Sources of Genetic Variation

HSC Biology | Free Study Notes

In this lesson

• how mutation creates new alleles

• how meiosis produces variation

• what crossing over does

• what independent assortment means

• how fertilisation increases variation

Why genetic variation matters

Genetic variation means differences in genetic information between individuals.

This variation is important because it:

• makes individuals different from one another

• provides the basis for natural selection

• helps populations respond to changing conditions

Mutation

Mutation is a change in the DNA sequence.

Why mutation matters

Mutation is important because it can create new alleles.

If no mutation occurred:

• no completely new alleles would enter the population

• variation would be more limited over time

  
  

Where mutation fits in variation

Mutation is a source of genetic variation in both:

• asexual reproduction

• sexual reproduction

HSC marking guidance states that mutation during DNA replication is a source of genetic variation that can arise in offspring of both asexual and sexual reproduction. 

Important point

Mutation introduces new genetic possibilities, but not all mutations are beneficial. Some are neutral and some are harmful.

Meiosis

Meiosis is a type of cell division that produces gametes with half the chromosome number.

Why meiosis matters for variation

Meiosis is important because it does not just reduce chromosome number. It also creates new combinations of alleles.

How meiosis increases variation

Variation in meiosis comes mainly from:

• crossing over

• independent assortment

The syllabus specifically requires students to model meiosis including crossing over of homologous chromosomes. 

Crossing over

Crossing over is the exchange of genetic material between chromatids of homologous chromosomes during Meiosis I.

What happens in crossing over

• homologous chromosomes pair up

• parts of chromatids are exchanged

• new combinations of alleles are created on the chromatids

Why crossing over matters

Crossing over increases variation because it produces chromosomes that are not exactly the same as the original maternal or paternal chromosomes.

HSC marking guidance states that crossing over leads to a new combination of alleles on each chromatid. 

Independent assortment

Independent assortment means homologous chromosomes line up in random order and orientation in Meiosis I.

What happens

Because the homologous pairs line up randomly:

• each gamete receives a different combination of maternal and paternal chromosomes

Why this matters

Independent assortment creates many possible chromosome combinations in gametes, increasing genetic variation.

Homologous chromosomes line up in random order and orientation and separate in Meiosis I, resulting in different combinations of parental chromosomes in the gametes. 

Fertilisation

Fertilisation is the fusion of male and female gametes.

Why fertilisation matters for variation

Each gamete is already genetically unique because of meiosis. When fertilisation happens:

• one unique gamete from one parent combines with one unique gamete from the other parent

• this creates a new genotype in the offspring

Key idea

Fertilisation increases variation because the combination of gametes is random.

HSC marking guidance states that the chance of any one gamete from one individual combining with another gamete from a second individual at fertilisation leads to very high variation in offspring.

How these sources work together

These sources of variation do not act separately.

Mutation

Creates new alleles.

Meiosis

Shuffles alleles into new combinations.

Crossing over

Recombines genetic material between homologous chromosomes.

Independent assortment

Produces different combinations of chromosomes in gametes.

Fertilisation

Combines two unique gametes to form a new genotype.

Together, these processes explain why sexually produced offspring are genetically different.

Comparing the sources of variation

Source of variation	What it does
Mutation	Creates new alleles
Meiosis	Produces genetically varied gametes
Crossing over	Creates new allele combinations on chromatids
Independent assortment	Produces different combinations of chromosomes in gametes
Fertilisation	Combines two unique gametes

Why sexual reproduction produces more variation

Sexual reproduction leads to much more genetic variation than asexual reproduction because:

• meiosis produces unique gametes

• fertilisation combines gametes randomly

• mutation can introduce new alleles

This is why sexual reproduction is strongly linked to variation in offspring. HSC marking guidance makes this comparison directly. 

Worked example

Exam-style question

Explain how meiosis and fertilisation contribute to genetic variation in offspring.

Worked answer

Meiosis contributes to genetic variation because crossing over and independent assortment create gametes with different combinations of alleles. Fertilisation increases variation further because one unique gamete from one parent combines randomly with one unique gamete from the other parent, producing a new genotype.

Why this works

This answer:

• includes two key processes

• explains how each increases variation

• links both clearly to offspring genotype

Common mistakes

• Saying mutation only happens in sexual reproduction.

• Forgetting that mutation is the source of new alleles.

• Confusing crossing over with independent assortment.

• Saying fertilisation creates new alleles rather than new combinations of alleles.

• Treating meiosis as only reduction division, without mentioning its role in variation.

Quick quiz

1. What is mutation?

2. Why is mutation important for genetic variation?

3. What is crossing over?

4. What does independent assortment mean?

5. How does fertilisation increase variation in offspring?

HSC Biology Home