Lectures 34/35 Population Genetics

Question 1

What is the locus?

Answer 1

The location of a gene on a chromosome

Question 2

What is the gene?

Answer 2

The unit of heredity

Question 3

What is an allele?

Answer 3

A variant of a gene or locus

Question 4

What is a haplotype?

Answer 4

Combination of alleles in a gamete

Question 5

What is heterozygosity?

Answer 5

A measure of variation

Question 6

What are the different types of polymorphism?

Answer 6

Morphological (rare)
Chromosomal (number and arrangement)
Immunological (Blood groups, antigenic groups)
Proteins (Electrophoretic ability)
Proteins or DNA Sequence

Question 7

What equation can be used to determine the frequency of genotypes in a population

Answer 7

p^2 + 2pq + q^2

Question 8

How can the equation p^2 +2pq + q^2 be used to calculate allele frequencies?

Answer 8

f(A)= p^2 + 0.5*f(Aa)

Question 9

How can we calculate genotype frequencies?

Answer 9

p^2= frequency of one homozygote
q^2= frequency of other homozygote
2pq= frequency of heterozygotes

Question 10

How can heterozygosity be measured?

Answer 10

1-f(homozygotes)

Question 11

What is the purpose of the hardy-weinberg equilibrium?

Answer 11

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

Question 12

What are the assumptions made by the hard-weinberg equation?

Answer 12

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

Question 13

What are the features of the impact of mutation on population genetics?

Answer 13

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

Question 14

How can the decrease in allele frequency be determined via mutations?

Answer 14

p(n)=p(0)e^(-nu)
u=rate of mutation
p0= initial allele frequency
pn=allele frequency after n generations

Question 15

What is the impact of inbreeding on population genetics?

Answer 15

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

Question 16

How does random drift effect heterozygosity?

Answer 16

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

Question 17

In the extreme case of inbreeding, self-fertilization what is the numerical effect on heterozygosity?

Answer 17

Heterozygosity is halved each generation

Question 18

How can the decline of loss of heterozygosity be determined?

Answer 18

H(t)=H(0)(1-1/(2N))^t
N=population size
t=number of generations

Question 19

What are the events that can cause random differentiation of populations?

Answer 19

Bottleneck where there is a reduced size of the population over time
Founder effect where there is reduced size over space

Question 20

What is homozygosity by descent?

Answer 20

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)

Question 21

What is the inbreeding co-efficient and how can it be calculated?

Answer 21

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

Question 22

Why can inbreeding be harmful clinically?

Answer 22

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

Question 23

How can the risk of inbreeding resulting in the expression of deleterious alleles be calculated?

Answer 23

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

Question 24

What is migration and gene flow?

Answer 24

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

Question 25

How can the new allele frequency caused by migration and gene flow be calculated?

Answer 25

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)

Question 26

What is the effect on gene flow on differentiation?

Answer 26

Acts as a homogenizing force

Question 27

What is selection with reference to population genetics?

Answer 27

When alleles have differential rates of survival and reproduction causing changes in allele frequencies and genotypes in a population

Question 28

How can selection in a population be detected?

Answer 28

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

Question 29

How does selection interact with allele frequencies?

Answer 29

There is a sigmoidal relationship so selection is rapid at intermediate frequencies but slow and low and high frequencies

Question 30

Why do recessive deleterious alleles persist in populations?

Answer 30

There can be a recessive refuge in heterozygotes provided that they do not show any symptoms of the condition

Question 31

What is the role of codominance on the frequency of hetero and homzygotes?

Answer 31

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

Question 32

What is the neutral theory?

Answer 32

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

Question 33

What are neutral mutations?

Answer 33

Mutations in non-coding DNA regions

Mutations which code for the same or a similar amino acid