Download - Evolution [part 1]
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VARIATION & MUTATION
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Variation:
describes the differences in characteristics shown by organisms of the same species
can be: discontinuous continuous
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Discontinuous Variation:
Produces:individuals with clear-cut differences
with no intermediates between them
examples: blood group in humans wing length in Drosophila sex tongue rolling
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Discontinuous Variation: Characteristics are usually controlled by:
1-2 major genes which have: two or more allelic forms
their phenotypic expression is relatively unaffected by environmental conditions
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Continuous Variation:
many characteristics in a population show a complete gradation from one extreme to the other without a break
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Continuous Variation: Examples:
Mass Height Intelligence Colour of organs & organisms
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Continuous Variation:
Characteristics are produced by the combined effects of:
many polygenes and the environment
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Influence of the environment
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Sources of variation inasexually reproducing organisms :
replication of DNA is nearly perfect - i.e. little possibility of variation in
genotype
any apparent variation between organisms is thus almost certainly the result of environmental influences
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Sources of Variation: Meiosis
Allows for new combinations of genes
Creates variation within a population
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Four Sources of variation in:
sexually reproducing organisms:
1. Crossing over 2. Random orientation of bivalents
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Four Sources of variation in:
sexually reproducing organisms:
4. Random
fertilisation
3. Random orientation of sister chromatids
Metaphase plate
Metaphase plate
OR
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DO NOT:Crossing over
generate the major changes in genotype necessary to give rise to new species
Random orientation
Random fertilisation
MUTATIONSgenerate changes in genotype
necessary to give rise to new species
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MUTATIONS
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A Mutation is:
a change in the amount, arrangement or structure of the DNA of an organism
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A mutation produces a change in the genotype & is passed on: when a cell nucleus divides by:
mitosis or meiosis from the mutant cell
Mutant daughter cellsMutant daughter cells
Mutant cell
Mutant cell
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Which type of mutation can be inherited by the offspring?
germinal
somatic Occur in somatic cells:
are NOT passed on the offspring
Occur in gamete cells:
are passed on to the offspring
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Normal progeny can be produced if mutations occur in gamete cells because:
the effect produced by defective allele is masked by the dominant allele
DOMINANT RECESSIVE
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A Mutation may result in:
the change in appearance of a characteristic of a population
e.g. red eyes in Drosophila appeared in 1909
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e.g. dark-coloured moth appeared in 1848
The "typica" form of the moth.
The "carbonaria" form.
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Mutations can:
occur in: any gene at any timebe:
1. SPONTANEOUS
2. INDUCED
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Spontaneous Mutations:
are permanent changes in the genome that occur without any outside influence
occur because the machinery of the cell is imperfect
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Spontaneous Mutations arise because:
a) DNA polymerase makes errors in replication
b) meiosis is not perfect:
– non-disjunction
- translocationscan occur
One chromatid goes to each
daughter cell.
Both chromatids are sent to one daughter cell, the other gets
none. TranslocationChromosome 21 fuses
with 14.
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Induced Mutations:
occur when some outside agent causes a permanent change in DNA
mutagens: anything that causes a mutation examples:
• Asbestos
• Tar from tobacco
• UV radiation
• Pesticides
• Caffeine UV light causes adjacent
thymines to cross link
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Mutations can be:
1. Chromosomal mutations
2. Gene mutations or point mutations
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MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
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CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of the DNA
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME STRUCTURE
ANEUPLOIDYLoss or gain of a
single chromosome
EUPLOIDY/POLYPLOIDYThe increase in entire
haploid sets of chromosomes
AUTOPOLYPLOIDYResults from an increase in
the number of chromosomes within the same species
ALLOPOLYPLOIDYChromosome number within
a sterile hybrid doubles, producing fertile hybrids
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
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Changes in Chromosome structure [not in syllabus]
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MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
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Aneuploidy:
loss or gain of a single chromosomeresults from non-disjunction
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Down Syndrome: A meiotic error
Cause: non-disjunction at either:
anaphase I anaphase II
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anaphase I anaphase II
Homologous chromosomes fail to
separate
Sister chromatids fail
to separate
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Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
Fertilisation after non-disjunction in the mother results in a zygote with an extra
chromosome
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Down’s Syndrome is a common form of chromosomal mutation in humans:
resulting from non-disjunction
Why is Down’s syndrome also called
trisomy 21?
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Trisomy 21
Three copies of Chromosome 21
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Causes of Down Syndrome:
1. 96% of the cases: non-disjunction of
chromosome 21 during anaphase of meiosis
2. 3-4% of the cases: translocation (movement) of:
chromosome 21 to chromosome 14, or less commonly to chromosome 22
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Relationship Between Age and Aneuploidy
Older mothers more likely to produce aneuploid eggs
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Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical Traits
XXY, XXYY, XXXY maleKlinefelter syndrome
sterility, small testicles, breast
enlargement
XYY male XYY syndrome normal male traits
Klinefelter syndrome
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Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical Traits
XO femaleTurner
syndrome
sex organs do not mature , sterility,
short stature
XXX female Trisomy Xtall stature, learning disabilities, limited
fertility
Turner syndrome
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Question: [SEP, 2009]
Suggest explanations for each of the following observations.
1. Meiosis generates biological diversity in eukaryotes. (2)
New combinations of alleles arise through crossing-over and random orientation of both bivalents and sister chromatids.
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Question: [SEP, 2009]
Suggest explanations for each of the following observations.
2. Klinefelter’s Syndrome, where individuals would be characterised by a XXY combination of sex chromosomes, is caused by an anomalous event during meiosis. (2)
Due to non-disjunction of sex chromosomes, one cell would result in having two copies of the X chromosome rather than one. This can happen at either anaphase I or II.
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MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
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Euploidy (Polyploidy)
polyploids are gamete and somatic cells containing multiples of the haploid number of chromosomes
2n = 6 = 2x (diploid)
2n = 9 = 3x (triploid)
2n = 12 = 4x (tetraploid)
Homologous chromosomes
One extra set
Two extra sets
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Plants commonly exhibit polyploidy: 95% of ferns 70% of angiosperms many of the fruits & grain are polyploid plants
are polyploid
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Polyploidy is often associated with:
advantageous features such as
increased:- Size- Hardiness- Resistance to
disease
having advantageous features is called hybrid vigour
DiploidPolyploid
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Hybrid vigour in corn
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Polyploids with odd numbered chromosome sets are usually sterile
as they produce mostly aneuploid gametes
Somatic cell:Full set of chromosomes = 9
Gamete:Will contain the INCORRECT number of chromosomes
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Benefit of Odd Ploidy-Induced Sterility
Seedless fruit watermelons and bananas
Not all watermelons are red. This triploid, seedless variety has sweet, yellow flesh.
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Polyploidy is more common in:
plants than animals
Reason:
the increased number of chromosomes makes normal gamete formation during meiosis much more prone to errors
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Why are polyploid plants still capable of reproduction?
As most plants are capable of vegetative propagation.
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MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
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Autopolyploidy:
• a condition that may arise naturally or artificially
• results from an increase in the number of chromosomes within the same species
2n = 64n = 12
2n
4n
Failure of cell divisionin a cell of a growing
diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.
Gametes produced by flowers on this
branch will be diploid.
Offspring with tetraploid karyotypes may be viable
and fertile—a new biological species.
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Autopolyploidy can be induced by:
the drug colchicine – spindle formation is inhibited and chromatids fail to separate during anaphase
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How is a 4n cell produced from a 2n one?
DNA replicates but cytoplasm does not split
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Allopolyploidy
chromosome number within a sterile hybrid doubles,
producing a fertile hybrid
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Let’s explain ‘Allopolyploidy’ using Spartina as an example
Spartina alterniflora
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Spartina (cord grass): an example of allopolyploidy
Spartina alterniflora2n = 62
Spartina maritima2n = 60
1n 1870 in Townsend harbor in southern England:
a sterile hybrid of two Spartina species was found and was called Spartina townsendii
Spartina townsendii2n = 61
Gametes: n = 30 Gametes: n = 31 STERILE HYBRID
X
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Spartina townsdendii went through a process of genome duplication
22 years later, a new vigorous and fertile form was recorded – it was named S. anglica S. maritima
2n = 60
S. alterniflora
2n = 62
n = 30 n = 31Gametes:
S. townsendii
2n = 61
4n = 122
S. anglica
Sterile hybrid:
Allopolyploidy
A NEW speciesS. anglica
Fertile hybrid :
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Allopolyploidy has been described as ‘instant evolution’. Explain.
A new species is produced in a short time – over one generation
S. maritima
2n = 60
S. alterniflora
2n = 62
n = 30 n = 31Gametes:
S. townsendii
2n = 61
4n = 122
S. anglica
Sterile hybrid:
Allopolyploidy
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How can S. townsendii reproduce, if it is sterile?
Vegetatitve propagation by producing:
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Why is S. townsendii sterile?
Meiosis cannot occur properly as chromosomes are NOT
homologous in S. townsendii
Consider this example:
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No homologous chromosomes in sterile hybrid:
Species 1 Species 2
2n = 4 2n = 6
Sterile hybrid: 2n = 5
Gametes:
X
X
n = 2 n = 3
BUT what if chromosome
number doubles in a sterile hybrid?
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Now homologous chromosomes are present!!
Sterile hybrid: 2n = 5
Chromosome doubling:
Allopolyploidy
4n = 10
Can this new organism produce viable gametes?
YES
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Why is the new species fertile?
Sterile hybrid:
2n = 5
Fertile new species:
2n = 10
Meiosis can occur properly as homologous chromosomes are
present
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Individual becomes fertile after chromosome doubling
Gametes:
n = 5
2n = 10
X
X
a new species is produced which is fertile with polyploids like itself but infertile with parental species
2n = 10
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Meiotic error in anther during pollen grain formation
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Sterile hybrid
AB
Meiotic error
AABB
X
ABD
Meiotic error
T. turgidum(Emmer wheat)
(2n = 28)
X
Sterile hybrid
Spot where allopolyploidy
occurred during the
development of T. aestivum.
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a b c d e
Primitive wheat (a) crossed withwild grass (b) to produce an infertile hybrid.
Chromosome doubling produces a fertile hybrid (c)
which is crossed with wild grass(d) to produce an infertile hybrid
Chromosome doubling results in fertile hybrid (e)
This hybrid is a cultivated wheat used for flour production
Many crop plants result from doubling of chromosome number
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Question: [SEP, 1999]
1. What is polyploidy? (2)
Gamete and somatic cells containing multiples of the haploid number of chromosomes.
2. Suggest TWO mechanisms that may result in polyploidy. (4)
Allopolyploidy – chromosomes double in a sterile F1 hybrid.
Autopolyploidy – duplication of chromosomes of a single species after fertilisation.
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Question: [SEP, 2003]
Two closely-related plants, A and B, have chromosome sets AA and BB respectively. When Plant A is crossed with Plant B a hybrid, possessing chromosome set AB, is produced. Such hybrids are sterile.
a) Why are hybrid plants sterile? (3)
As there are no homologous chromosomes which is a prerequisite for meiosis to occur.
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Question: [SEP, 2003]b) Fertile polyploidy hybrids might develop from
these sterile plants. Describe a sequence of events that would enable this to occur. (3)
Chromosomes replicate but cytokinesis does not occur. This results in a cell having homologous chromosomes and so meiosis can occur.
c) Given the results observed in these crosses, would you conclude that Plant A and Plant B belong to the same species or to different species? Justify your answer. (3)
Belong to different species. Offspring produced are sterile.
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MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
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MUTATIONChange in the amount, structure or arrangement of the DNA of
an organism
GENE/POINT MUTATIONSDescribe a change in the structure of DNA at a single locus
INSERTION
INVERSION
DELETION
SUBSTITUTION
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Fig. 6 Gene or point mutation
1) INSERTION: the addition of an extra nucleotide
A GT G C A TA TT G A C A G
2) DELETION: involves the loss of a nucleotide
A GT G C A T A TT C A G
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Fig. 6 Gene or point mutation
4) SUBSTITUTION:
a particular base is substituted by another (e.g. sickle-cell anaemia)
A GT G C A T A TT G T A G
3) INVERSION: two nucleotides become arranged in the wrong order
A GT G C A T T TA G C A G
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What is a:
Silent mutation:
an alteration in a DNA sequence that does not result in an amino acid change in a polypeptide
Frameshift mutation:
is caused by insertions or deletions of nucleotides which shifts the codon triplets of the genetic code of mRNA and causes a misreading during translation
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A silent mutation: no effect on amino acid sequence
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A frameshift mutation:
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Sickle Cell Anaemia:
in humans is an example of base substitution
affects a base in one of the genes involved in producing haemoglobin
at position 14 in the DNA: the base thymine is
replaced by adenine
Amino acid sequence
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Sickle Cell Anaemia:
at low oxygen tensions, haemoglobin S crystallises in the red cells distorting them into a sickle shape
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Sickle Cell Anaemia:
many sickle cells are destroyed in the circulation
Result: oxygen-carrying capacity of the blood is lowered
incidence is very high in Africa and Asia
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Sickle Cell Trait is the heterozygous condition
heterozygotes are resistant to some forms of malaria
Incidence of Malaria Incidence of Sickle cell anaemia
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Inheritance of sickle cell anaemia:
Carrier parents : HbAHbS x HbAHbS
F1 phenotypes: 25% normal : 50% carriers : 25% sufferers
F1 genotypes: HbAHbA HbSHbA HbSHbA HbSHbS
Gametes:
HbAHbAHbS
HbSx
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Essay Titles
1. Variation among organisms and between individuals of the same species is purely the result of genetic differences. Discuss this statement. [SEP, 2003]
2. “Random mutations are the driving force of evolutionary change”. Discuss. [SEP, 2009]
3. Compare and contrast the roles of genetic mutations and meiosis in generating genetic diversity. [MAY, 2010]
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END OF SECTION