Mendelian Inheritance

HSC Biology | Study Notes

Mendelian inheritance is a key part of NSW Biology Stage 6, Module 5, Heredity. This topic matters because Module 5 requires students to investigate inheritance patterns, including the inheritance of autosomal, sex-linked, codominant, incompletely dominant and multiple alleles, and to use tools such as Punnett squares to predict offspring outcomes. HSC marking guidelines also regularly assess correct parental genotypes, phenotypic ratios and working using Punnett squares. 

In this lesson

• what alleles are

• the difference between genotype and phenotype

• what dominant and recessive mean

• how Mendelian inheritance works

• how to use Punnett squares

What is Mendelian inheritance?

Mendelian inheritance is the basic pattern of inheritance first described by Gregor Mendel.

It explains how characteristics are passed from parents to offspring through alleles.

At this level, Mendelian inheritance mainly refers to traits where:

• one gene controls a characteristic

• each parent contributes one allele

• dominant and recessive relationships can be used to predict offspring

Alleles

An allele is an alternative form of a gene.

What this means

A gene may exist in different versions.

For example, a gene for flower colour might have:

• one allele for purple

• one allele for white

Why alleles matter

The combination of alleles an organism inherits affects its genotype and may affect its phenotype.

Genotype

A genotype is the combination of alleles an organism has for a particular trait.

Examples of genotype

Using B for a dominant allele and b for a recessive allele:

• BB

• Bb

• bb

These are all genotypes.

Key idea

The genotype describes the genetic makeup, not the visible characteristic.

Phenotype

A phenotype is the observable characteristic shown by an organism.

Examples of phenotype

Examples may include:

• purple flowers

• white flowers

• black fur

• white fur

Key idea

The phenotype is what you can observe, while the genotype is the allele combination causing it.

Dominant and recessive

Dominant alleles

A dominant allele is expressed in the phenotype if at least one copy is present.

Example

If B is dominant, then:

• BB shows the dominant phenotype

• Bb also shows the dominant phenotype

Recessive alleles

A recessive allele is only expressed if two copies are present.

Example

If b is recessive, then:

• bb shows the recessive phenotype

Key idea

A recessive trait only appears when the organism is homozygous recessive.

Homozygous and heterozygous

These terms are often used in Mendelian inheritance questions.

Homozygous

Homozygous means the two alleles are the same.

Examples:

• BB

• bb

Heterozygous

Heterozygous means the two alleles are different.

Example:

• Bb

HSC multiple-choice questions frequently test recognition of heterozygous offspring and correct parental genotypes.

How Mendelian inheritance works

Each parent contributes one allele for each gene to the offspring.

Simple pattern

If the parents are:

• Bb and Bb

then each parent can pass on:

• B

• or b

The offspring combinations depend on which allele from one parent combines with which allele from the other.

Punnett squares

A Punnett square is a grid used to predict the possible genotypes and phenotypes of offspring.

Why Punnett squares matter

Punnett squares help students:

• organise allele combinations clearly

• predict offspring genotypes

• predict offspring phenotypes

• calculate expected ratios or probabilities

HSC marking guidelines often reward correct parental genotypes, suitable working and correct phenotypic ratios using Punnett squares.

How to set up a Punnett square

Example cross

Cross:Bb × Bb

Step 1: write one parent across the top

• B

• b

Step 2: write the other parent down the side

• B

• b

Step 3: fill in the boxes

	B	b
B	BB	Bb
b	Bb	bb

Step 4: interpret the result

Genotypes:

• 1 BB

• 2 Bb

• 1 bb

Phenotypes:

• 3 dominant

• 1 recessive

Genotypic ratio and phenotypic ratio

Genotypic ratio

The genotypic ratio shows the proportion of allele combinations.

In the example above:

• 1 BB : 2 Bb : 1 bb

Phenotypic ratio

The phenotypic ratio shows the proportion of visible traits.

In the example above:

• 3 dominant : 1 recessive

Why Mendelian inheritance is useful

Mendelian inheritance helps explain:

• how simple traits are passed on

• why offspring can differ from parents

• how probabilities of traits can be predicted

It is the starting point for more complex inheritance patterns later in Module 5.

Worked example

Exam-style question

Two heterozygous black rabbits are crossed. Black fur, B, is dominant to white fur, b. Use a Punnett square to predict the offspring phenotypes.

Worked answer

Parents:Bb × Bb

	B	b
B	BB	Bb
b	Bb	bb

Genotypes:

• 1 BB

• 2 Bb

• 1 bb

Phenotypes:

• 3 black

• 1 white

So the expected phenotypic ratio is 3 black : 1 white.

Why this works

This answer:

• identifies the parental genotypes

• uses a correct Punnett square

• gives both genotype and phenotype outcomes

Common mistakes

• Mixing up genotype and phenotype.

• Forgetting that a dominant allele is expressed in a heterozygous genotype.

• Saying a recessive phenotype can appear in Bb.

• Filling in Punnett squares incorrectly by copying whole parent genotypes into each box.

• Giving a phenotypic ratio when the question asks for genotype, or vice versa.

Quick quiz

1. What is an allele?

2. What is the difference between genotype and phenotype?

3. What does dominant mean?

4. What does recessive mean?

5. What is the expected phenotypic ratio in a Bb × Bb cross?

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