The Hardy-Weinberg Law of Genetic Equilibrium
When does evolution happen?
Tutorial Link before starting…
There is a lesson to be learned from Alice in Wonderland and looking through the
“looking glass”.
Sometimes it is far easier to ask the complementary question:
When does Evolution NOT happen?
G. Hardy and W. Weinberg independently proposed that the frequency of alleles
and genotypes in a population will remain constant from generation to generation
if the population is stable and in genetic equilibrium.
Five conditions are required in order for a population to remain constant and
unchanging.
(No evolution).
First of all define Hardy-Weinberg equilibrium: The steady-state relationship between relative frequencies of two or more
alleles in an idealized population; both the allele frequencies and the genotype frequencies will remain constant from generation to
generation in a population breeding at random in the absence of evolutionary forces.
A change in the gene pool is by definition “evolution”. Define “gene pool”: The total aggregate of genes in a population at
any one time.
The five conditions are required in order for a population to remain at Hardy-Weinberg equilibrium are: Link
1.
A large breeding population
2.
Random mating
3.
No change in allelic frequency due to mutation
4.
No immigration or emigration
5.
No natural selection
Follow Key Concepts 1 through 6 to answer the following: (These are online)
Two different scenarios of the effect of earthquakes on a gene pool: _______________________________________
_____large or small breeding populations will have significantly different effects on the gene pool. _____________
An example of random mating: __coral dispersing sperm without direction, fertilization is by chance__________
An example of assortive mating: ___blister beetles choosing mates of similar size__________________________
An example of a mutation: ____introducing new alleles in a population_____________________
An example of isolation: ____being isolated from other alleles___________
An example of immigration: ___pollen carrier in by the wind_______
An example of selection: ___changing the environment to favor one allele or trait__________________________
An example of no selection: ___removing any advantage of having any particular trait_______________
Looking at predominant hair color in Stockholm and in Seoul, could one immediately determine which allele is
dominant? Yes /No explain___ Unless we knew the populations were in H-W equilibrium…we couldn’t be able
to compare the populations based upon merely looking at the populations. One might assume the blondes in
Stockholm were dominant. Then what about the black hair Seoul? _______________
What is the Hardy Weinberg equation? ____p2 + 2pq + q2 = 1______________________
Sample Problem 1:
Calculate q2 __.25____ Find q.__.5__ Find p.__.5___ Find 2pq.__.5____
Sample Problem 2:
In a certain population of 1000 fruit flies, 640 have red eyes while the remainder has sepia eyes. The sepia eye
trait is recessive to red eyes. How many individuals would you expect to be homozygous for red eye color?
__160______
q2 = 1000-640 = 360/1000 = .36
sqrt.36 = .6 = q
p = 1 – q = 1 - .6 = .4
p2 = (.4)(.4) = .16
16% of 1000 = 160
Sample Problem 3:
Phenylketonuria (PKU) is a human metabolic disorder that results in mental retardation if it is untreated in
infancy. In the United States, one out of approximately 10,000 babies is born with the disorder. Approximately
what percent of the population are heterozygous carriers of the recessive PKU allele? __________
q2= 1/10,000 = 0.0001
q=
= 0.01
p = 1 - q = 1 - 0.01 = 0.99
The carriers are heterozygous. Therefore, 2pq = 2 (0.99) (0.01) = 0.0198= 1.98%
Time out!!! Whoa Nelly! This can be confusing!
Homozygous individuals stricken with PKU never have children. Go back to page 1 and list any HardyWeinberg Equilibrium condition(s) not being met with the PKU example cited above.
____Natural selection would be the biggest problem here. There would also be the condition of random
mating… there is no mating. There is, of course, random mutations occurring all the time.
____________________________
Time out – continued.
Let’s compare two Mugwump populations which inherit coat colour accordingly: BB is bronze, Bb is speckled
(excellent camouflage against lichen) and bb is white. An isolated population has 10 000 individuals: 3600
Bronze - 4800 Speckled - 1600 White ; for a total of 10 000 in the population A different isolated population
also has 10 000 individuals 3000 Bronze – 6000 Speckled – 1000 White ; again for a total of 10 000
individuals. First Population: p = _____.6_________ & q = ______.4______
Second Population: p = ___.68___ & q = ___.32___
One population is in HW equilibrium, while the other population is decidedly NOT in Hardy-Weinberg
Equilibrium. Identify which is which.
The first / second population is in Hardy Weinberg Equilibrium
(circle one)
The first / second population is NOT in Hardy Weinberg Equilibrium
(circle one)
Provide a possible explanation why one population is not in Hardy Weinberg Equilibrium.
____There could have been random mutation but most likely there was some sort of genetic drift___________
Homozygous recessive sickle cell anaemia is lethal and stricken individuals rarely have children. Yet in some
regions of the world, the frequency of the sickle cell allele (q) is increasing over time. Why would that be?
Explain.
______Selection in some areas favors the heterozygous form. These individuals keep the allele in the population
and possibly increase its frequency. _____________________________
In other regions of the world, the frequency of the sickle cell allele (q) is remaining constant over time. Does
unchanging p & q constancy indicate Hardy Weinberg Equilibrium has been achieved?
Explain. Yes / No ___ Unchanging allele frequencies indicate that H-W equilibrium has been achieved. _________
What about Sickle Cell Anemia North America? Is p & q for Sickle Cell Anemia changing in North America?
Explain.
_________One would have to assume that p & q are changing in North America due to immigration, mutations,
and non-random mating. ______
Elusive Mugwumps can be very shy.
Here is a very rare photo of a bronze Mugwump.
Allelic Frequency vs. Genotypic Frequency:
If you are told that the frequency of a recessive allele in a population is 10%, what is q for this population?
______.1_____________
If you observe a population and find that 16% show the recessive trait, you know the frequency of the aa
genotype. This means you know q2. What is q for this population? __.4___
A class of 12 AP biology students gathered the following data:
Initial class frequencies: AA 3 Aa 6 aa 3 Final class frequencies: AA 2 Aa 5 aa 5
1. What are the initial p and q? .5/.5
2. What are the final p and q? .65/.35
3. Is the population in Hardy-Weinberg equilibrium? Explain your response. ____No. The allele frequencies
change. There must be some sort of genetic drift._________
Here is a virtual experiment that demonstrates evolution over 200 years during the Industrial Revolution in
England: Link
What underlying law of nature has produces this change in numbers? ___Natural Selection_______
500 light colored moths and 500 dark colored moths are released into a polluted forest. After 2 days the
moths were recaptured, make a prediction about the number of each type of moth that would be captured.
________400 dark, 100 light… just an example. ___________________
Another Tutorial Link before continuing:
Four evolutionary pressures that cause populations to change:
________genetic drift, gene flow, non-random mating, natural selection
________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
Two causes of “Genetic Drift” would be:
the ______Bottleneck_______ Effect & the___Founder_______ Effect
Natural selection changes allele frequency, and therefore the accumulation of favorable traits, in three ways:
Stabilizing, directional, or disruptive (diversifying) selection
Identify the three kinds of graphs:
A ______Stabilizing ____ selection
B ______Directional _____ selection
C _______Disruptive _____ selection
Explain what could have caused two separate populations in graph 3.
___________having the intermediate form is not beneficial, such as in the case for beak thickness in finches_______
ABC
Witness yourself how a new species can evolve: Link
Five extraordinary examples of adaptive radiation sharing a common ancestor (click on gallery)
____________________________ __________________________ _______________________
____________________________ __________________________
Quiz:
Tom Mueller RHS