Lecture 7: Hardy-Weinberg principle Flashcards
Evolution is driven by what 4 processes: NGGM
- Natural selection
- Genetic drift
- Gene flow
- Mutation
DEF: increases the frequency of alleles
that contribute to reproductive success in a particular environment
Natural selection
DEF: causes allele frequencies to change
randomly
Genetic drift
DEF: occurs when individuals leave one
population, join another, and breed
Gene flow
DEF: modifies allele frequencies by continually
introducing new alleles
Mutation
The Modern Synthesis refers to the era in the
early 1900s. What is one product of this era? (a model)
The Hardy–Weinberg principle
Hardy–Weinberg principle is the mathematical ___ _______ for study of evolutionary processes
Null hypothesis
In 1908, G. H. Hardy and Wilhelm Weinberg
– Wanted to know what happened in an entire
____ when all individuals—and thus all possible
genotypes—bred
– Like Darwin, engaged in _______ ________
Population, population thinking
Hardy and Weinberg imagined that all of the
alleles from all gametes in each generation go into
a single group
– called a ____ _____ and then combine randomly
Gene pool
They calculated what would happen if
– pairs of _____ picked randomly, many times, and
each pair combined to produce _____
Gametes, offspring
The calculations predict the _____ of the
offspring that the population would produce,
– as well as the ______ of each genotype
Genotype, Frequency
Deriving the Hardy–Weinberg Principle:
- a gene with two alleles, A1 and A2
- The frequency of A1 is represented by p
- the frequency of A2 is represented by q
- Because there are only two alleles, p + q = 1
- three genotypes are possible: A1A1, A1A2, and A2A2
- The frequency of the A1A1 genotype is p^2
– The frequency of the A2A2 genotype is q^2
– The frequency of the A1A2 genotype is 2pq
T or F
True
What is the Hardy–Weinberg equation2
p^2 + 2pq + q^2 =1
When alleles are transmitted via meiosis and
random combination of gametes. The ____ ________DO NOT change
Allele frequencies
((Therefore when allele frequencies are calculated for the next generation, the frequency of A1 is still p and the frequency of A2 is still q)
The Hardy–Weinberg model is based on what 5
assumptions about the behaviour of populations
and alleles?
- No natural selection at the gene in question
- No genetic drift affecting the gene in question
- No gene flow
- No mutation
- Random mating with respect to the gene in
question
DEF: all members of the parental generation contribute equal numbers of gametes to the gene pool
No natural selection at the gene in question
DEF: i.e., random allele frequency
changes) affecting the gene in question—alleles not
picked by chance because assumes population is large
No genetic drift affecting the gene in question
DEF: no new alleles are added by immigration
or lost through emigration
No gene flow
DEF: no new alleles are introduced to the
gene pool
No mutation
DEF: no mate choice; gametes combine
randomly Copyright
Random mating with respect to the gene in
question
2 conclusions of the Hardy-Weinberg
principle:
- The allele frequencies in a population will not
change, generation after generation - If the allele frequencies in a population are given
by p and q, the genotype frequencies will be
given by p^2, 2pq, and q^2
T or F
True
Hardy–Weinberg Principle Serve as a ____ Hypothesis
Null
CASE STUDY 1: Are MN Blood-Type Alleles in
Humans in Hardy–Weinberg Equilibrium
NO ANS
What are the 2 alleles that human populations have for the MN blood group?
M and N, which code for protein on surface of red blood cell
M and N are _____ (heterozygotes have both M and N versions of protein on red blood cells)
Codominant
The genotype of a person can be determined from
blood samples
- Genotype may be ___, __, or __
MM, MN, or NN
Analysis to determine if the population is in
Hardy–Weinberg equilibrium based on four steps
NO ANS
Define Estimate genotype frequencies
by dividing total number of individuals with each genotype by the total number of individuals in the sample
Define Calculate observed allele frequencies
from the observed genotype frequencies
Ex. Example: The frequency of the M allele = frequency of MM homozygotes + half the frequency of MN heterozygotes.
Define calculate the genotypes expected
Use the observed allele frequencies according to the Hardy–Weinberg principle
Define compare
Statistically compare the observed and expected
values
In a population of 2,345 people located in Inuit,
Greenland: 1,958 had genotype MM, 366 had
genotype MN, and 21 had NN. Calculate:
1. The observed genotype frequencies (MM, MN, NN) in the
population
2. The observed allele frequencies of M and N in the
population
3. The expected genotype frequencies if the population
were in Hardy-Weinberg equilibrium
4. Compare the observed and expected genotype
frequencies
NO ANS
The observed and expected MN genotype
frequencies were almost identical. Thus:
– For every population examined, the genotypes at the
MN locus are in Hardy–Weinberg equilibrium
– The MN blood groups were not being affected by any
of the four evolutionary processes
– Mating was random with respect to this gene
– Null hypothesis is accepted
NO ANS
Natural selection occurs when:
- Individuals with certain phenotypes produce more
surviving offspring than do individuals with other
phenotypes
– If certain alleles are associated with these favoured
phenotypes,
▪ they increase in frequency while other alleles decrease
in frequency
– The result is evolution
▪ Hardy–Weinberg model assumption has been VIOLATED
T or F
True
DEF: the number and relative
frequency of alleles that are present in a particular
population
Genetic variation
Maintaining genetic variation is important
– Selection can occur only if heritable variation exists in a
population
– Lack of variation can make populations less able to
respond successfully to changes in the environment,
and their average fitness will decline
– If environmental change is severe, the population may
become extinct
T or F
True
Natural selection occurs in a wide variety of
patterns or modes (4 points)
- Directional selection
- Stabilizing selection
- Disruptive selection
- Balancing selection
DEF: changes the average
phenotype in the population in one direction
Directional selection
If it continues, the favoured alleles will approach a
frequency of __ while disadvantageous alleles will
approach a frequency of ___
1.0, 0.0
Alleles that reach a frequency of 1.0 are termed ___, and those that reach 0.0 are termed ____
Fixed, lost
When disadvantageous alleles decline in frequency,
____ selection is said to occur
Purifying
Directional selection in cliff swallows:
- A six-day cold spell caused the birds’ primary food,
insects, to disappear and many birds died of starvation
– Survivors were much larger on average than the birds that died
Researchers suggested that the larger birds
survived because they had bigger fat stores and
did not get as cold as smaller birds.
* Larger birds thus were less likely to die from
– Exposure to cold
– Starvation
T or F
True
DEF: reduces genetic variation in a trait
- Reduces both extremes in a population
– No change in the average value of a trait over time
Stabilizing selection
Stabilizing selection in human newborns:
- Babies of average birth weight survived best
– Mortality was high for very small and large babies
* Persuasive evidence of stabilizing selection on
birth weight in this population
– Alleles associated with high or low birth weight were subject to _____ selection
– Alleles associated with intermediate birth weight
increased in frequency
T or F
Purifying, True
DEF:
- Intermediate phenotypes are selected against and
extreme phenotypes are favoured
– Has the opposite effect of stabilizing selection
– Increases variation in a trait
Disruptive selection
Disruptive selection in black-bellied seedcrackers:
- Individuals with very short or long beaks survived
– Birds with intermediate phenotypes died
T or F
True
Food (i.e., seed size) is the agent that causes
natural selection
– Nearby only two seed sizes available: small and large
– Small-beaked birds eat small seeds efficiently and
large-beaked birds eat large seeds efficiently
– Birds with intermediate beaks have trouble with small
and large seeds
▪ Alleles associated with medium-sized beaks are subject
to purifying selection
▪ Disruptive selection maintains high overall variation in
this population
T or F
True
Disruptive selection can cause _____, the
formation of new species
Speciation
If individuals with one extreme of a trait mate
preferentially with individuals that have the same
extreme trait, and individuals with the other extreme
values of the trait mate with members of the opposite- sex with the same extreme values
– Selection would result in ___ distinct populations, which may eventually form ___ new species
two, two
DEF: occurs when no single allele has a distinct advantage
- There is a balance among several alleles in terms of
their fitness and frequency
Balancing selection
- Heterozygous individuals have higher fitness than
homozygous individuals do—called _________ ________
Heterozygote advantage
- Consequence is that selection on different alleles is
balanced - Genetic variation is maintained in the population
- Certain ____ are favoured at different times or in
different places
- Although allele frequencies change locally, overall
genetic variation in the population is maintained
Alleles
- Certain alleles are favoured when they are rare but not
when they are common
- _________ ________ selection
Frequency-dependent selection
Ex. Rare alleles for colouration in guppies are
favoured because predators learn to recognize common colours
– Thus, alleles for common colours decrease in
frequency and those for rare alleles increase
– Over the short term, allele frequencies shift, but
variation is maintained over the long term
T or F
True
