A change in the base sequence of DNA.

Name two causes of mutation.

Spontaneous errors in DNA replication, and mutagens such as ultraviolet radiation or X-rays.

When is a mutation passed to offspring made by sexual reproduction?

When it occurs in a cell that forms gametes.

What is independent assortment?

The random orientation of homologous pairs in metaphase I, so that maternal and paternal chromosomes are shuffled into gametes.

What is the formula for the number of chromosome combinations in gametes?

2^n, where n is the haploid number of chromosomes (ignoring crossing over).

Where do chiasmata form?

Between non-sister chromatids of homologous chromosomes in prophase I.

What happens in crossing over?

Non-sister chromatids break and rejoin, exchanging sections, so alleles are in new combinations.

Why does crossing over increase variation?

It makes chromatids that carry new combinations of alleles, so gametes from the same chromosome pair differ.

What is random fertilisation?

The chance fusion of any male gamete with any female gamete.

Why do meiosis and fertilisation not create new alleles?

They only shuffle alleles that already exist into new combinations.

Sources of genetic variationEdexcel A-Level Biology B: Flashcards

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What is a mutation?

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What is a mutation?: A change in the base sequence of DNA.
Which process is the only source of new alleles?: Mutation.
Name two causes of mutation.: Spontaneous errors in DNA replication, and mutagens such as ultraviolet radiation or X-rays.
When is a mutation passed to offspring made by sexual reproduction?: When it occurs in a cell that forms gametes.
What is independent assortment?: The random orientation of homologous pairs in metaphase I, so that maternal and paternal chromosomes are shuffled into gametes.
What is the formula for the number of chromosome combinations in gametes?: $2^n$ , where $n$ is the haploid number of chromosomes (ignoring crossing over).
Where do chiasmata form?: Between non-sister chromatids of homologous chromosomes in prophase I.
What happens in crossing over?: Non-sister chromatids break and rejoin, exchanging sections, so alleles are in new combinations.
Why does crossing over increase variation?: It makes chromatids that carry new combinations of alleles, so gametes from the same chromosome pair differ.
What is random fertilisation?: The chance fusion of any male gamete with any female gamete.
How many different zygotes can form from two parents with $n = 4$ (ignoring crossing over)?: $16 \times 16 = 256$ .
Why do meiosis and fertilisation not create new alleles?: They only shuffle alleles that already exist into new combinations.

Exam questions on Sources of genetic variation

An orchard owner notices one branch of an apple tree bearing red apples while the rest of the tree bears green apples. Cuttings taken from the red branch and grown into new trees all produce red apples. Genetic testing shows a new allele for fruit colour is present in every cell of the red branch.
Explain why every tree grown from a cutting of the red branch produces red apples.2 marks
A species of beetle has a diploid number of 8 (four pairs of homologous chromosomes). A breeder wants to know how many genetically different offspring two unrelated beetles could produce. Ignore crossing over unless told otherwise.
Two unrelated beetles of this species mate. Calculate the number of genetically different zygotes that random fertilisation could produce from their gametes, ignoring crossing over.2 marks
Two parents who are both heterozygous at many gene loci have four children. No two of the children, apart from any identical twins, would be expected to have the same genotype, even though all of the children have the same two parents.
Explain how meiosis produces gametes that differ genetically from each other.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).