Germ-Line and Somatic Mutations

HSC Biology | Free Study Notes

In this lesson

• what germ-line mutations are

• what somatic mutations are

• the key differences between them

• how they affect inheritance and disease risk

• common examples

What is a mutation?

A mutation is a change in the DNA sequence.

Mutations can happen in different types of cells. This is important because the effect of the mutation depends on where it occurs.

The two main types on this page are:

• somatic mutations

• germ-line mutations

Somatic mutations

A somatic mutation is a mutation that occurs in a body cell, not in a gamete.

What this means

Somatic mutations affect the individual in whose body the mutation occurs.

They may affect:

• one cell only

• a group of body cells descended from that cell

• a particular tissue or part of the body

Effect on offspring

A somatic mutation is not passed to offspring.

This is one of the most important differences between the two types. HSC marking guidance states this directly. 

Germ-line mutations

A germ-line mutation is a mutation that occurs in a sex cell or in cells that give rise to gametes.

What this means

Because germ-line cells are involved in reproduction, a mutation here may become part of the DNA of the offspring.

Effect on offspring

A germ-line mutation may be passed to offspring.

This is the key inheritance difference between germ-line and somatic mutations. 

Key differences

Feature	Somatic mutation	Germ-line mutation
Where it occurs	Body cells	Sex cells or cells that form gametes
Effect on offspring	Not passed to offspring	May be passed to offspring
Main effect	Affects the individual only	Can affect the next generation

Inheritance

Somatic mutation and inheritance

Somatic mutations are not inherited by offspring because they are not present in the gametes.

A 2021 HSC marking guideline explains that a trait was identified as somatic because it was not present in the parents or the offspring of the affected male. If it had been a germ-line mutation, it would be more likely to appear in offspring. 

Germ-line mutation and inheritance

Germ-line mutations are important in heredity because they can be transmitted to the next generation.

A 2019 HSC multiple-choice question also showed that a germ-line mutation may cause no noticeable phenotype in offspring if it occurs in non-coding DNA, showing that inheritance and visible effect are not always the same thing. 

Disease risk

Somatic mutations and disease risk

Somatic mutations can increase disease risk in the individual.

For example, HSC marking guidance gives mutation in skin cell DNA leading to skin cancer as a somatic mutation example. 

Key idea

Somatic mutations can be serious, but their effect is usually limited to the person in whom they occur.

Germ-line mutations and disease risk

Germ-line mutations can increase disease risk in offspring because the altered DNA may be inherited.

HSC marking guidance gives mutation in a sex cell leading to haemophilia as an example of a germ-line mutation. 

Key idea

A germ-line mutation can contribute to inherited disorders because it becomes part of the offspring’s genetic makeup.

Examples

Example of a somatic mutation

• mutation in a skin cell leading to skin cancer 

  
  

Example of a germ-line mutation

• mutation in a sex cell leading to haemophilia 

HSC-style example with twins

A 2021 HSC marking guideline used twin development to show the difference:

• a mutation occurring before germ cells form may appear in somatic and germ-line cells and can be passed to offspring

• a mutation occurring only in somatic cell lines affects the individual but cannot be passed on 

Why this topic matters in Module 6

This topic is important because Module 6 is about genetic change. To understand the effect of mutation, students need to know whether a mutation:

• affects only one individual

• can enter the next generation

• can contribute to inherited disease

• can introduce new alleles into a population

That is why the syllabus specifically separates somatic and germ-line mutations. 

Worked example

Exam-style question

Explain one difference between a somatic mutation and a germ-line mutation.

Worked answer

A somatic mutation occurs in a body cell and is not passed to offspring. A germ-line mutation occurs in a sex cell or a cell that forms gametes, so it may be inherited by offspring. 

Why this works

This answer:

• identifies the location of each mutation

• explains the inheritance difference

• uses the key terms clearly

Common mistakes

• Saying all mutations are inherited.

• Mixing up body cells and gametes.

• Forgetting that somatic mutations can still cause disease in the individual.

• Assuming a germ-line mutation must always cause a visible phenotype.

• Describing location without explaining the effect on offspring.

Quick quiz

1. What is a somatic mutation?

2. What is a germ-line mutation?

3. Which type of mutation can be passed to offspring?

4. Give one example of a somatic mutation.

5. Why can a germ-line mutation affect inherited disease risk?

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