozygous organisms are often called "carriers" for recessive traits.

Genetic Notation and Terminology

When geneticists discuss inheritance, they use specific notation to make things clear:

Genotype: The genetic makeup of an organism (e.g., AA, Aa, or aa).
Phenotype: The observable characteristics resulting from the genotype.
Capital letters (A): Represent dominant alleles.
Lowercase letters (a): Represent recessive alleles.

Example: Human Eye Colour

Brown eye colour (B) is generally dominant over blue (b).

BB: Homozygous dominant - Brown eyes
Bb: Heterozygous - Brown eyes (but carries the blue allele)
bb: Homozygous recessive - Blue eyes

This is a simplified model, as eye colour is actually determined by multiple genes!

Predicting Inheritance with Punnett Squares

Punnett squares are a brilliant tool that helps us predict the possible genotypes and phenotypes of offspring from a genetic cross.

How to Use a Punnett Square

Let's look at an example where we cross two heterozygous pea plants (Tt) for tallness, where T = tall (dominant) and t = short (recessive):

Write the possible gametes (sex cells) from each parent along the top and side of a grid
Fill in the grid by combining the alleles
Count the different possible genotypes and phenotypes

	T	t
T	TT	Tt
t	Tt	tt

Results:

1/4 (25%) TT (homozygous dominant) - Tall plants
2/4 (50%) Tt (heterozygous) - Tall plants
1/4 (25%) tt (homozygous recessive) - Short plants

Phenotype ratio: 3 tall : 1 short

Different Types of Inheritance Patterns

🐾 Complete Dominance

One allele completely masks the other in heterozygotes.

Example: Pea plant height (T/t)

TT - Tall
Tt - Tall
tt - Short

🦊 Codominance

Both alleles are expressed equally in heterozygotes.

Example: Red and white flower colour (R/W)

RR - Red flowers
RW - Red AND white patches
WW - White flowers

🐃 Incomplete Dominance

Heterozygotes show a blend or intermediate phenotype.

Example: Snapdragon flower colour (R/W)

RR - Red flowers
RW - Pink flowers
WW - White flowers

Genetic Disorders and Carrier Status

Understanding homozygous and heterozygous genotypes is crucial for understanding genetic disorders, especially those that follow recessive inheritance patterns.

Case Study: Cystic Fibrosis

Cystic fibrosis is caused by a recessive allele (f). People with the following genotypes:

FF: Homozygous dominant - Unaffected
Ff: Heterozygous - Unaffected carrier (can pass the allele to offspring)
ff: Homozygous recessive - Has cystic fibrosis

If two carriers (Ff) have children, there's a 25% chance each child will have cystic fibrosis (ff).

Practical Applications: Solving Genetic Problems

Let's work through a practical example to see how we can use our knowledge of homozygous and heterozygous inheritance.

Example Problem: Tongue Rolling

The ability to roll your tongue into a tube is determined by a dominant allele (R), while being unable to roll your tongue is recessive (r).

If a heterozygous tongue-roller (Rr) has children with someone who cannot roll their tongue (rr), what proportion of their children will be able to roll their tongues?

	R	r
r	Rr	rr
r	Rr	rr

Results:

2/4 (50%) Rr (heterozygous) - Can roll their tongue
2/4 (50%) rr (homozygous recessive) - Cannot roll their tongue

Therefore, 50% of their children will be able to roll their tongues.

Summary: Homozygous vs Heterozygous

📖 Key Points to Remember

Homozygous means having two identical alleles (AA or aa)
Heterozygous means having two different alleles (Aa)
Dominant alleles (A) are expressed when present in either homozygous or heterozygous form
Recessive alleles (a) are only expressed in the homozygous form
Punnett squares help us predict inheritance patterns
Understanding these concepts is crucial for genetic counselling and breeding programs

⚠ Common Mistakes to Avoid

Confusing genotype (genetic makeup) with phenotype (observable traits)
Assuming all traits follow simple dominant/recessive patterns
Forgetting that heterozygous individuals (Aa) can pass either allele to offspring
Thinking that dominant traits are more common in a population (they're not necessarily)
Forgetting that many traits are influenced by multiple genes and environmental factors

Real-World Application: Selective Breeding

Farmers and breeders use knowledge of homozygous and heterozygous inheritance to develop plants and animals with desired traits. For example:

Breeding homozygous dominant (AA) individuals ensures all offspring will show the dominant trait
Identifying heterozygous carriers (Aa) helps avoid passing on recessive disorders
Creating hybrid varieties by crossing different homozygous lines can produce offspring with hybrid vigour

These principles have been used for thousands of years, long before we understood the genetics behind them!

🧠 Test Your Knowledge!