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QUESTIONS:
2014; 1a
DNA Structure
Base Phosphate Sugar -­‐ Deoxyribose
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• 
• 
• 
The bases are the source of genetic variation and are
what make different strands of DNA unique
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One of these monomers is pictured above and is
called a ‘nucleotide’. A nucleotide is made up of a
sugar (deoxyribose), and a base
There are four different bases: Adenine, Thymine,
Guanine and Cytosine
• 
The sugar and phosphate are the same across all
nucleotides and link together to make a strong
‘backbone’ for the bases to attach to
These bases adhere to a strict set of rules known as
the ‘complementary base pairing’ rules that help
ensure processes such as DNA replication can occur
• 
These rules state that Adenine can only bind to
Thymine and Cytosine can only bind to Guanine
DNA is classified as a ‘polymer’. This means it is
made up of thousands of repeating units known as
‘monomer’s
© StudyTime 2015
Practice Question
Use the unlabeled diagram below to discuss the structure of a strand of DNA
d
DNA is made up of thousands of repeated units. These units are arranged in pairs in a ‘double helix’ structure that resembles a ladder of bases. Each repeaBng unit is known as a nucleoBde and has an idenBcal sugar and phosphate group. The nucleoBdes can however differ in which base is aGached to them. If a nucleoBde contains the base ‘Adenine’, its neighboring nucleoBde must contain the base ‘Thymine’, and vice versa. Similarly, if a nucleoBde contains the base ‘Guanine’, its neighboring nucleoBde must contain ‘Cytosine’. This is according to the complementary base pairing rules and defines the geneBc diversity between differing strands of DNA. © StudyTime 2015
QUESTIONS:
2014; 1b
2013; 2a
2012; 1a, b
Genes, Alleles,
Chromosomes
Gene Allele Chromosome • 
An organism contains thousands of nucleotides
arranged in a very long chain of DNA
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The location of a particular gene is the same across
all chromosomes
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In order for all of the DNA to fit into the nucleus, this
DNA is arranged into smaller pieces of wound up
DNA known as ‘chromosomes’. Humans contain 46
chromosomes each consisting of tightly wound DNA
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Organisms of the same species can have different
versions of these genes. Different forms of a
particular trait (i.e ‘green eyes’) are known as
different ‘alleles’
• 
Chromosomes are further divided into sections
known as ‘genes’. Each gene is responsible for the
‘coding’ of a particular trait (i.e ‘eye colour’)
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Therefore, chromosomes are made up of DNA, which
is made up of genes. These genes have different
expressions known as alleles.
© StudyTime 2015
Practice Question
Use the words alleles, DNA, chromosomes, genes and nucleus to complete the
following sentence:
d
______ reside in the ________ and are made up of wound_______. This is
arranged in sections called ________ which have different versions known as _____
Chromosomes reside in the nucleus and are made up of wound DNA. This is
arranged in sections called genes which have different versions known as alleles.
© StudyTime 2015
QUESTIONS:
2013; 2b
Alleles
Bb BB
Heterozygous • 
Alleles are different versions of a particular trait,
such as different eye colours
• 
Alleles are determined by the unique sequence of
bases found in a particular gene in an organism
• 
Organisms such as humans which form as a result of
sexual reproduction contain two separate alleles for
each trait. These alleles determine how the trait is
expressed on the organism
Homozygous Dominant • 
Dominant alleles are denoted using a capital letter
and mask the effects of the second allele. This
means that a dominant allele will always be
expressed
• 
Recessive alleles are denoted by a lower case letter
and are able to be masked by a more dominant allele.
This means that a recessive allele will only be
expressed if both alleles present are of the recessive
type
• 
An organism with two identical alleles for a particular
gene is known as homozygous (i.e BB). An organism
with two differing alleles is heterozygous (Bb)
© StudyTime 2015
Practice Question
Define the following as heterozygous or heterozygous and state which allele will be
expressed
d
B = Black hair
b = Blonde hair
Alleles Present Heterozygous/Homozygous Allele Expressed BB Homozygous dominant Black hair bb Homozygous recessive Blonde hair Bb Heterozygous Black hair © StudyTime 2015
Mutations
Change in base sequence • 
Mutations are the only way of creating totally new
alleles in a species
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Mutations originate from a change to one or more
bases in the intended base sequence of an
organism’s DNA
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The change in base sequence means that the amino
acid sequence and hence the protein created by the
DNA will differ to the one intended
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Mutations can be either harmful or beneficial,
although harmful ones are more common
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It is easier for a beneficial mutation to become
established in a population. Once it does, it becomes
known as an allele
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Although mutations can occur in any cell of an
organism, only those occurring on gametes (sperm
or egg) can be passed on to offspring during
reproduction
© StudyTime 2015
Practice Question
Too much exposure to direct sunlight can lead to a mutation causing skin cancer.
Explain what this mutation causes at the DNA level and discuss why skin cancer isn’t
directly inheritable.
d
Too much exposure to direct sunlight can cause a base to change in the DNA of a cell. Because DNA codes for specific proteins, any change to the DNA sequence can directly effect what the organism is able to create. A cell that is effected by the mutaBon has had its DNA sequence altered to the point where it becomes dangerous to itself. Skin cancer isn’t directly inheritable as the mutaBon occurs on a skin cell. As skin cells are not involved in reproducBon, a mutaBon confined to non-­‐sex cells is not passed on to offspring. © StudyTime 2015
QUESTIONS:
2014; 1a, b, c
2013; 3a
Sexual Reproduction
23
23 • 
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• 
All organisms perform either asexual or sexual
reproduction, with some able to perform both. Both
types or reproduction have their own advantages and
disadvantages
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Humans perform sexual reproduction. This means
that human offspring have two parents and genetics
that are a combination from both
Sexual reproduction is advantageous as it means
offspring are never genetically identical to their
parents. This is useful as it adds genetic diversity to a
population and hence increases survival rates
• 
During sexual reproduction, offspring inherit a set of
alleles for each gene from each parent
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Because of this, gametes have half the number of
chromosomes usually found in an individual (23)
Sexual reproduction can be disadvantageous as it
carries a high energy cost, and requires a
reproducing individual to find a mate
© StudyTime 2015
Practice Question
State whether the following concepts are characteristic to sexual or asexual
reproduction
Concept High energy cost Sexual/Asexual t
Sexual Gametes with half the number of intended chromosomes Sexual Necessity to find mate Sexual Offspring geneBcally idenBcal to parent Asexual Faster Asexual Offspring not idenBcal to parent Sexual © StudyTime 2015
QUESTIONS:
2014; 1d, 2a
2013; 1c, d
2012; 2b
Genotypes/Phenotypes
Bb
Black Hair
Genotype • 
The allele combination of an organism for a
particular gene can be described in two different
ways: using either the genotype or the phenotype
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The genotype describes the actual alleles present,
whilst the phenotype represents the result of the
allele combination
• 
The ‘genotype’ is represented as two letters, each
describing an allele from either parent. Dominant
alleles are in capital letters while recessive alleles are
lower case
Phenotype • 
The phenotype is represented as words and
describes the physical outcome of the genotype
• 
Genotype and Phenotype ratios can describe the
potential outcomes for offspring calculated using a
punnett square. They describe the likelihood of each
potential genotype and phenotype across the
offspring of two individuals
© StudyTime 2015
Practice Question
For each allele combination, give the phenotype and genotype. Then give the overall
Genotype and phenotype ratios for the offspring
Allele 1 Allele 2 Genotype Phenotype Black Hair (B) Black Hair (B) BB Black Hair Blonde Hair (b) Black Hair (B) Bb Black Hair Blonde Hair (b) Bb Black Hair Black Hair (B) Blonde Hair (b) Blonde Hair (b) bb t
Blonde Hair Genotype raBo = 1 : 2 : 1 Phenotype raBo = 3 : 1 © StudyTime 2015
QUESTIONS:
2014; 1c
2013; 3c, 4a, b
• 
• 
Genetic Variation/
Adaptations
Genetic variation is a concept highly important to the
survival of a species. It means that not all members
of a population are susceptible to the same diseases/
predators and hence are unlikely to become extinct if
the environment changes
Genetic variation arises due to the processes
involved in Meiosis, and the nature of sexual
reproduction combining two sets of alleles
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During an environmental change, those with
beneficial genetics will live to reproduce and pass on
their alleles
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Those that lack beneficial alleles will not survive to
reproduce. The harmful alleles will therefore die out.
This process is called ‘natural selection’
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The beneficial alleles which go on to establish in the
population go on to be called ‘adaptations’ and aid
organisms in that particular environment
© StudyTime 2015
Practice Question
A new predator is introduced to a population. Those with blue coloring are able to
blend into their surroundings to avoid being seen, whilst the orange organisms are
not. Discuss what will happen to the amount of blue and orange individuals in the
population over time.
d
v
The organisms with blue coloring have a phenotype which is favorable to their
environment. This means they each have an allele combination which contributes
towards their survival. Because they have a greater survival rate, organisms with blue
coloring have a much greater chance of reproducing. This means that more blue color
alleles will be passed on to future generations.
Because of this, the number of blue individuals in the population will increase over time
while the number of orange will decrease.
© StudyTime 2015
QUESTIONS:
2014; 4c
2012; 2a
Punnett Squares
B b B BB Bb b Bb bb • 
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Punnett squares are a way of working out the
likelihood of different genotypes arising from a set of
parents
By counting the relative numbers of each genotype
and phenotype represented, punnett squares are a
convenient way of calculating genotype and
phenotype ratios
The outer cells of the square represent the alleles
present in each of the parents. (It doesn’t matter
where you put each parent, as long as you are
consistent)
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The inner cells of the square are then filled in by
combining the two adjacent alleles together
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Remember that a capital letter represents a
dominant allele and a lower case letter represents an
allele that is recessive. When filling in the punnett
square, always write the capital letter first
© StudyTime 2015
Practice Question
Two individuals heterozygous for hair colour (Bb) reproduce to create a series of
offspring. Draw a Punnett square for this cross and use it to form an expected
phenotype and genotype ratio for the offspring
1.  Maternal alleles = B, b
2.  Fill in punnett square:
Paternal alleles = B, b
B B b B BB Bb b b Bb bb B b 3.
Genotype ratio = 1 : 2 : 1
Phenotype ratio = 3 : 1
© StudyTime 2015
QUESTIONS:
2014; 4a, b
2013; 1a, b
Pedigree Charts
Unaffected Male
Affected Male
Unaffected Female
Affected Female
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Pedigree charts are special family trees that give
information on the presence of a specific trait across
multiple generations
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Different shapes are used to represent males and
females. This makes it easy to tell whether a specific
trait is more likely to affect males or females
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They are often used to track the presence of a
disease or a specific allele through a family
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A key is provided with most pedigree charts. This
details how to read whether an individual is affected
or not
Each shape on the pedigree chart represents an
individual, with horizontal lines showing breeding
partners and vertical lines leading to offspring
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By looking at offspring, pedigree charts can be used
to ‘work backwards’ to figure out the genotypes of
parents
© StudyTime 2015
Practice Question
Use the pedigree chart and key below to identify the genotype of each parent for a
particular disease. P = affected, p = unaffected
d
Unaffected Male
Affected Male
Unaffected Female
Affected Female
1.  The mother has the unaffected phenotype
2.  As the unaffected phenotype is recessive, she must not have a dominant allele
3.  The mother’s genotype is ‘pp’
4.  The father has the affected phenotype
5.  As the affected phenotype is dominant, he could have either the ‘Pp’ genotype, or ‘PP’
6.  The father has unaffected offspring. In order for this to occur, he must pass on a
recessive allele. The father therefore has the genotype ‘Pp’
© StudyTime 2015
QUESTIONS:
2014; 3b
Pure Breeding
B b b Bb bb Test cross for a
heterozygous (not
pure breeding)
individual
b Bb bb • 
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Because a dominant allele will lead to an individual
expressing the dominant phenotype regardless of the
other allele, it can be hard to tell whether a dominant
individual has the heterozygous or homozygous
dominant genotype
Knowing the actual genotype is useful to breeders
and farmers as an individual with the homozygous
dominant genotype is ‘pure bred’
This terms comes from the idea that two pure-bred
individuals can only reproduce to create individuals
with the dominant phenotype
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The best way to work out the genotype of an
individual is to run a ‘test cross’ using them. This
involves breeding the unknown individual with a
homozygous recessive individual
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If any of the offspring from the cross exhibit the
recessive phenotype, the unknown individual must be
heterozygous
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If all of the offspring exhibit the dominant phenotype,
it can be indicated that the unknown individual has
the homozygous dominant genotype. The more
offspring there are created by the cross, the more
sure we can be of this fact
© StudyTime 2015
Practice Question
A farmer has cows with both the recessive allele for black colour (B), or the dominant
allele for brown colour (b). He wants to know whether one of his black cows has the
heterozygous or homozygous dominant genotype. Explain, with the aid of punnett
squares, how he could do this.
1. 
2. 
3. 
4. 
In order to work out the unknown genotype, the farmer should use a test cross
This involves crossing the unknown individual with a homozygous recessive individual
If any of the offspring are brown coloured, the unknown individual is heterozygous
If all of the offspring are black, there is a chance the unknown individual is
Pure breeding. The more crosses are done, the more sure we can be.
B b b Bb bb b Bb bb B B b Bb Bb b Bb Bb Heterozygous test cross
Homozygous Dominant test cross
© StudyTime 2015
QUESTIONS:
2013; 3b
2012; 3a
Cell Division
4n Mitosis
2n n • 
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2n n Cells do not grow in size. Instead, they replicate to
form identical cells in the processes of tissue growth
and repair
Mitosis is the cell replication responsible for growth
and repair. It involves a single parent cell splitting
into two identical ‘daughter’ cells
Before Mitosis occurs, all of the DNA inside the
parent cell is replicated in order for a copy to go into
the daughter cell
n Meiosis
n • 
Meiosis is the cell replication responsible for the
creation of gametes (sperm and egg cells)
• 
The process of Meiosis begins the same way as
Mitosis, but involves an extra cell division with no
DNA replication
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This results in four daughter cells with half the
number of chromosomes as the original parent cell
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We say that regular cells contain ‘2n’ chromosomes,
whilst sex cells contain ‘n’ chromosomes – with ‘n’
representing the number of chromosomes unique to
that species
© StudyTime 2015
Practice Question
Describe what type of cell division an egg cell would undergo, and use the diagram
below to discuss the number of chromosomes the daughter cells will have
As egg cells are a type of gamete, they will undergo Meiosis. This is because they are only
involved in reproduction, and have no need to perform growth or repair.
Meiosis involves two cell divisions but only one DNA replication. This results in daughter
cells with half the number of chromosomes present in the parent cell.
It is beneficial for gametes to contain half the number or chromosomes present in a
regular cell as they are designed to combine with each other to form a zygote. The zygote
contains the regular number of chromosomes and goes on to form an individual during
sexual reproduction.
© StudyTime 2015