Techniques: Population Genetics
Background
A population is a group of organisms of the same species found within a localized area.
Individual organisms within the population do not evolve; however, populations do. When
evolution occurs at the population level, this is known as microevolution. The gene pool
represents the total genes in the population. In a stable, non-evolving population, both the allelic
frequency and the genotypic frequency of the gene pool will remain constant if certain conditions
are met.
The Hardy-Weinberg theorem describes these equilibria in mathematical terms. The frequency of
the dominant allele is represented by p, and the recessive allele frequency by q, such that
p+q=1
The frequencies of the genotypes in the population are represented by p2 (homozygous
dominant), 2pq (heterozygous) and q2 (homozygous recessive), where p2 + 2pq + q2 = 1.
To calculate allelic and genotypic frequencies of a particular trait in a population, start with
q2: the frequency of the homozygous recessive individuals (q2).
q = √ q2
p=1–q
The frequency of the homozygous dominant (p2) and heterozygous genotypes (2pq) can then be
calculated.
The conditions that are required to maintain a non-evolving population are:
 Mutations do not occur
 The population size is large
 There is no gene flow, that is, no immigration or emigration within the localized area
 Mating is totally random.
 No natural selection occurs, that is all genotypes are equal in their reproduction success.
Since it is highly unlikely that all of these conditions can be met, this becomes the basis for
evolution in the population. Natural selection, which gives leads to adaptive changes, is the
primary factor that leads to evolutionary change. Examples include camouflage colorations in
animal that enable them to blend in with the environment, escaping predation, and antibiotic
resistance that develop in bacteria. However, there may be non-adaptive reasons why gene
frequency may change in a population. Two important factors are genetic drift, and gene flow.
Genetic drift is the fluctuation of genotypic and allelic frequency that occurs in small populations
and is due to chance, rather than natural selection. The bottleneck effect is a type of genetic drift
that occurs due to a disaster, for example, an earthquake or forest fire. Due to chance, the
composition of the gene pool of the survivors may not represent the gene pool of the original
population. A second example of genetic drift is the founder effect. When a few members of the
original population become isolated and colonize a new location, this newly formed population
may not represent the allele frequency of the original population.
If a population is not totally isolated then gene flow may occur, for example, wind may carry
pollen from one region to another with a slightly different population. As a result this may lead
to changes in the composition of the gene pool.
A note on bitter tasting ability and single-nucleotide polymorphism
Mammals are believed to distinguish only five basic tastes: sweet, sour, bitter, salty, and umami
(the taste of monosodium glutamate). Taste recognition is mediated by specialized taste cells that
communicate with several brain regions through direct connections to sensory neurons. Taste
perception is a two-step process. First, a taste molecule binds to a specific receptor on the surface
of a taste cell. Then, the taste cell generates a nervous impulse, which is interpreted by the brain.
For example, stimulation of “sweet cells” generates a perception of sweetness in the brain.
Recent research has shown that taste sensation ultimately isdetermined by the wiring of a taste
cell to the cortex, rather than the type of molecule bound by a receptor. So, for example, if a
bitter taste receptor is expressed on the surface of a “sweet cell,” a bitter molecule is perceived
as tasting sweet.
A serendipitous observation at DuPont, in the early 1930s, first showed a genetic basis to taste.
Arthur Fox had synthesized some phenylthiocarbamide (PTC), and some of the PTC dust
escaped into the air as he was transferring it into a bottle. Lab-mate C.R. Noller complained that
the dust had a bitter taste, but Fox tasted nothing – even when he directly sampled the crystals.
Subsequent studies by Albert Blakeslee, at the Carnegie Department of Genetics (the forerunner
of Cold Spring Harbor Laboratory), showed that the inability to taste PTC is a recessive trait that
varies in the human population. In the figure above, Albert Blakeslee is using a voting machine
to tabulate results of taste tests at the AAAS Convention, 1938. (Courtesy Cold Spring Harbor
Laboratory Research Archives)
Bitter-tasting compounds are recognized by receptor proteins on the surface of taste cells. There
are approximately 30 genes for different bitter taste receptors in mammals. The gene for the PTC
taste receptor, TAS2R38, was identified in 2003. Sequencing identified three nucleotide
positions that vary within the human population – each variable position is termed a single
nucleotide polymorphism (SNP). One specific combination of the three SNPs, termed a
haplotype, correlates most strongly with tasting ability.
Analogous changes in other cell-surface molecules influence the activity of many drugs. For
example, SNPs in serotonin transporter and receptor genes predict adverse responses to antidepression drugs, including PROZAC® and Paxil®. In our example, PTC tasting actually
conforms to classical Mendelian inheritance, and illustrates the modern concept of
pharmacogenetics – where a SNP genotype is used to predict drug response.
Purpose
The purpose of this exercise is to learn the Hardy-Weinberg theorem, calculate allelic and
genotypic frequencies in the classroom and stable populations, and determine factors that
influence the gene pool in a population.
Materials per team
PTC (Phenylthiocarbamide) paper
Control paper
Red and white beans
250 ml Beaker
Procedures
A. Hardy-Weinberg Law
1. All the students in the class will represent a population
2. Each student should obtain PTC paper and control paper. The ability to taste this
substance is an autosomal trait. Tasting (T) is dominant to non-tasting (t).
3. First place a control paper at the tip of your tongue and then follow with the PTC paper.
The PTC paper will taste bitter if you are a taster (TT or Tt).
4. Record the individual results and class results. In addition, calculate allelic and
genotypic frequencies according to the Hardy-Weinberg law in Section A of the lab
report.
5. For a second autosomal trait, determine the number of individuals who have unattached
earlobes (E), the dominant trait, and the number with attached earlobes (e), the recessive
trait.
6. Record this data, calculate the allelic and genotypic frequencies, and complete Section A
of the lab report.
B. Genetic Drift
1. Take 50 red beans and 50 white beans and place them in a 250 mL. beaker. These beans
represent the allelic frequency for color in a gene pool, with red (R) = 0.5 and white (r) =
0.5.
2. To simulate random mating, cover your eyes and choose two beans from the beaker.
Record the genotype of the “offspring” in the table in section B of the lab report.
3. Return the beans to the beaker, mix thoroughly, and repeat an additional 49 times,
making sure to return the beans to the beaker each time. This represents the first
generation of this population.
4. Calculate the genotypic frequency and allelic frequency in this population.
5. If these numbers have changed from the original population, create a new gene pool
using the newly calculated allelic frequency for R and r.
For example: If your allele frequency changes to R = 0.6 and r = 0.4, then place 60 red
beans and 40 white beans in the beaker.
6. As in the previous steps, perform 50 simulated random matings again for the second
generation. Record the data in the table in section B of the lab report.
7. Complete section B of the lab report.
WORKSHEET
Population Genetics
1. The overall formula for the Hardy-Weinberg theorem is:
__________________________________________
2. In a population, the frequency for the dominant allele is represented as ____________ and
the frequency for the recessive allele is __________________.
3. Using symbols of the Hardy-Weinberg law, list the genotypic frequencies in a population
a. Homozygous dominant: _______________________
b. Heterozygous:
_______________________
c. Homozygous recessive: _______________________
4. Define the following terms:
a. gene pool: __________________________________________________
b. genetic drift: __________________________________________________________
c. gene flow: ____________________________________________________________
d. microevolution: _______________________________________________________
5. List the five conditions that must be met for the Hardy-Weinberg equilibrium to be
maintained in a population.
a. __________________________________________________________________
b. __________________________________________________________________
c. __________________________________________________________________
d. __________________________________________________________________
e. __________________________________________________________________
A. Hardy-Weinberg Law
1. Record your results from the PTC tasting in the following table.
Number of Tasters (TT, Tt)
Number of Non-tasters (tt)
Total
a. Record your individual result with the PTC paper. _________________________
b. Calculate the frequency of non-tasters in the population. ____________________
Note: Use decimals to the hundredth place for all your calculations.
c. Calculate the frequency of the non-taster allele. ___________________________
d. What is the formula used to calculate the frequency of the taster allele?
__________________________________________________________________
e.
Using this formula, calculate the frequency of the taster allele. _______________
f. Calculate the frequency of the:
i. Homozygous dominant individuals in the population: ________________
ii. Heterozygotes in the population: _________________________________
g. Based on the class size, determine the number of individuals who would be:
i. Homozygous dominant tasters: __________________________________
ii. Heterozygous tasters: __________________________________________
iii. Non-tasters: _________________________________________________
2. Record your results of earlobe position in the following table.
Number with unattached earlobe (EE, Ee)
Number with attached earlobes (e)
Total
a. Record whether you have attached or unattached earlobes. ________________________
b. Calculate the frequency of students with attached earlobes in the population. _________.
As before, use decimals to the hundredth place for all your calculations.
c. Calculate the frequency of the attached earlobe allele. ____________________________
d. What is the formula used to calculate the frequency of the unattached earlobe allele?
________________________________________________________________________
e.
Using this formula, calculate the frequency of the unattached earlobe allele. __________
f. Calculate the frequency of the:
i. Homozygous dominant individuals in the population: ______________________
ii. Heterozygotes in the population:_______________________________________
g. Based on the class size, determine the number of individuals who would be:
i. Homozygous dominant: ______________________________________________
ii. Heterozygous: _____________________________________________________
iii. Homozygous recessive: ______________________________________________
3. Among Caucasians the frequency of cystic fibrosis is 1/2500 live births. Cystic fibrosis is an
autosomal recessive genetic disease more commonly found among Caucasians. You may use
either decimals or fractions for your response.
a. Calculate the allelic frequency for this gene. _____________________________
b. Calculate the frequency of the normal allele. _____________________________
c. What is the frequency of “carriers” in the population? ______________________
B. Genetic drift
1. Record the number of individuals resulting from “random mating” in the following table:
Red/Red
Red/White
White/White
Allelic
Allelic
frequency (R)
frequency (r)
First generation
Second generation
2. The original allele frequency for red is: ________________ , for white is: ______________
3. Calculate the expected genotype frequency of:
a. Red/red
b. Red/white
c. White/white
4. Using the data from the first generation, calculate the frequency for the red (R) and white (r)
alleles and actual genotype frequencies.
a. Frequency of R allele: __________________________________________
b. Frequency of r allele: ___________________________________________
c. Frequency of red/red: ___________________________________________
d. Frequency of red/white: __________________________________________
e. Frequency of white/white: ________________________________________
5. If the results are different from the original allelic frequency describe why this has occurred?
_____________________________________________________________________________
6. Using the data from the second generation, calculate the frequency for the red (R) and white (r)
alleles
a. R allele: __________________________________________
b. r allele: ___________________________________________
c. Red/red: ___________________________________________
d. Red/white: __________________________________________
e. White/white: ________________________________________
7. Define the following terms and give an example of each.
a. Founder effect: _____________________________________________________
__________________________________________________________________
b. Bottleneck effect: ___________________________________________________
__________________________________________________________________
8. What process is responsible for adaptive evolutionary changes? Explain.
___________________________________________________________________________