Population Genetics Chapter 21

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Population Genetics Chapter 21
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Population Genetics is the study of the genes in
a population.a population is all the
individuals of one species in a given area.
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Macroevolution is looking at the Big Picture of
evolution.the creation of new species and the
extinction of old ones (example lung fishes
evolving into primitive amphibians)
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Microevolution/adaptations are when the genetic
code of one individual changes and enables the
individual to survive better. (example natural
immunity from diseases, mutation that results in
camouflage)
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Genetic codes can differ between individuals of
the same species. For instance the amino acid
leucine can be the result of the codons UUA, UUG,
CUU, CUC, CUA, CUG.blood groups have lots of
variances for blood types and blood
proteinsorganisms have many different
alternatives of enzymes and may behave
differently.
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If an individual has alternate codes in their DNA
it is considered heterozygous. On one strand of
DNA I may have UUG for leucine, but on the
homologous strand I may have CUU. I am said to
be heterozygous for that gene locus. This is a
green light for natural selection to occur.
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How do we know if our genes are homo or
heterozygous for certain amino acids? Run a gel
electrophoresis and compare how far the proteins
migrate across the gel
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Hardy Weinberg Equilibrium
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In a given population of animals random mating
will not produce crazy genotypic ratios. They
should be constant/consistent.
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Necessary AssumptionsPopulation is very
large.Random matingNo mutationNo gene input
from external sources.No selection occurring
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Hardy Weinberg Equationp2 2pq q2 (p q)2
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part to memorizep2 2pq q2

homo dom hetero homo recp frequency
of the dominant alleleq frequency of the
recessive allelep2 percent of the homo dom.
phenotype (outward appearance)q2 percent of
the homo rec. phenotype (outward appearance)2pq
percent of the hetero phenotype (outward
appearance)
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frequency of specific events
of total events Ex of tongue
rollers in class of students in class
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if you are contrasting 2 events the decimal
percentages should equal 1.p q 1So if the
frequency of p .75, then the frequency of q
.25
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Example You visited Gatorland Zoo in Florida
and counted the number of turtles that had a
notch in their upper shell (carapace).
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Out of 100 turtles 16 did not have a notch in
their upper shell. 84
did have the notch in their upper shell.
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16 did not have a notch in their upper shell.
84 did have the notch in their upper shell.So
p2 2pq showed the dominant phenotype notched
shell p2 would represent the ones that were
homo dominant 2pq would represent the ones that
were heterozygousboth added together equal the
84 showing the notched trait.q2 represents the
recessive phenotype that do not have the
notch.So q2 represents the 16
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Now here comes the fun. In AP Biology you are
expected to be able to calculate the frequencies
of the alleles p and q. Then you will be able to
pinpoint the frequencies of the homo dom and
hetero phenotypes instead of grouping them
together.
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First set up three columns to show your
workColumn 1 Column 2 Column q2
p2 2pq
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16 did not have a notch in their upper shell.
84 did have the notch in their upper shell.
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Hardy-Weinberg suggests that if random breeding
is present, then in a population of turtles these
frequencies should remain constant. However, if
certain factors interfere, such as
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Mutations Migration into or out of an
areaGenetic drift Non-random mating Or
artificial selection take place then it may
upset the Hardy-Weinberg Equilibrium.
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Mutations somewhat rare but have dramatic
consequences in some cases. Because mutations
are rare and usually affect a single individual,
they dont normally upset the Hardy-Weinberg
Equilibrium.
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Migration occurs when individuals in a
population leave or enter an other area with
another population.If successful mating occurs
then genetic alternatives can begin to show.In
some cases the individual stays behind but their
gametes travel to another population such as
plants releasing pollen into the wind, or clams
squirting their sperm and eggs into the water
column.
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Gene Flow Tends to homologize allele frequencies.
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Genetic Drift is when there are random
fluctuations or loss of alleles in a population
due to a small population becoming isolated.
Example the TV show LOST.
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Founder principle occurs when individuals of a
species become isolated and start a new gene
pool. Ex. If the characters on LOST all started
breeding.
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Bottleneck Effect is when a natural disaster
like a flood, drought, earthquake, or tsunami
occurs and genetic variability is restricted to
the surviving individuals. Example It is
thought that the reason why we only have one
species of cheetahs today is because a great
catastrophe decimated the population and only a
few of the same species survived (made it through
the bottleneck).
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Inbreeding occurs when blood relatives have
sex. It can upset the Hardy-Weinberg
Equilibrium. Inbreeding tends to increase
homozygous traits, especially those that are homo
recessive. Inbred animals tend to acquire homo
recessive traits that can be damaging to their
survival. Plants can be inbred by
self-pollination.
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Outcrossing is the opposite of inbreeding and
results in more heterozygous genes/traits.
Plants outcross by cross-pollination.
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Selection is leaving behind more offspring than
your competitors. Leaving behind a legacy.
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artificial selection is when a breeder chooses
which animals will be bred together.
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natural selection is when the environment or
Mother Nature allows for random mating and
selects the fittest to breed.
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Selection PressuresAvoiding PredatorsMatching
Climatic ConditionPesticide Resistance
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Eugenics is the practice of selectively
breeding humans to gain a dominant race. Society
has frowned upon this practice so far.
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Directional Selection moves in the direction
from a large group of homozygous individuals to
the other extreme of homozygous individuals.
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Stabilizing Selection moves from having too many
homo dom and homo recessive towards gaining more
heterozygous individuals.
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Disruptive Selection moves from having a lot of
heterozygous individuals to homo recessive and
homo dominant.
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Good Luck on the Test