Non-Mendelian Inheritance

HSC Biology | Free Study Notes

In this lesson

• what non-Mendelian inheritance means

• how co-dominance works

• how incomplete dominance works

• what multiple alleles are

• what sex linkage means

  
  

What is non-Mendelian inheritance?

Non-Mendelian inheritance refers to inheritance patterns that do not fit the simplest dominant and recessive pattern of one allele completely masking another.

In these cases:

• both alleles may be expressed

• the phenotype may be blended

• a gene may have more than two possible alleles in the population

• inheritance may depend on whether the gene is on a sex chromosome

Co-dominance

Co-dominance happens when both alleles are fully expressed in the phenotype of a heterozygous individual.

What this means

Neither allele is hidden. Both are shown at the same time.

Example

A 2025 HSC multiple-choice question describes a red camellia crossed with a white camellia, where all offspring show both red and white petals. This was identified as co-dominance. 

Another HSC-style pattern

2019 HSC marking guidance describes a 1:2:1 ratio with three phenotypes as typical of co-dominant alleles, where both alleles are expressed in the heterozygous offspring. 

Key idea

In co-dominance:

• both alleles are visible

• the phenotype is not blended

• the heterozygote shows both traits

Incomplete dominance

Incomplete dominance happens when neither allele is completely dominant, so the heterozygous phenotype is intermediate between the two homozygous phenotypes.

What this means

The phenotype looks like a blend of the two parental traits.

Example idea

If one allele gives red flowers and one gives white flowers, the heterozygous offspring may appear pink.

HSC link

2019 HSC marking guidance notes that a 1:2:1 ratio may also represent incomplete dominance, where the two alleles are blended in the heterozygous phenotype. 

Key idea

In incomplete dominance:

• neither allele fully masks the other

• the heterozygote has an intermediate phenotype

Co-dominance and incomplete dominance compared

Pattern	Heterozygous phenotype
Co-dominance	Both alleles are fully expressed
Incomplete dominance	Intermediate or blended phenotype

Multiple alleles

Multiple alleles means that a gene has more than two possible alleles in the population.

Important point

An individual still only has two alleles for that gene, one from each parent, but the population may have several possible allele forms.

Example from NSW materials

A Year 12 problem set describes dog coat colour with five recognised alleles for one gene, arranged in a dominance series. It then asks for the maximum number of different alleles in one individual dog, with the correct answer being two. 

Why this matters

Multiple alleles increase the number of possible genotypes and phenotypes in a population.

Sex linkage

Sex linkage means the gene is located on a sex chromosome, usually the X chromosome.

Why this matters

Because males and females have different sex chromosomes, sex-linked inheritance can produce different inheritance patterns from autosomal inheritance.

HSC example

2020 HSC marking guidance explains a sex-linked inheritance example using:

• male genotype: XAY

• female genotype: XaXa

It shows how offspring outcomes differ depending on whether the trait is sex linked or not. 

Key idea

In sex-linked inheritance:

• males often show recessive X-linked traits more easily because they only have one X chromosome

• inheritance patterns may differ between male and female offspring

Why these patterns matter

These inheritance patterns are important because many real traits do not follow a simple dominant-recessive model.

Module 5 specifically expects students to interpret:

• autosomal inheritance

• sex linkage

• co-dominance

• incomplete dominance

• multiple alleles 

Punnett squares and non-Mendelian inheritance

Punnett squares can still be used in non-Mendelian inheritance, but the interpretation is different.

In co-dominance

You may see:

• both alleles expressed in the heterozygote

• often a 1:2:1 phenotypic ratio

In incomplete dominance

You may also see:

• a 1:2:1 phenotypic ratio

• but the heterozygote is intermediate, not showing both traits separately

In sex linkage

Punnett squares must use sex chromosomes, such as:

• XA

• Xa

• Y

In multiple alleles

Punnett squares still work, but there are more allele possibilities in the population.

Worked example

Exam-style question

Explain the difference between co-dominance and incomplete dominance.

Worked answer

In co-dominance, both alleles are fully expressed in the heterozygous phenotype, so both traits are seen at the same time. In incomplete dominance, neither allele is completely dominant, so the heterozygous phenotype is intermediate between the two homozygous phenotypes.

Why this works

This answer:

• defines both patterns clearly

• compares them directly

• focuses on the heterozygous phenotype

Common mistakes

• Saying co-dominance and incomplete dominance are the same.

• Forgetting that an individual can only have two alleles, even in a multiple-allele system.

• Mixing up sex-linked and autosomal inheritance.

• Saying co-dominance produces a blended phenotype.

• Forgetting to use sex chromosomes properly in sex-linkage Punnett squares.

Quick quiz

1. What is non-Mendelian inheritance?

2. What happens in co-dominance?

3. What happens in incomplete dominance?

4. What are multiple alleles?

5. What does sex linkage mean?

HSC Biology Home