Chapter 16
Mutation
A mutation is a change in the structure or amount of an organism’s genetic material.
Mutations are:


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Spontaneous
Random
Rare
The mutation rate of any one gene is the number of mutations that occur at that gene
site (locus) per million gametes. The mutation rate varies for different genes and
different species.
Mutant alleles are usually recessive and will not show up in the phenotype unless two
mutant alleles meet as a pair in some future generation
Mutant alleles are expressed in the phenotype of the first generation only if they are


Dominant (achondroplasia)
Sex-linked (haemophilia)
Mutagenic Agents
Mutation rates can be artificially increased by exposing an organism to mutagenic
agents such as
Chemical agents
Irradiation
eg. mustard gas
eg. X-rays
UV light
Gamma rays
Mutation is the only source of new variation. Although most mutations are harmful
or even lethal, very occasionally a mutation gives some advantage to the organism
(mutant) resulting in the variation upon which natural selection acts.
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Chapter 16
Chromosome Mutations
These result in changes to the number of chromosomes or to the structure of the
chromosome.
1. Changes to the number of chromosomes
Changes to the number of chromosomes result from non-disjunction during meiosis.
If a spindle fibre fails to carry chromosome material to the correct pole during division,
one of the resulting cells will have one chromosome too many, the other cell one
chromosome less than normal. (See Fig 16.1 p 109 Torrance ‘New Higher Biology’).
This results in the formation of abnormal gametes. Often these gametes will be unable
to form viable embryos because the organism cannot cope with the alteration in genetic
material.
However, a small number of humans survive for a short time with an extra chromosome
17 or 18. People born with an extra chromosome 21 are affected by Down Syndrome.
Several conditions result from abnormal numbers of sex chromosomes resulting from
non-disjunction of one chromosome pair.
Turner’s Syndrome : Infertile female, 45 chromosomes, lacking 1 X (45, XO)
Klinefelter’s Syndrome : Infertile male, 47 chromosomes, extra X (47, XXY)
The above conditions in humans result from non-disjunction of one chromosome pair
but occasionally all the spindle fibres fail during meiosis and gametes are formed with
the diploid number of chromosomes (2 sets in one gamete). In humans, fertilisation
involving such a gamete could not form a viable embryo but plants formed in this way
can survive and are of economic significance.
Complete non-disjunction and polyploidy
Polyploidy is a condition in which an individual possesses one or more sets of
chromosomes in excess (extra) of the normal diploid number. In crop plants this often
confers increased vigour. (Bigger crop yields due to increased seed or fruit size).
If a polyploid plant has an uneven number of chromosome sets, it will be sterile
(because homologous chromosomes cannot pair up at meiosis and non-viable
gametes will be produced). However, because no seeds can be formed, these plants
produce seedless fruits which can be useful.
Fertile polyploid plants can be formed from two different species if there is fusion of
diploid gametes from each. The polyploid is able to undergo meiosis in the normal
way because each chromosome does have a homologous partner. One chromosome
set in this polyploid plant will be made up of a complete set of chromosomes from each
of the parent species acting together. (Refer back to meiosis diagram on pp80-81 and
think about it).
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Chapter 16
2. Changes to the structure of chromosomes
These changes affect whole regions of a chromosome and will involve many
genes.
(There are 30,000 genes in humans shared between the 23 chromosomes which form
one chromosome set. Remember we have 2 chromosome sets in all our diploid cells
and get one copy of each gene from Mum and one copy from Dad. If the gene has
different forms, different information , we call the different forms alleles. Only sexlinked genes in males do not have a partner.)
Changes to the structure of a chromosome will involve changes to the number of genes
present or to the sequence (order) of these genes on the chromosome. These changes
are most likely to happen when chromatids break at chiasmata and do not cross over
and rejoin in the normal way. There are 4 ways changes in chromosome structure can
occur:




Deletion
Duplication
Translocation
Inversion
Use pp 112-113 in Torrance ‘New Higher Biology’ to copy and think about the diagrams
for each of these 4 changes and note what the consequences are for the organism.
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Chapter 16
Gene mutations
Gene mutations involve changes in one or more of the nucleotides in a strand of DNA.
It results in changes to the type of base (substitution) or the sequence of bases within
a gene.
There are 4 types of gene mutation




Substitution
Inversion
These are ‘point’ mutations
Deletion
These are ‘frameshift’ mutations
Insertion
Refer to p115 Torrance . Carefully review diagrams and ensure you realise the
different consequences for the amino acid sequences in the resulting protein
molecule produced by a ‘point’ and a ‘frameshift’ mutation. If at all unclear, discuss
with teacher as soon as possible.
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