Sources of genetic variationEdexcel A-Level Biology B: Revision notes

Section 1

Mutation: the source of new alleles

A mutation is a change in the base sequence of DNA. A mutation in a gene can create a new allele (a different version of that gene). Mutations arise spontaneously and at random, for example from errors during DNA replication, and the rate is increased by mutagens such as ultraviolet radiation, X-rays and some chemicals.

Mutation is the only process that produces completely new alleles, so it is the original source of all genetic variation. Many mutations have no effect on the phenotype, some are harmful and a few are beneficial. A mutation is only inherited by offspring made by sexual reproduction if it occurs in a cell that forms gametes. A mutation in a body cell is passed on to daughter cells by mitosis, for example in a cutting grown from a branch.

Key termsmutationallelemutagen

Common mistake

Do not say that meiosis or crossing over creates new alleles. They only make new combinations of alleles that already exist.

Section 2

Independent assortment in meiosis I

In metaphase I, the homologous pairs (bivalents) line up on the equator of the spindle. Each pair is orientated at random, so which chromosome of a pair goes to which pole does not depend on how the other pairs are orientated. This is independent (random) assortment and it gives a mixture of maternal and paternal chromosomes in each gamete.

For a haploid number of $n$ , the number of different chromosome combinations in the gametes is $2^n$ (ignoring crossing over). In humans, $n = 23$ , so one person can make over 8 million different combinations.

Key termsindependent assortmenthomologous pair

Exam tip

Always show the working $2^n$ in a calculation, for example $2^4 = 16$ for a diploid number of 8.

Section 3

Crossing over in prophase I

In prophase I, homologous chromosomes pair up as bivalents. Non-sister chromatids break at the same points and rejoin to the other chromatid. The points where they join are called chiasmata. The result is an exchange of sections of chromatid, so the alleles on each chromatid are new combinations.

The chromatids are now genetically different, so even the two gametes that come from the same chromosome pair carry different combinations of alleles. Crossing over therefore increases variation beyond that from independent assortment.

Key termscrossing overchiasmanon-sister chromatids

Section 4

Random fertilisation

During sexual reproduction, random fertilisation means that any male gamete can fuse with any female gamete. Since each gamete already has a different combination of alleles, this gives a very large number of possible zygotes.

For two parents who each make $2^n$ types of gamete, the number of possible zygotes (ignoring crossing over) is $2^n \times 2^n$ . For a diploid number of 8, that is $16 \times 16 = 256$ .

Key termsrandom fertilisation

Section 5

Putting it together

Mutation produces new alleles.
Crossing over and independent assortment produce gametes with new combinations of existing alleles.
Random fertilisation combines the gametes in many different ways.

A good answer says which process produces new alleles and which only produces new combinations.

That's the notes covered.

Carry on to the next subtopic.

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).