Week 4 Selection, Gene Flow and Mutation Flashcards
If an allele is dominant then the heterozygote has the same phenotype as the …… for the dominant allele
If an allele is dominant then the heterozygote has the same phenotype as the homozygote for the dominant allele
If an allele is recessive then the heterozygote has the same phenotype as the …..
If an allele is recessive then the heterozygote has the same phenotype as the other homozygote
If A is dominant then the heterozygote has the same fitness as the ….. for that allele
If A is dominant then the heterozygote has the same fitness as the homozygote for that allele
If A is recessive then the heterozygote has the same fitness as the ……
If A is recessive then the heterozygote has the same fitness as the other homozygote
What is additive fitness?
If A is dominant but the traits have “additive fitness” then the heterozygote has a fitness value that is intermediate between the two homozygoes
What is Directional Selection?
one allele is favoured
What is Disruptive Selection?
extremes of a trait are
favoured
What is Stabilizing Selection?
variation is reduced
What is Balancing Selection?
multiple alleles are maintained in the population
What is Fluctuating Selection?
the direction of selection changes over time
How can we explain the shape of this curve?
- Fitness point of view?*
- What does this mean for deleterious recessive allels?* Examples of this?
When A is at high frequency B is rare, and therefore B is most often present in heterozygotes
There is nothing to differentiate fitness of AA from AB individuals, and so there is very little phenotypic variation for selection to operate on
It is difficult to eliminate deleterious recessive alleles from a population (Ellis-van Creveld syndrome)
How can we explain the shape of this curve?
- Fitness point of view?*
- What does this mean for dominant discorders compared to recessive ones?* Examples of this?
Even when the A allele is at high frequency the B allele is always ‘visible’
From a fitness point of view, selection is always acting to drive out B alleles
Dominant disorders can be driven out of a population more easily than recessive disorders, and hence there are less of them around (marfan syndrome)
What is heterozygote advantage (overdominace)?
What is the graph curve for heterozygote advantage?
Heterozygote advantage:
How can we explain the shape of this curve?
There is a balance of the selection for A and against A
What is an example of heterozygote advantage?
Sickle-cell anemia
The sickle-cell allele (HbS) is autosomal recessive; meaning only homozygotes are affected
However, HbS also confers partial resistance to malaria, meaning in certain parts of the world the heterozygote has the highest fitness
What is the graph curve for heterozygote advantage when taking into account fitness?
What is heterozygote disadvantage (underdominance)?
What is the graph curve for heterozygote disadvantage?
What is frequency dependent selection?
when the fitness of a genotype depends on the frequency of the genotype in the population
Frequency dependent selection: A genotype is favoured when it is common or rare?
rare
Frequency dependent selection: When the genotype is favoured, the frequency of
alleles which contribute to it increase or decrease?
Increase
Frequency Dependent Selection: As these alleles increase in frequency, the genotype
increases or decrease in frequency?
increase
Frequency Dependent Selection: The fitness of the genotype decreases as it becomes more common or rare?
common
Frequency Dependent Selection: The decrease in fitness causes the alleles which contribute to the genotype to increase or decrease?
decrease
What is gene flow?
the processes by which individuals genes (or alleles) move from one population to another
- Gene flow can be one-directional or multi-directional*
- Movement of individuals does not necessarily imply movement of genes*
In the absence of gene flow populations tend to become genetically …. from one another
genetically differentiated from one another
What is panmixia?
random mating
A panmictic population is one where all individuals are potential partners
What is barrier to gene flow?
What is differentiation?
Mainly visible in neutral loci, which are evoluing under drift alone
What can happen to the gene flow in homogenises populations?
can recover lost genetic variation
What can be experienced in a population which is isolated?
limited or zero gene flow, we expect drift to lead to differentiation between populations
Smaller numbers of differences are expected between close branches, larger differences between more distant branches
What is Mutation?
The processes producing genetic variation
DNA changes either have no effect or cause harm, but occasionally a mutation can improve an organism’s chance of surviving and passing the beneficial change on to its descendants
What is point mutation? & example
one base exchanged for another
– Sickle-cell disease: GAG is mutated to GTG and glutamic acid becomes valine
– Many cancers
– Color blindness
What is insertion mutation? & example
extra base pair(s) inserted
– Huntington’s Disease - abnormal region of CAG repeats (4p16.3)
What is deletion mutation? & example
base pair(s) lost
– Duchenne muscular dystrophy (Xp21)
What is frameshift mutation? & example
Applies to insertions and deletions. Anything which changes the amino acid sequence being coded fo
– Chrohn’s disease
Label a chromosome:
What are the larger mutational events that can occur?
• Large-scale deletion/insertion events
• Duplication
• Inversion
• Translocation
very large:
• Polysomy (Trisomy 21)
• Whole genome duplication
Each gene copy experiences mutation at a rate. What is the symbol for rate?
μ
In a population of 2N genes the total number of mutations is therefore, μ becomes what?
2Nμ
The chance of any one new allele going to fixation is what?
1/(2N)
Therefore…the probability of a new mutant allele going to fixation under drift alone is what?
1/(2N) * 2Nμ = μ
The rate of substitution is dependant or independent of the population size?
independant
