Download LAB EXERCISE: Population Genetics

LAB EXERCISE: Population Genetics
Laboratory Objectives
After completing this lab topic, you should be able to:
1. Define the following terms – natural selection, recessive lethal allele, gene pool,
population genetics, and mutation.
2. Relate changes in allele frequency to natural selection.
3. Explain why it is difficult to eliminate recessive alleles from a population.
Introduction
Natural selection deals with the idea that those individuals within a population with genes that
make them better adapted to their surroundings are more likely to survive, reproduce, and pass
their genes on the next generation. In biological terms, nature selects those individuals most fit to
survive. Over time, the process of natural selection leads to an increase in the frequency of
“good,” or beneficial, genes in a population and a decrease in the frequency of “bad,” or harmful,
genes. New alleles for a gene come into being as a result of mutation, which is a change in the
DNA of an individual. Mutations may have no effect, be beneficial, or cause harm.
There are many degrees of injury caused by harmful alleles. Some such as
nearsightedness, may cause minor inconveniences but could shorten life because of an inability
to see well (auto accidents). Others, such as the allele for sickle-cell anemia, may cause severe
symptoms and shorten life for many people. Some harmful alleles, such as the allele that causes
Tay-Sachs disease, are lethal in the homozygous recessive condition. Homozygous recessive
individuals contain 2 recessive alleles for a particular characteristic. All homozygous recessive
individuals with 2 Tay-Sachs alleles die. Severely harmful alleles are always rare in a
population; they have a low allele frequency. The allele frequency is a mathematical statement
about how common a specific allele is in the population. The study of the change in allele
frequency in populations is known as population genetics.
In this exercise, we will use an artificial population to follow the frequency of a particular
harmful allele over several generations. We will assume that there are two alleles present in the
population: a dominant normal allele and a recessive lethal allele. Those individuals who inherit
2 recessive alleles must get 1 from each of its parents. When organisms receive 2 recessive lethal
alleles, they die without reproducing and do not pass on their alleles to the next generation.
Therefore, you might expect the number of recessive lethal alleles in the population to decrease
over several generations.
The allele frequency of the recessive lethal allele should decrease as the recessive lethal
allele becomes less common and the dominant normal allele becomes more common. In this
exercise, we will conduct a simulation (an artificial setup that approximates the real situation) to
determine how natural selection will affect the allele frequency of a recessive lethal allele in a
population.
1 During this exercise, you will:
1. Use white beans to represent the dominant normal alleles and red beans to represent the
recessive lethal alleles in a population.
2. Simulate sexual reproduction and natural selection over 10 generations.
Procedure
Generation 1
Use white beans to represent the normal dominant allele (one form of a gene) and red beans to
represent the recessive lethal allele. White = A; red = a
1. Set up a gene pool of 100 genes (80 white alleles and 20 red alleles). The gene pool is all of
the genes in the population of a species. Use the box/bag provided as a container for your beans.
2. Shake the beans in the container and randomly draw two beans at a time (to simulate the allele
combinations in the individuals) and set out the pairs of bean on your bench top. The mixing of
the beans simulates the mixing of genes that occurs during sexual reproduction as a result of
meiosis and random mating. The bean “pairs” you’ve just drawn from the box/bag represent the
individual organisms of the first generation of organisms.
3. Tally the “allelic” combinations you just drew out of the container. We count the beans twoat-a-time because diploid individuals inherit 2 alleles, 1 from each parent. As you count, you will
have three possible combinations: two white beans (AA), red and a white bead (Aa), or two red
beans (aa). Also tally the total number of “A” and “a” alleles.
Genotype
AA
Aa
aa
Phenotype
Organism lives
Organism lives
Lethal; organism dies
4. Two red beans represents a lethal combination. This organism will die. Simulate the deaths of
these homozygous recessive individuals by removing them from the population. (Take the two
red beans out of your population and separate them from the remaining beans.) After you have
examined all the pairs of beans,
a. determine the number of red beans remaining.
b. determine the total number of beans remaining.
c. record your results in Table 1 on page 4.
d. calculate the percentage of red beans remaining in your population (number of red beans
remaining divided by the total number of beans remaining).
% red beans remaining =
total red beans still in the population
total of all beans still in the population
2 e. record this new allele frequency in the last column of Table 1.
Generations 2-10
1. To begin the next generation, we will assume that the survivors reproduce and as a result the
population will grow. In many populations of organisms, the number of individuals being born
and the number of those dying are about the same and the population is considered stable. We
will assume that in our hypothetical “bean” population the reproductive capability of the
organisms keeps the population of total alleles at 100 for each generation and that the surviving
alleles (red and white beans) reproduce equally well. You have already calculated the percentage
of red beans remaining after the first generation (percentage means number out of 100).
Therefore, your second generation starts with 100 beans but with the new percentage of red
beans you have just calculated. To bring the total number of beans back up to 100, while
maintaining the new percentage of red beans: do the following:
a. Count the red beans remaining in the container.
b. Add enough red beans to the container so that the total number of red beans is equal
to the percentage of red beans you calculated in Table 1.
c. After you have added the red beans, add enough white beans to have a total population
of red and white beans equal 100. For example, if you calculated that you have 30% red
beans remaining, add enough red beans to make 30 red beans and 70 white beans to make
100 total beans. If you are unsure, see the following example.
Example
Percent of red beans at start:
20%, because you had 20 red out of a total of 100.
Red beans removed by random mating:
4
Number of red beans left:
20 – 4 = 16
Total number of beans left:
100 – 4 = 96
Red beans ÷ total beans × 100 = % red:
16 ÷ 96 = 0.166 × 100 = 16.6%, or 17* rounded off
*Record value on Table 1.
To prepare for the next round of mating,
1. Add 1 red bean to bring the red total to17.
2. Add 3 white beans to bring the overall total to 100.
2. Starting with the newly reproduced population which is the new generation 2,
a. Shake the beans and count off the beans, two by two.
b. Remove the homozygous recessive lethal organisms (those with two red beans), as you
did for the first generation.
c. Calculate the percentage of red beans remaining among the survivors, as you did after
generation 1.
d. Record your data in Table 1.
e. Repeat this process until you have completed 10 generations.
3. Graph the results of your simulation on the graph paper provided (Generation time vs. Allelic
frequency at the end of each generation).
3 BIOL100
Laboratory Assignment 8: Population Genetics
Name: ____________________________________________
Table 1: Allele Frequency Data
Generation
% Red Beans
at Start of Run
Number
of Red Beans
at End of Run
1
2
3
4
5
6
7
8
9
10
4 Total
Number
of Beans at
End
of Run
New Allele
Frequency
% Red Beans
at End of Run
(Red ÷ Total × 100)