Lecture 8: Population Genetics Flashcards
What is a population?
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.
What is a gene pool?
All of the alleles PRESENT and CARRIED by the population
What is population genetics?
Changes in the frequency of the alleles in a gene pool
over time.
Locus vs Allele
Define both
- Locus:
– location of a gene/marker on the chromosome - Allele:
– one variant form of a marker at a particular locus
Locus vs Allele …draw image of Homologous Chromosomes..
SLIDE 4
What is Hardy Weinberg Equation?
“The Hardy-Weinberg law can be used to find the frequency of disease alleles in a population and the carrier frequency”
Allele Frequencies for genotypes AA, Aa, aa
- 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
Hardy Weinberg Equation
* Frequency of allele “A: in a population is called “p”
- 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
Hardy Weinberg Equation
– Frequency of allele “a” in a population is called “q”
- 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”
Punnet square …MALE (sperm) x FEMALE (Eggs)
EQUATION: 2 x Aa = 2pq
punnet square ..slide 8
What are the HARDY WEINBERG EQUATION? =2
p2 + 2pq + q2 = 1
OR
(p+q)2=1
UNDERSTANDING the Hardy Weinberg Equation…
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
How can alleles for recessive disorders persist in the population?
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
An Autosomal Recessive Disease…2
- 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
What is the frequency of carriers (= heterozygotes) ?
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
Allelic frequencies will remain constant over time if
the following conditions are met: 6
HOW OFTEN MET?
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…
Human population genetics
There are a NUMBER OF PARAMETRES which INFLUENCE THE TRANSMISSION OF GENES FROM GENERATION TO GENERATION.
WHAT ARE THEY? 6
- Selection
- Founder effect / Genetic bottleneck
- Genetic drift
- Effective population size
- Endogamy
- Consanguinity
Human population genetics - UNDERSTANDING “SELECTION”
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.
Human population genetics … SELECTION: “HETEROZYGOTE ADVANTAGE ..
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.
Human Population Genetics … the founder effect
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.
The founder effect look at image ..slide 18
Sample of original population…
Founding population A - DESCENDANTS
Founding population B - Descendants
HUMAN POPULATION GENETICS… UNDERSTANDING GENETIC DRIFT
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
Look at graphs on gENETIC DRIFT
SLIDE 20
GENE FREQUENCY VS GENERATIONS
Human Population Genetics … Understanding ENDOGAMY
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.
Human Population genetics.. Understanding CONSANGUINITY
– 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.
Effect of “Inbreeding”…INDIVIDUAL VS POPULATION
- 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
Hardy Weinberg Equation information van be used for ? 2
“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
