Download Ch. 15 Notes

opulation Genetics & Evolution
Ch. 14 & 15
Topic Overview
• How do we measure allele frequencies in
populations?
• These measurements are used to analyze
the genetic structure of populations.
• Evidence from several disciplines is
providing insight into the origin of species
and their dispersal over the earth.
Ch. 14: When Allele
Frequencies Stay
Constant
Population Genetics
• Genes can be considered at the POPULATION level.
• Population = An interbreeding group of the
same species in a given geographical area
• POPULATION GENETICISTS don’t care about
individuals or families, but are interested in the GENE
POOL that is available.
• Gene pool = The collection of all alleles in the
members of the population
• If we look at a POPULATION and determine the GENE
POOL, we can then determine the ALLELE FREQUENCY
for each.
• Allelic frequency is frequency of a allele that is
present in the population
Allelic Frequencies
• Codominant alleles can be measured directly
(MM, MN, NN etc.)
• Recessive/Dominant allelic frequencies cannot
be measured directly based on ‘visual
phenotypes’
• DNA based methods can convert
recessive/dominant traits into ones where DNA
markers are used to identify the alleles
• Mathematical formulas such as HardyWeinberg Law can be used to determine allelic
frequencies
Allele Frequencies Change
• ALLELE FREQUENCIES can change when:
1. One genotype confers a REPRODUCTIVE
ADVANTAGE.
2. MIGRATION occurs between populations.
3. Subgroups in a population become
REPRODUCTIVELY ISOLATED
4. MUTATION changes ALLELES or introduces new ones
5. NATURAL SELECTION occurs.
• A small change in ALLELE FREQUENCY is a
MICROEVOLUTION, and over time, MACROEVOLUTION (the
formation of a new species) can occur.
• Gene Flow = Movement of alleles between populations
when people migrate and mate
Rare Genetic Disorders
• Tests for about 900 genetic disorders are available
through public and private testing laboratories.
• There is no testing for many rare diseases.
• In the US = affect about 1 in 1,500 people or fewer.
• Even though there may be a low number o those
affected with recessively inherited disease in a
population, heterozygotes can be quite frequent.
• Ex: 1 in 20 members are heterozygotes if 1 in 1,500
people are affected.
Journal #26
• If your family history showed to presence of a
rare disease that is fatal in early childhood and
testing was not available to determine if you
carry the mutation, what would you do?
1. Take a chance that you mate is not a
heterozygote
2. Go ahead and have children knowing that if
your mate is a heterozygote, there is only a
25% chance that a child will be affected.
3. Decide not to have children.
4. Decide to adopt.
Why use Hardy-Weinberg?
• Without DNA testing, the frequency of recessive alleles
in the population cannot be measured directly.
• Estimating the frequency of heterozygotes in a
population is an important part of genetic counseling.
• Godfrey Hardy & Wilhelm Weinberg independently developed
a formula that can be used to determine the recessive allele
frequency.
• This formula is used by geneticists, clinicians, field biologists,
population geneticists, and others to measure the frequency
of alleles and genotypes in populations, without the need to
use DNA testing.
Hardy-Weinberg equilibrium, also referred to as the
Hardy-Weinberg principle, is used to compare allele
frequencies in a given population over a period of
time. It provides a framework for understanding how
populations evolve.
Hardy-Weinberg equilibrium
• Describes populations that are not evolving.
• Allele frequencies remain constant from
generation to generation.
Hardy-Weinberg equilibrium
• The population of alleles must meet five rules in order to
be considered “in equilibrium”
• Genotype frequencies stay the same if five conditions
are met.
1.
2.
3.
4.
5.
very large population: no genetic drift (reproductive isolation)
no emigration or immigration: no gene flow
no mutations: no new alleles added to gene pool
random mating: no sexual selection
no natural selection: all traits aid equally in survival
Hardy-Weinberg equilibrium
describes populations that are not
evolving.
• Real populations rarely meet
all five conditions.
• Real population data is
compared to a model.
• These models are used to
study how populations
evolve.
Using the Hardy-Weinberg Law
• The Hardy-Weinberg equation is used to predict genotype
frequencies in a population
• Frequency of the dominant (A) allele : p
• Frequency of the recessive (a) allele: q
• Because the sum of p and q represent 100% of the alleles
for that gene in a population : p + q = 1
• Frequency of homozygous dominants (AA genotype): p2
• Frequency of homozygous recessive (aa genotype): q2
• Frequency of heterozygotes (Aa genotype):
2pq
• These represent 100% of the genotypes for this gene in the
population, so
• p2 + 2pq +q2 = 1
Hardy-Weinberg Equation
• The EQUATION is only possible in an IDEALIZED POPULATION.
• Expressed genes (EXONS) are rarely in H-W EQUILIBRIUM
because NATURAL SELECTION is OCCURING.
• For “non-coding”genes (which aren’t expressed –
INTRONS) the DNA is often in EQUILIBRIUM
• 95% of the DNA is “junk” DNA
• Disproved the assumption that dominant traits would become
more common, while recessive traits would become rarer because
the alleles are maintained in the heterozygous individual.
• Equilibrium helps maintain genetic variability.
• CARRIER RISK
Using the Hardy-Weinberg Law
•
For autosomal recessive diseases, the homozygous recessive class is used to determine
the frequency of alleles in a population - Its phenotype indicates its genotype.
• CF affect 1 in 2500 caucasian newborns (european decent)
• Homozygous recessive geneotype (aa) = q2
q2 = __1__ = 0.0004
2,500
q = 0.0004
q = 0.02 = 1/50
Because p + q = 1, once we know the frequency of the a allele is 0.02 (2%), we
can calculate the frequency of the dominant allele A by subtraction.
p=1–q
p = 1 – 0.02
p = 0.98 or 98%
In this population, 98% of the alleles for gene A are dominant (A) and 2% are
recessive (a).
Heterozygote = carrier frequency
2pq = 2 (0.98)(0.02) = 0.039 or 3.9%
• Or approximately 1/25 or 1 in 25 Caucasians carry the gene
for cystic fibrosis.
Individuals who are Heterozygote for
rare recessive traits aren’t that rare.
What are the odds of CF in the
caucasian population?
• Carrier 2pq = 1/25 for CF
• Two heterozygotes mate
• (1/25) * (1/25) = 1/625 = 0.16 %
• If they mate they have a ¼ chance of producing an
affected child.
• So, the chance that they will mate and produce an
affected offspring is:
• (1/25)(1/25)(1/4) = 1/10,000 or 1 in 10,000
individuals has this disorder.
Applying H-W to Sex Linked Traits
• SEX LINKED TRAITS are slightly different because of the male only needing
to receive one “dose” from a parent.
For females, the standard Hardy-Weinberg equation
applies
•
•
p2 + 2pq + q2 = 1
• However, in males the allele frequency is the
phenotypic frequency
•
•
•
•
•
• p + q= 1
With 1/10,000 males having hemophilia,
q = 0.0001
p =1 - 0.0001 = 0.9999
Carrier Frequency (females) 2pq = 2(0.001)(0.9999) =
0.00019 = ~1/5000
Affected females = q2 = (0.0001)(0.0001) =
• Only 1/100,000,000 females have hemophilia
• Often q approaches 0 if a trait is rare enough, which makes
the carrier frequency simply TWICE the frequency of the
trait.
• A POLYMORPHISM is a DNA sequence that VARIES in at least
1% of a POPULATION at a particular GENE LOCUS
• It is too frequent to be the result of MUTATION
• This makes possible DNA Profiling
Deviations from H-W
• Genetic drift changes allele frequencies due to chance alone.
• Gene flow moves alleles from one population to another.
• Mutations produce the genetic variation needed for evolution.
• Sexual selection selects for traits that improve mating success.
• Natural selection selects for traits advantageous for survival.
• In nature, populations evolve.
– expected in all populations most of the
time
– respond to changing environments
SPECIATION
KEY CONCEPT
New species can arise when populations are isolated.
The isolation of populations can lead to
speciation.
• Populations become isolated when there is no gene flow.
• Isolated populations adapt to their own environments.
• Genetic differences can add up over generations.
• Reproductive isolation can occur between isolated populations.
– members of different
populations cannot mate
successfully
– final step to becoming
separate species
•
Speciation is the rise of two or more species from one existing species.
Populations can become isolated in
several ways.
• Behavioral barriers can cause isolation.
• called behavioral isolation
• includes differences in courtship or mating behaviors
• Geographic barriers can cause isolation.
– called geographic isolation
– physical barriers divide population
•
Temporal barriers can cause isolation.
– called temporal isolation
– timing of reproductive periods prevents mating
Ch 15: Changing
Allele Frequencies
Hardy Weinberg Review
• Used to study of alleles in a population.
• Allele frequencies have to be constant.
• Large gene pool, w/ No mutation, migration, natural selection
& only nonrandom mating.
p2 + 2pq + q2 = 1 or 100%
EX: BB + Bb +bb = 1
• Used to determine carrier frequency for genetic disorders that
aren’t be tested for.
• Changing allele frequencies permit both LONG AND SHORTTERM CHANGES to occur in a species/population.
• Gene frequencies are affected by both NATURAL and
UNNATURAL events.
• Natural events such as severe storms, EARTHQUAKES and
DISEASES can shrink the gene pool.
• Unnatural events (caused by MAN) can also have
immediate and lasting affects on the available genes in a
population. (wars, DEFORESTATION, nuclear testing,
OVER-FISHING)
Conditions That Change Allele Frequencies
•Nonrandom mating
•Migration
•Genetic drift
•Mutation
•Selection
Nonrandom mating
• Nonrandom (SELECTIVE) Mating
• Nonrandom mating indicated individuals of one
genotype reproduce more often with each other.
• May be due to
• Ethnic or religious preferences
• Isolated communities
• Cultures in which consanguinity (marriages between
relatives – cousins) is more prominent
• EX: The males of some bird species
must make ELABORATE NESTS to lure in
potential female partners.
• Sexual selection selects for traits that improve mating success.
• Humans choose their mates based on
physical appearance, ethnic background,
intelligence and shared interests.
• Some traits ARE RANDOMLY MIXED- MOST
BLOOD TYPES are not affected by
preferential mating. (Mr. Smith didn't meet
Mrs. Smith by asking if they had compatible
blood types.) These randomly mixed traits
are often considered UNIMPORTANT or are
NOT OBSERVED at all.
Examples
• Arnold
• chinese immigrant to south africa with teeth disorder (fall out by
age 20). He had 7 wives. Now: of 356 descendants 70 w/disorder
• Genghis Khan
• 1 – in every 200 males living between Afghanistan & northeast
china share his Y chromosome
• Albanism in Hopi Indians
• men w/ albanism stay back to help women rather than get
sunburned = more contact w/ women = more babies
• Tay Sachs among Ashkenazim Jews
• use disorder databanks to decide not to marry
• Endogamy or marriage within the
community
Migration
• Individuals migrate and move genes from one area to
another.
• Genetic effects of migration are reflected in current
populations.
• Changes in allele frequency can be mapped across
geographic or linguistic regions
• Allele frequency differences between current
populations may be correlated to historical events or
cultural differences.
• Native American alleles started to become MIXED with
European and African alleles after the arrival of Columbus
in 1492.
• Migration may also reduce the VARIETY or frequency of
alleles in the "HOME" population.
• Gene flow moves alleles from one population to another.
• A CLINE is a change in allele frequency
from one population to a neighboring
one. There are linguistic & geographic
clues to the gene frequencies.
• Examples:
• Along the Nile : genetic variations due
to language & cultural barriers in ancient
neighboring kingdoms.
• A mountain range may create a cline.
Genetic Drift
• The "change in allele frequency that occurs
when a SMALL GROUP separates from the
LARGER is termed genetic drift."
• Genetic Drift occurs when a population
becomes SUFFICIENTLY SMALL TO CHANGE
THE NORMAL allele frequencies. The change
in population can be the result of behavior
(the AMISH) or natural disasters (the LEMURS
in the movie "Dinosaurs").
• Genetic drift changes allele frequencies due to chance alone.
FOUNDER EFFECT
• The founder effect is the result of small
groups leaving a population and
creating new populations (if those poor
lemurs in "Dinosaurs" would have
started a new population, it would have
suffered from the founder effect).
• They maintain their original small gene
pool.
• Increases incidence of otherwise rare
traits.
Founder Effect
• Examples:
• French Canadians lack many breast
cancer genes
• DUNKERS = physical features (attached
earlobes)
• Amish & Mennonites (many recessive
disorders)
POPULATION BOTTLENECKS
• Extreme cases of founder effect are called
POPULATION BOTTLENECKS.
• Large population size is drastically reduced in
size.
• Rebounds in population size occur with
descendants of a limited number of survivors.
• After a population bottleneck occurs there are a very
LIMITED NUMBER OF SOME ALLELES and a high
frequency of others to pass to the next generation.
This can have severe a long-lasting effects.
• EX: CHEETAHS (can graft skin from one to another
without rejection), & EUROPEAN JEWS (Tay Sachs)
Mutation
• Allele frequencies change in response to
mutation.
• Can introduce new alleles.
• Can convert one allele to another
• Mutation has a minor impact unless
couple with another effects such as small
population size
• Mutations produce the genetic variation needed for evolution.
Natural Selection
• Is the differential survival and
reproduction of individuals with a
particular phenotype.
• Natural selection may result in
• An increase (positive selection) or
• decrease (negative selection) in the
frequency of an allele.
• Natural selection selects for traits advantageous for survival.
• In nature, populations evolve.
– expected in all populations most of the
time
– respond to changing environments
SPECIATION
KEY CONCEPT
New species can arise when populations are isolated.