Page 22 - CB1.3 Genetic Varation sciPAD

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22 CB 1.3 GENETIC VARIATION
PRINCIPLES OF GENETIC VARIATION
Mutation as a Cause of Variation
Previously you have seen that meiosis and sexual reproduction result in new combinations of
genes in individuals within a population. However, neither meiosis nor sexual reproduction can
create new genes or alleles.
To create a new gene or allele, the base sequence (i.e. the A-T-C-G sequence) of a section of
DNA needs to change. When a change occurs, it’s referred to as a mutation.
The normal base sequence from an allele results in a
protein that leads to a dark-coloured phenotype.
A single mutation in the allele for coat colour has resulted
in a different protein being produced. This results in a
white-coloured phenotype.
G C G G
A T G G A
C
G
C G C C
T A
T
A
C C T
G C G G
A G G A
C
G
C G C C
T
T
A
C
G
C C T
Mutations can occur naturally during DNA replication when a cell is dividing. Sometimes a mistake
is made as the genetic code is being copied. The rate of mutation can be increased by exposure
to ‘mutagens’. Common mutagens include some chemicals (such as those found in cigarettes
and fatty foods), ultraviolet light (from the Sun), and radiation (from x-rays, or radioactive materials,
etc.).
If a mutation happens when a somatic (body) cell is dividing, other cells will usually do the job
of the damaged cell. However, if a mutation occurs in a gametic cell (a sex cell, i.e. a sperm or
ovum), it will be inherited by the offspring and all of its cells will carry the mutation.
‘Mutation’ sounds like a word that means only bad things. However, mutations can be ‘neutral’
or ‘silent’ (i.e. they have no observable effect on an organism). Some mutations may be harmful
and may affect the survival of the organism.
One of the most common harmful effects of mutations in humans is cancer. Cancer is caused by
a mutation that makes cells divide and grow at an abnormally fast rate. As the cancerous cells
continue to grow and divide they produce a mass of tissue called a ‘tumour’.
Occasionally, a ‘beneficial mutation’ will give the organism a survival advantage over other
members of its species, enabling it to live on in conditions where others die.
1. State the three causes of variation within a population. 

2. Define the term ‘mutation’
. 


3. (a) Define the term ‘mutagen’
. 


(b) Give three examples of mutagens. 


4. Outline the differences between a ‘silent’ mutation, a ‘beneficial’ mutation and a ‘harmful’
mutation. 





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