Download Hardy-Weinberg Equilibrium: Bean Love

Hardy-Weinberg Equilibrium: Bean Love
Your team has a container filled with a species that looks remarkably like a bean. Its
name is Beanus gooberensis, and our population is polymorphic for coat color. Some
individuals are red, and some are white, though all are members of the same species.
Red coat color (W) is dominant to white coat color (w), which is recessive.
1. Select two red B. gooberensis from your container. These will be your parental (P)
beans, and we will decide that each of them has the genotype Ww. What is the
frequency of each allele in this parental population of two beans (Adam and Eve)?
W:
w:
2. Construct a Punnett square for the parental cross and determine all possible
genotypes among the babies.
What are the frequencies of each genotype?
WW:
Ww:
ww:
3. What is the frequency of each allele in the babies?
W:
w:
Select four B. gooberensis from the jar to represent the expected babies.
4. Now let the babies go wild! To determine all possible matings of siblings in your
first generation, allow each individual to mate ONLY ONCE (1) with itself and (2) with
each of its three siblings. There are ten possible pairings:
WW x WW
WW x WW1
WW x WW2
WW x WW
Ww1 x Ww1
Ww1 x Ww2
Ww1 x ww
Ww2 x Ww2
Ww2 x ww
5. For each of these matings, do a Punnett square:
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ww x ww
Hardy-Weinberg Equilibrium: Bean Love
6. Record the number of individuals of each genotype for all Punnett squares
combined:
WW:
Ww:
Ww:
7. What is the frequency of each allele in the entire second generation?
W:
w:
8. Have these frequencies changed from the original parental frequencies?
Explain.
Causes of Microevolution
Hardy-Weinberg equilibrium describes the genetics of ideal populations that rarely
exist in nature, and only if the following five conditions are met by that population:
1.
2.
3.
4.
5.
The population is infinitely large
There is no migration of individuals into or out of the population
Individuals mate randomly within the population
The gene in question does not mutate (changing the number of alleles)
No genotype has a reproductive advantage over another
In a population in Hardy-Weinberg equilibrium, the particular gene being examined
should remain within a defined range of relative proportions, and undergo no change in
allele frequency from one generation to the next.
However, if this does not hold true--and relative allele frequencies change from one
generation to the next --then one or more of the five conditions listed above is/are not
being met. And that's when things get interesting. We will perform a few exercises to
demonstrate how some of these factors (#2 and #5 for now) can change allele
frequencies in populations.
Genetic Drift
Let's consider the first rule for HW equilibrium: An infinitely large population. We
can think about it in terms of probability, as in a coin toss.
If you flip a coin, there's a 50:50 chance that it will come up heads. Still, if you
flipped a coin 10 times and it came up heads 8 times, you might reasonably chalk up
the variation from the expected (5 heads and 5 tails) to chance. However, if you went
on to flip 100,000 times, and the coin came up heads 80,000 times, you might just as
reasonably wonder if the coin was haunted. It's not very likely that with so many coin
flips, you'd still get that 80:20 ratio, intead of closer to 50:50. The Main Lesson: the
larger your sample size, the lower the risk of deviation from an expected result due only
to chance.
One of the Hardy-Weinberg requirements is a very large sample size ("infinitely
large population"). A small subset of a large population might not contain an accurate
sample of the allele frequencies present in the large population, and hence, the allele
frequencies of their offspring will not be the same as that of the original large
population. Genetic drift is a change in the gene pool of a small population due to
random sampling error.
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Hardy-Weinberg Equilibrium: Bean Love
Genetic drift can occur via the bottleneck effect, in which the population size is
drastically reduced by some outside factor (earthquake, pathogen, habitat destruction,
etc.). Due to random sampling error, the few survivors might not have the same allele
frequencies as the original large population.
Similarly, genetic drift can also occur if a small subset of a large population
colonizes an isolated region and is permanently separated from the original "parent"
population. This is known as Founder Effect.
Exercise: The Effects of Small Population Size on Allele Frequency
Your container of B. gooberensis contains a population of 100 individuals. We've
counted them out so that 75 express the dominant allele (pigmentation/red skin) and 25
express the recessive allele (no pigmentation/white skin).
1. Calculate the frequency of each allele in this original population:
W:
w:
2. One member of your team should hold the container, while another member looks
away and chooses five individuals at random. These will become your founding
colony. Once you have chosen your individuals, calculate the frequency of each
allele in your new little population.
W:
w:
3. Do these frequencies differ from those of the original population?
How do you predict this might affect allele frequencies after several generations of
mating within the new, small population?
4. Perform the same exercise for founding populations numbering 20, 40, 50 and 80
individuals. Do each of these populations' gene frequencies vary from those of the
original population of 100? If so, which shows the greatest divergence from the
original population's relative genotype frequencies? Is there a relationship between
population size and degree of deviation from the original allele frequencies?
Explain.
5. In the example you just performed, which of the five Hardy-Weinberg criteria is not
being met?
Gene Flow
Gene flow occurs when individuals of the same species mate together, mixing their
genes. Populations that are physically separated from each other--though they are the
same species--have no gene flow. They are more likely to change and differentiate
from each other than are populations that can still mingle and share their genes.
Sometimes populations are only partially separated from each other. Members from
one population can leave (emigrate) and join another population (immigrate). When this
happens, those members bring their genes with them, and that means there's gene flow
between the populations. This can change the allele frequency of both the population
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Hardy-Weinberg Equilibrium: Bean Love
losing its members, and the population gaining members, especially if the two
subpopulations (called demes) don't have the same allele frequencies to begin with.
Exercise: The Effects of Immigration and Emigration on Allele Frequency
Your original container of B. gooberensis contains a population of 100 individuals.
We've counted them out so that 75 express the dominant allele (pigmentation/red skin)
and 25 express the recessive allele (no pigmentation/white skin).
1. Calculate the frequency of each allele in this original population:
W:
w:
Your facilitator will now bring another deme to your station. It may or may not
have the same frequencies of dominant and recessive alleles as your original
one. Let's see what happens when some members of the new population
immigrate into your population.
2. One member of your team should hold the container, while another member looks
away and chooses ten individuals at random from the NEW population. Now
look at your new arrivals and see what color they are. If there are red beans,
let's assume for now that half of them are WW and half are Ww (since we
can't tell for sure, just by looking)
3. You now have a population of 110 beans, but the allele frequencies of W and
w might not be the same as before. If you assume that half each of your
NEW red beans are WW and Ww, and that NEW white beans are ww (a no
brainer), then calculate the NEW allele frequencies in your population, with
the addition of the new immigrants.
W:
w:
Is this the same as the frequencies you calculated before?
Explain:
4. Let's now assume that no further immigration takes place. Use the HardyWeinberg equation to calculate the expected frequencies of WW, Ww, and
ww individuals after a few generations of mating.
WW:
Ww:
ww:
5. Give me a one-word definition for the change in allele frequencies that you
have just witnessed!
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