Population Genetics

Question 1

Population and Evolutionary Genetics

Answer 1

is the study of genetic variation in populations and in evolution:
- how genetic variation arises, how it varies, how it is
maintained.
In genetics, a population is defined as a localised group of interbreeding individuals of the same species.
All the alleles of a gene in a population make up the gene pool.
Many traits show variation in a population, called
polymorphism - multiple “morphs” or forms of a trait.

Question 2

Variation or polymorphism in traits can be examined at different levels:

Answer 2

Morphological
Physiological
Biochemical
Most genes are polymorphic – more than one allele present in population.
- If only one - called monomorphic and allele is fixed in
the population.

Question 3

Biochemical Polymorphism - alcohol dehydrogenase

Answer 3

Example: Alcohol dehydrogenase enzyme – breaks down ethanol.
In Drosophila comes in different forms called allozymes:
- migrate differently in gel electrophoresis – called Fast and Slow forms.
- Before DNA methods were available, allozymes were used widely to study variation in populations.

Question 4

Can define genetic structure of a population by

Answer 4

frequencies of different genotypes, or by frequencies of alleles.
Example: one gene, two alleles that show incomplete
dominance for flower colour.
Hypothetical population of 500 individuals:
320 red, 160 pink, 20 white.
Note: When alleles are incompletely or co-dominant can
determine genotype from phenotype - so know genotype of
every individual in population.

Question 5

Incomplete dominance

Answer 5

Heterozygote has intermediate phenotype
Genotypic and phenotypic ratios of F2
coincide:
1: 2: 1

Question 6

Genotype frequencies:

Answer 6

Number in the population with that genotype/ total population

Question 7

Frequency of alleles

Answer 7

p + q = 1

Question 8

DNA marker alleles are co-dominant – so

Answer 8

can use to work out allele frequencies
Example: CCR5 gene in humans (C Chemokine Receptor 5)
– receptor for HIV.
Mutation with 32bp deletion – null allele, resistant to HIV.
- can genotype with PCR.

Question 9

The Hardy-Weinberg Principle

Answer 9

Describes gene pool of a population that is not evolving.
i.e. the allele and genotype frequencies remain constant from generation to generation
(so also called H-W equilibrium).
-If mating is random, every male gamete unites at random with every female gamete,
- frequencies of pairings depend on the allele frequencies e.g. ferquency of allele CR is 0.8 there’s a 80% chance an egg pairing with sperm has CR allel

Can think of all the alleles being in a “bin” or pool, and reproduction occurring by selecting two at random

Question 10

If gametes unite randomly, how can you calculate genotype frequencies in next generation:

Answer 10

p = freq CR
q = freq CW
p + q = 1
Expected genotype frequencies:
CRCR      CRCW       CWCW
   p2     +     2pq     +   q2       = 1

These frequencies are
the same as in previous generation

Question 11

General Hardy-Weinberg equilibrium

Answer 11

For two alleles A and a.
Let p = freq A, q = freq a

Genotypic frequencies will be:
A2 + 2Aa +a2 = 1

Question 12

Applying the H-W Principle

Answer 12

In many cases dominance is complete, so can’t determine genotype of all individuals.
But can still use H-W theory to calculate allele frequencies and estimate genotype frequencies.
e.g. may want to estimate carrier frequency for recessive human disorder.
Example: a human recessive disorder albinism occurs in
1/10,000 births.
What is the expected frequency of carriers?
p2 + 2pq +q2 = 1
q = √1/10000
q = 0.01
p = 1-q
p = 0.99
2pq = 0.0198

Question 13

Genotypes will stay in H-W equilibrium only if

Answer 13

1. the population is very large
2. there is no gene flow
3. there is no natural selection
4. there is no mutation
5. there is random mating

If any of these do not apply then allele and genotype
frequencies will change – microevolution.
The mechanisms that most commonly alter allele frequencies
are due to violations of conditions 1-3

Question 14

Genetic Drift

Answer 14

If population size is not very large, genotype and allele
frequencies can change due to random sampling effects, called genetic drift.

e.g. out of 10 flower 5 leaves offspring then in generation 2 only 3 leaves offspring = all red

In small populations genetic drift acts faster and with greater consequences
- causes fixation of one allele or the other (randomly).

Question 15

WHAT Certain circumstances can increase effect of drift:

Answer 15

(1) Bottleneck – sudden dramatic decrease in population size - Bottlenecks can be due to natural disasters.
(2) Founder Effect – isolation of a few individuals to form new population

In either case, certain alleles may
be over represented in new
population, others under
represented.

Question 16

Founder effect

Answer 16

Founder effect is where a large population has been founded by
a small population.
The allele frequencies of the large population depend on alleles of the ‘founders’ – this is a random effect
Founder effect can occur in human populations if a population starts from very few individuals.

Question 17

2. Gene Flow

Answer 17

Migration of individuals into and out of a population can alter allele frequencies if genotypes migrate differentially.
Different effects if unidirectional or bidirectional.
If bidirectional tends to reduce differences between
populations.

Question 18

3. Natural Selection

Answer 18

If a particular genotype is better suited to an environment these individuals will produce more offspring than others, and
contribute more to next generation.
This will change allele frequencies. We say this genotype has greater “relative fitness” than others,
and other genotypes are “selected against”.

Question 19

Balancing Selection

Answer 19

Sometimes natural selection maintains two or more forms in population, called balancing selection.
E.g. Heterozygote advantage
- heterozygote more fit than both homozygotes under certain
conditions.
E.g. Frequency-dependent selection
- the least common genotype is the most fit

Question 20

4. Effect of mutation

Answer 20

Mutation is an evolutionary force as it creates new variation.
mutation rate = μ
- probability of mutation to a different allele per gene per generation
- mutation rates are generally around 10-5 to 10-8
Recurrent mutation can change allele frequencies
But mutation is extremely slow at changing allele frequencies, and so cannot account for rapid genetic changes.

Question 21

5. Non random mating

Answer 21

Bias towards choosing a similar mate
= positive assortative mating
Bias towards choosing a dissimilar mate
= negative assortative mating
Humans commonly show positive assortative mating for
skin colour and height.
Positive assortative mating increases the homozygosity of the
population (reduces the numbers of heterozygotes)

Question 22

Assortative mating Vs inbreeding

Answer 22

Assortative mating is between similar but unrelated individuals.
Inbreeding is between related individuals.
Inbreeding increases the number of deleterious recessive
individuals (aa) in a population.
Inbreeding also increases the homozygosity of the population
(reduce the numbers of heterozygotes).