SOURCE OF VARIATION: MUTATION

Is a change in a gene or chromosome

A mutation is a spontaneous change in a gene or chromosome

•Gene mutations may arise when a gene fails to make an exactcopy of itself during replication prior to meiosis or mitosis

•Chromosome mutations can result from:

-Damage to, or loss of a chromosome

-Incomplete separation of chromosomes at meiosis leading to extra chromosomes in one gamete

-Part of a chromosome becoming attached to another chromosome (translocation)

- Doubling the whole set of chromosomes

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Chromosome mutations in humans usually result in spontaneous abortion of the foetus

But a proportion survive e.g.

Downs syndrome: The affected person has one extra chromosome in their genome (i.e. 47 instead of 46 chromosomes)

This results in characteristic facial features, varying degrees of mental impairment and, usually, a very cheerful disposition

Klinefelters syndrome. The affected male has an extra X chromosome (XXY)

The person appears to be a normal male but he is infertile

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-A mutated gene which is dominant, or inherited as a homozygousrecessive, will affect the whole organism

-A mutated gene which does not result in abortion or early deathof an organism will be inherited by the offspring

-Very rarely, a mutation will be beneficial and will be inherited bythe offspring

Gene mutations in humans may result in:

Haemophilia Cystic fibrosisAlbinism

Colour blindness

Sickle cell anaemia

Dwarfism

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-If a gene mutation takes place in a gamete which contributes toa zygote, the mutation will affect the whole organism

-A gene mutation in a body cell will affect only that cell and any cells derived from it.

•It is mutations in body cells which give rise to cancers.

-These mutations usually affect the genes which control cell division

-The control on cell division is reduced so that the cell dividesrepeatedly, giving rise to a tumour

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Some environmental effects may increase the mutation rate.

These include radiation (X-rays, ultra-violet, radioactive materials) and carcinogenic chemicals such as those in tobacco smoke

These are all known as mutagens

-Mutagens which affect body cells may produce cancers.

-Mutagens affecting the reproductive organs may result in defective offspring

Mutagens

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Question 1

Chromosome mutations can occur as a result of…

(a) gain of an extra chromosome

(b) doubling of the entire set of chromosomes

(c) faulty replication of DNA

(d) breaking of a chromosome

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Question 2

Gene mutations can occur during DNA replicationprior to …

(a) mitosis

(b) meiosis

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Question 3

Sickle cell anaemia results from …

(a) a change in a nucleotide triplet

(b) a change in a single nucleotide

(c) a change in a single amino acid

(d) a chromosome mutation

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