Lectures 34/35 Population Genetics Flashcards
What is the locus?
The location of a gene on a chromosome
What is the gene?
The unit of heredity
What is an allele?
A variant of a gene or locus
What is a haplotype?
Combination of alleles in a gamete
What is heterozygosity?
A measure of variation
What are the different types of polymorphism?
Morphological (rare) Chromosomal (number and arrangement) Immunological (Blood groups, antigenic groups) Proteins (Electrophoretic ability) Proteins or DNA Sequence
What equation can be used to determine the frequency of genotypes in a population
p^2 + 2pq + q^2
How can the equation p^2 +2pq + q^2 be used to calculate allele frequencies?
f(A)= p^2 + 0.5*f(Aa)
How can we calculate genotype frequencies?
p^2= frequency of one homozygote q^2= frequency of other homozygote 2pq= frequency of heterozygotes
How can heterozygosity be measured?
1-f(homozygotes)
What is the purpose of the hardy-weinberg equilibrium?
To provide a null hypothesis in genetics against which to test if a factor is having an effect on a populations genetics or whether this would have happened normally
What are the assumptions made by the hard-weinberg equation?
No mutation occurs
No movement of individuals into or out of the population
The population is significantly large that there is no random genetic drift
Mating is panmictic (no inbreeding)
no selection
What are the features of the impact of mutation on population genetics?
They are the only true source of genetic novelty
While rare in coding genes due to the high fidelity of DNA replication they can be substantial on a population basis
Occur randomly without benefit to the cell
However mutations will only effect the allele frequencies of a population if it is a recurrent mutation (at the same gene locus) with no reversion as this will decrease the frequency of the allele as it is lost
How can the decrease in allele frequency be determined via mutations?
p(n)=p(0)e^(-nu)
u=rate of mutation
p0= initial allele frequency
pn=allele frequency after n generations
What is the impact of inbreeding on population genetics?
Can result in a loss of heterozygosity
which frequently causes inbreeding depression as their is an increased chance that deleterious and lethal alleles which are recessive will have an impact
There is reduced ecological and evolutionary adaptiveness so populations are more vulnerable to environmental changes
How does random drift effect heterozygosity?
It will result in a change in allele frequencies solely from chance however this will only occur if the population is small
Fixation or continued heterozygosity is unpredictable
In the extreme case of inbreeding, self-fertilization what is the numerical effect on heterozygosity?
Heterozygosity is halved each generation
How can the decline of loss of heterozygosity be determined?
H(t)=H(0)(1-1/(2N))^t
N=population size
t=number of generations
What are the events that can cause random differentiation of populations?
Bottleneck where there is a reduced size of the population over time
Founder effect where there is reduced size over space
What is homozygosity by descent?
When you have two genes at a given locus that are descended from a single source, such as may occur in inbreeding. This adds to the homozygosity achieved by the population through mating of unrelated partners (p^2+q^2)
What is the inbreeding co-efficient and how can it be calculated?
The inbreeding coefficient is the probability homozygosity by descent and can be calculated through the equation F=(1/2)^n where n is the number of outbred individuals
Why can inbreeding be harmful clinically?
It can lead to the expression of deleterious recessive alleles this can lead to clinical conditions such as
Sickle Cell anemia, cystic fibrosis, phenylketonuria, tay-sachs disease, alkaptonuria
How can the risk of inbreeding resulting in the expression of deleterious alleles be calculated?
If there is random breeding then the risk is =p^2
If there is inbreeding then the risk =Fp for full siblings this means p/4 and p/16 for first cousins
What is migration and gene flow?
Movement of alleles into one population from another (this process can contribute to a loss of alleles but only if the population is very small)
Process results in an admixture of the allele frequencies of both populations to cause the new allele frequency
How can the new allele frequency caused by migration and gene flow be calculated?
Initial allele frequency of recipient population= Pt
Initial allele frequency in migrants= P
Proportion of next generation of recipient population contributed to by migrants=m
new allele frequency:
P(t+1)= (1-m)Pt+mP=Pt+m(P-Pt)
Difference= m(P-Pt)
What is the effect on gene flow on differentiation?
Acts as a homogenizing force
What is selection with reference to population genetics?
When alleles have differential rates of survival and reproduction causing changes in allele frequencies and genotypes in a population
How can selection in a population be detected?
Through detecting fitness, this can done by calculating the expected count of the genotype via the hardy-weinberg equation and then determining the ratio of observed/expected=fitness
This can then be standardised by:
fitness/fitenss of refrence genotype
How does selection interact with allele frequencies?
There is a sigmoidal relationship so selection is rapid at intermediate frequencies but slow and low and high frequencies
Why do recessive deleterious alleles persist in populations?
There can be a recessive refuge in heterozygotes provided that they do not show any symptoms of the condition
What is the role of codominance on the frequency of hetero and homzygotes?
When there is co-dominance of genes then the fitness of heterozygotes can be different to the fitness of homozygotes
If heterozygotes are less fit then this is an unstable condition (underdominance) with only small changes in allele frequencies leading to the extinction or fixation of the allele
If heterozygotes are more fit then this is a stable condition of balanced polymorphism where equilibrium will be returned to if there is a small change in allele frequencies
What is the neutral theory?
Because diversity is greater than predicted, the neutral theories suggests this is due to many alleles being equally fit allowing a large proportion of diversity to be produced through random mutation and random genetic drift
For neutral alleles:
Rate of fixation=rate of mutation=rate of evolution
What are neutral mutations?
Mutations in non-coding DNA regions
Mutations which code for the same or a similar amino acid
