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Alleles and Complimentation
(Lecture Five, Alessandro Bianchi)
Genetic Analysis
Used to understand a biological phenomenon
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The mechanism of this determination can
be uncovered by genetic analysis of molecular processes that link the phenotype to genotype
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Allele: one of a series of possible alternative forms of a gie gene
Gene: a hereditary unit that occupies a specific locu within the genome or chromosome
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Dominance
Dominance vs
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If a white flower is produced, then
the white flower’s allele is dominant
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Therefore, dominance refers to allelic interaction
and affects phenotupe, not inheritance, as you will
always inherit both allele
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In diploid organisms:
The phenotupe of the recessive allele have an
effect on the phenotype when that allele is in
homozygous state
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This means the recessive
allele will be detected as there is no other copy to
be more dominant and mask the effect of the
recessive allele
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In the diploid
form, if the yeast is starved it will undergo meiosis
to create four spores of two mating types; alpha or
A
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In the haploid form:
Alleles are immediately recognised, even if they’re
recessive
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Red strains carry gene mutations in biosynthesis of adenine for red phenotype
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Tools of Genetic Analysis

Mutagenesis:
We need lots of mutations/alleles to be able to perform genetic analysis
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Above shows 9 mutants assayed then found to be associated with
phenotype P
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We then try and find
what order they are in the pathway
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We cross the different
mutation strains to make a diploid
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If
we see a different phentoype, a wild type, this
means the mutants are on different genes
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Complimentation Test:
We can make double mutant diploids for all the
mutation combinations, to systematically deduce
which combinations still produce the same
phenotype
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However, there is a limitation, if one of these alleles
is dominant
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To
avoid this we sort which is dominant and
recessive by breeding them with a wild
type
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This can occur when a
protein has different domains, e
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A and
B
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TRF2

An example of a dominant-negative allele:
The above shows a cartoon of the telomeres
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They have each 2 DNA binding domains at the C terminus, both
proteins need both DNA binding sites to efficiently bind to the telomeres
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This
mutated TRF2 will be able to dimerise with the wild type, normal functioning TRF2 but it cannot
bind with telomeres to carry out their function
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Epistasis Analysis
An example of a dominant-negative allele:


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In
other words, we don’t know what
type of pathway we are dealing with;
linear or lots of branches?

How we obtain information about a genetic pathway?:
Cross mutatants to see how they interact genetically! An organism holding two alleles for each of the two
genes were examplifying, we want to see how they interact by what phenotype they produce, which is
known as epistasis, the analysis of how one gene affects another
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Epistatic analysis allows us to deduct the
chronological order of what genes effect what other genes in a pathway
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For example, you could study
the phenotype of sensitivity to UV irradiation
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They could be linear,
meaning one gene effects another in a pathway or they are in
parallel (right) when they only directly effect the phenotype
without effecting each other
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If you make a double mutant with A
and B
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Two Genetic Pathways
Substrate dependent pathway has steps needed in series (linear) to create a particular phenotype
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Effectively, this is a
series of positive reactions
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Switch regulatory pathways are a combination of positive and negative regulators that can be in an ‘on’ or
‘off’ state, which act on another
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A hypomorphic allele is one that has reduced effect on the phenotype, most likely by an epistatic allele
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Inhibition of telomerase will
shorten the chromosome
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Epistasis analysis of telomere length regulation:

We know this from looking at telomerase DNA, ones with black blots that stop higher up have long
telomeres
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So in the absence of these genes telomerase produced longer telomeres,
meaning these proteins have negative effects on telomerase when in action
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Also, because the phenotype is more strongly effected by non-functioning rif 2, then we have to say
there must be a gene X that induces the inhibitory effect of rif 1 on telomerase
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Because they have additive effects they must be in parallel
pathways, shown on the left, meaning they are not epistatic because one doesn’t affect another
but just adds to the effect
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Also, the rap1, rif 1, rif 2 triple mutant are the same length as the rap1 single
mutant, so rap1 is epistatic to rif 1 and rif 2
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Mutation makes telomerase less active producing shorter telomeres
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Cells with mutations in tel and any of rif2, rif2, or
rap1 will have very short telomeres as short as the
tel1 single mutant
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As shown above on the left
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