Lecture 8: Population Genetics

Question 1

What is a population?

Answer 1

A group of interbreeding individuals of the same species that inhabit the same space and time.

The key to a population is that they must be able to interbreed.

Question 2

What is a gene pool?

Answer 2

All of the alleles PRESENT and CARRIED by the population

Question 3

What is population genetics?

Answer 3

Changes in the frequency of the alleles in a gene pool
over time.

Question 4

Locus vs Allele

Define both

Answer 4

• Locus:
  – location of a gene/marker on the chromosome
• Allele:
  – one variant form of a marker at a particular locus

Question 5

Locus vs Allele …draw image of Homologous Chromosomes..

Answer 5

SLIDE 4

Question 6

What is Hardy Weinberg Equation?

Answer 6

“The Hardy-Weinberg law can be used to find the frequency of disease alleles in a population and the carrier frequency”

Question 7

Allele Frequencies for genotypes AA, Aa, aa

Answer 7

• Frequency of “A” in a population is called “p”
• Frequency of “a” in a population is called “q”

fA + fa = 1
p + q = 1

p = fAA + ½ fAa
q = faa + ½ fAa

….draw punnet square

Question 8

Hardy Weinberg Equation
* Frequency of allele “A: in a population is called “p”

Answer 8

• Frequency of allele “A: in a population is called “p”

– In a population of gametes, the probability that both the
egg and the sperm contain the “A” allele is p x p = p2

Question 9

Hardy Weinberg Equation

– Frequency of allele “a” in a population is called “q”

Answer 9

• Frequency of allele “a” in a population is called “q”

– In a population of gametes, the probability that both the
egg and the sperm contain the “a” allele is q x q = q2

– In a population of gametes, the probability that the egg
and the sperm contain different alleles is
* (p x q) + (q x p) = 2 pq

(p x q) = Male gices “A” and female gives “a”
+ = or
(q x p) = female gives “a” and male gives “A”

Question 10

Punnet square …MALE (sperm) x FEMALE (Eggs)

Answer 10

EQUATION: 2 x Aa = 2pq

punnet square ..slide 8

Question 11

What are the HARDY WEINBERG EQUATION? =2

Answer 11

p2 + 2pq + q2 = 1

OR

(p+q)2=1

Question 12

UNDERSTANDING the Hardy Weinberg Equation…

Answer 12

For a population to be in Hardy Weinberg Equilibrium,
the OBSERVED GENOTYPE frequencies must match those PREDICTED BY THE EQUATION p2 + 2pq + q2

— Determine “p”: equals frequency of AA + ½(frequency of Aa)

— Determine “q”: equals frequency of aa + ½(frequency of Aa)

Then
— p2 = predicted genotype frequency of AA
— q2 = predicted genotype frequency of aa
— 2pq = predicted genotype frequency of Aa

Statistical test required : chi-squared to test observed vs predicted

Question 13

How can alleles for recessive disorders persist in the population?

Answer 13

1 * Recessive disorders are quite rare

2 * ie. the alleles are relatively rare

3 * most recessive alleles are in heterozygotes

4 * Thus, rare disease-causing recessive alleles persist in the population in heterozygote carriers, even if
they are lethal when homozygous

Question 14

An Autosomal Recessive Disease…2

Answer 14

• Affects 1/100 people in a population (ie. 1%)
• The frequency of people with the aa genotype is 0.01
  – i.e. q2 = 0.01

Question 15

What is the frequency of carriers (= heterozygotes) ?

Answer 15

1 * q2 = 0.01 (given)

2 * Therefore q = 0.1 (take square root)

3 * p = 1-q = 0.9 (since p+q=1)

4 * 2pq = 0.18 = almost one in every five

Question 16

Allelic frequencies will remain constant over time if
the following conditions are met: 6

HOW OFTEN MET?

Answer 16

1 *RANDOM MATING

2 * ALLELIC FREQUENCIES are the SAME IN MALES AND FEMALES

3 * ALL GENOTYPES have EQUIVALENT FERTILITY AND VIABILITY

4 * MUTATION does NOT OCCUR

5 * MIGRATION INTO the POPULATION is ABSENT

6 * LARGE POPULATION so that ALLELIC VARIATION DO NOT OCCUR BY CHANCE

These IDEALISED CONDITIONS = are RARLEY MET but HW
HELPS us LEARN why ALLELE FREQUENCIES CHANGE…

Question 17

Human population genetics

There are a NUMBER OF PARAMETRES which INFLUENCE THE TRANSMISSION OF GENES FROM GENERATION TO GENERATION.

WHAT ARE THEY? 6

Answer 17

1. Selection
2. Founder effect / Genetic bottleneck
3. Genetic drift
4. Effective population size
5. Endogamy
6. Consanguinity

Question 18

Human population genetics - UNDERSTANDING “SELECTION”

Answer 18

SELECTION ..can occur when one genotype is more “fit” than another,

i.e., a person
with this genotype has lower susceptibility to disease and/or has higher fertility.

Question 19

Human population genetics … SELECTION: “HETEROZYGOTE ADVANTAGE ..

Answer 19

Heterozygote advantage
is a specific example of selection.

An individual heterozygous at a particular locus, i.e., who has inherited different alleles from each parent, exhibits greater biological fitness.

The best example of this
phenomenon is the greater fitness of HbA/HbS heterozygotes in malarial areas.

Question 20

Human Population Genetics … the founder effect

Answer 20

A disproportionate effect on gene frequencies that can occur when a new population is founded by a small group of individuals from a larger population,

e.g. a migrant
community.

NB In medical genetics, founder is also used to describe the first person in a
community who displays the symptoms of a particular genetic disorder.

Question 21

The founder effect look at image ..slide 18

Answer 21

Sample of original population…

Founding population A - DESCENDANTS

Founding population B - Descendants

Question 22

HUMAN POPULATION GENETICS… UNDERSTANDING GENETIC DRIFT

Answer 22

1 * Random changes in allele frequencies in the absence of selection

2 * More likely to occur in small populations than large ones
– The effective population size (Ne) : proportion of a population capable of reproduction,
e.g., females between 15 and 50 years of age. On average, Ne is 1/3 of the total
population size.
– Allele frequency can increase to fixation (100%), or can be extinguished (0%)
from the gene pool by random chance.

4 * Even in large pops, new alleles are subject to drift

5 * Genetic drift can result in a very good, dominant allele being eliminated from
the population

6 * The vast majority of good alleles/genes that arose during our evolution did not
end up in our gene pool

Question 23

Look at graphs on gENETIC DRIFT

Answer 23

SLIDE 20

GENE FREQUENCY VS GENERATIONS

Question 24

Human Population Genetics … Understanding ENDOGAMY

Answer 24

Endogamy
-. describes the preference for individuals to marry within their own specific community, e.g. cultural, religious, geographical
pressures.

• Through time this can result in community specific genetic profiles.

Question 25

Human Population genetics.. Understanding CONSANGUINITY

Answer 25

– is marriage between close biological relatives, most
commonly between first cousins who have 1/8 of their genes in common, and so on average their offspring are homozygous at 1/16 of all loci.

Question 26

Effect of “Inbreeding”…INDIVIDUAL VS POPULATION

Answer 26

• Individual: Increases the risk of having a child
  homozygous for a rare recessive allele
• Population: Results in an excess of homozygotes compared with random mating

Question 27

Hardy Weinberg Equation information van be used for ? 2

Answer 27

“The Hardy-Weinberg law can be used to find
the frequency of disease alleles in a population
and the carrier frequency”

This information can be used for
1 * Genetic Counselling

2 * Understanding the genetic basis of complex disease