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Population Genetics and
Evolution
Mr. Nichols
PHHS
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The Population
as a Genetic Reservoir
 Humans are not distributed randomly across the
world, but are clustered into discrete populations
Populations
 Populations
• Local groups of organisms belonging to a single
species, sharing a common gene pool
 Populations can be described by age structure,
geography, birth and death rates, and allele
frequencies
Population Diversity
 Populations are more diverse than individuals
• Only a group can carry all the alleles for traits
such as blood types A, B, AB, and O
• All the alleles in a population are the gene pool
 Gene pool
• The set of genetic information carried by the
members of a sexually reproducing population
Allele Frequency
 Allele frequency
• The frequency with which alleles of a particular
gene are present in a population
 The frequency of alleles in a population may
change from generation to generation
• Changes in allele frequency can cause change in
phenotype frequency; long-term change in allele
frequency is evolutionary change
How Can We Measure
Allele Frequencies in Populations?
 Population genetics studies allele frequencies in
populations, not offspring of single matings
 In some cases allele frequency in a population
can be measured directly
 In other cases, the Hardy-Weinberg Law is used
to estimate allele frequencies within populations
Codominant Allele Frequencies
Can Be Measured Directly
 Codominant allele frequencies can be measured
directly by counting phenotypes
• Phenotypes are equivalent to genotypes
 Example: The MN blood group
• LM and LN alleles are codominant and produce
three phenotypes, M, N, and MN
Recessive Allele Frequencies
Cannot Be Measured Directly
 With recessive alleles, there is no direct
relationship between phenotype and genotype
• Heterozygotes and dominant homozygotes have
the same phenotype
 Hardy and Weinberg independently developed a
mathematical formula to determine frequency of
alleles when one or more alleles are recessive
• Under certain specified conditions
The Hardy-Weinberg Law Measures
Allele and Genotype Frequencies
 Hardy-Weinberg Law
• Allele and genotype frequencies remain constant
from generation to generation when the
population meets certain assumptions
• There is a difference between how a trait is
inherited and the frequency of recessive and
dominant alleles in a population
Brachydactyly: A Dominant Trait
 What is the relationship between allele
frequency and phenotype frequency?
Assumptions of
the Hardy-Weinberg Law
 The population is large enough that there are no
errors in measuring allele frequencies
 All genotypes are equally able to reproduce
 Mating in the population is random
 Other factors that change allele frequency
(mutation and migration) can be ignored
Mathematics of
the Hardy-Weinberg Law
 For a population, p + q = 1
• p = frequency of the dominant allele A
• q = frequency of the recessive allele a
 The chance of a fertilized egg carrying the same
alleles is p2 (AA) or q2 (aa)
 The chance of a fertilized egg carrying different
alleles is pq (Aa)
Determining the Frequency of
Alleles in a New Generation
 Depends on the frequency of dominant and
recessive alleles in the parental generation
The Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
• 1 = 100% of genotypes in the new generation
• p2 and q2 are the frequencies of homozygous
dominant and recessive genotypes
• 2pq is the frequency of the heterozygous
genotype in the population
Calculating Frequency of Alleles
in a New Generation
 Given alleles in the parental generation are
a (q = 0.4) and A (p = 0.6)
Populations Can Be
in Genetic Equilibrium
 Genetic equilibrium
• When the allele frequency for a particular gene
remains constant from generation to generation
• Equilibrium in a population explains why
dominant alleles do not replace recessive alleles
 In equilibrium populations, Hardy-Weinberg law
can be used to measure allele and genotype
frequencies from generation to generation
Keep In Mind
 The frequency of recessive alleles in a
population cannot be measured directly
Using the Hardy-Weinberg Law
in Human Genetics
 The Hardy-Weinberg Law can be used to
• Estimate frequencies of autosomal dominant and
recessive alleles in a population
• Detect when allele frequencies are shifting in a
population (evolutionary change)
• Measure the frequency of heterozygous carriers
of deleterious recessive alleles in a population
Calculating the Frequency of Autosomal
Dominant and Recessive Alleles
 Count the frequency of individuals in the
population with the recessive phenotype, which
is also the homozygous recessive genotype (aa)
• The frequency of genotype aa = q2
• The frequency of the a allele is √q2 = q
• The frequency of the dominant allele (A) is
calculated p = 1 - q
Calculating the Frequency
of Alleles for X-Linked Traits
 For X-linked traits, females (XX) carry 2/3 of the
alleles and males (XY) carry 1/3 of the alleles
 The number of males with the mutant phenotype
equals the allele frequency for the recessive trait
• Frequency of an X-linked trait in males is q
• Frequency of the trait in females is q2
Calculating the
Frequency of Multiple Alleles
 In ABO blood types, six different genotypes are
possible (AA, AO, BB, BO, AB, OO)
• Allele frequencies: p (A) + q (B) + r (O) = 1
• Genotype frequencies: (p + q + r)2 = 1
 Expanded Hardy-Weinberg equation:
• p2 (AA) + 2pq (AB) + 2pr (AO) + q2 (BB) + 2qr
(BO) + r2 (OO) = 1
Frequencies of Heterozygotes
 For a genetic disorder inherited as a recessive
trait, most disease-causing alleles are carried by
heterozygotes
 The frequency of heterozygous carriers of
deleterious recessive alleles in a population is
used to calculate risk of having an affected child
Estimating the Frequency of
Heterozygotes in a Population
 Count the number of homozygous individuals in
the population (q2) and calculate the frequency
of the recessive allele q
 Calculate the frequency of the dominant allele p
(p = 1- q)
 Calculate the frequency for the heterozygote
genotype 2pq
Relationship between Allelic Frequency
and Genotype Frequency
 What are the chances of two heterozygotes
mating and having a child with a recessive trait?
• If 1 in 10,000 members of the population have the
disorder, then 1 in 50 is a heterozygote
• Chance of two mating is 1/50 x 1/50 = 1/2,500
• Chance of a given child being affected is ¼
• Chance of mating and having an affected child is
1/2,500 x ¼ = 1/10,000
Keep In Mind
 Estimating the frequency of heterozygotes in a
population is an important part of genetic
counseling
Measuring Genetic
Diversity in Human Populations
 Human populations carry a large amount of
genetic diversity
 Mutation generates new alleles, but has little
impact on allele frequency
 If the mutation rate for a gene is known, the
change in allele frequency resulting from new
mutations in each generation can be calculated
Replacement of a Recessive
Allele by Mutation Alone
 Mutation alone has a minimal impact on the
genetic variability present in a population
Genetic Drift Can
Change Allele Frequencies
 Forces such as genetic drift act on the genetic
variation in the gene pool to change the
frequency of alleles in the population
 Genetic drift
• Random fluctuations of allele frequencies from
generation to generation that take place in small,
isolated populations such as island populations or
socioreligious groups
Founder Effects
 Occasionally, populations start with a small
number of individuals (founders)
 Founder effects
• Allele frequencies established by chance in a
population that is started by a small number of
individuals (perhaps only a fertilized female)
Tristan da Cuhna
 An island population founded by a single family
Tristan da Cuhna
 Residents of the island of Tristan da Cuhna are
an example of isolation and inbreeding
• Show increased homozygosity for recessive traits
such as clinodactyly
 Clinodactyly
• An autosomal dominant trait that produces a bent
finger
Selection
 Wallace and Darwin identified selection as the
primary force that leads to evolutionary
divergence and the formation of new species
 Selection increases the reproductive success of
fitter genotypes
Natural Selection Acts
on Variation in Populations
 Natural selection acts on genetic diversity in
populations and is the major force in driving
evolution
 Natural selection
• Differential reproduction shown by some
members of a population that is the result of
differences in fitness
Fitness
 Better-adapted individuals have an increased
chance of leaving more offspring
 Fitness
• A measure of the relative survival and
reproductive success of a specific individual or
genotype
The Relationship between
Sickle-Cell Anemia and Malaria
 The allele for sickle-cell anemia is present in
very high frequencies in certain populations
• Many recessive homozygotes die in childhood
 The sickle-cell allele confers resistance to the
parasite Plasmodium, which causes malaria
• Selection favors survival and differential
reproduction of heterozygotes
Keep In Mind
 Mutation generates all new alleles, but drift,
migration, and selection determine the
frequency of alleles in a population
Natural Selection Affects the Frequency
of Genetic Disorders
 Rare lethal or deleterious recessive alleles
survive because the vast majority of them are
carried in the heterozygous condition
 Other factors can cause differential distribution
of alleles in the human population
• Migration, founder effects, mutations, selection
Lethal Alleles
 Almost all individuals with Duchenne muscular
dystrophy (DMD) die before reproducing
 The mutation rate for DMD is high, introducing
more DMD alleles
 The frequency of the DMD allele in a population
is balanced between alleles introduced by
mutation and those removed by deaths
Heterozygote Advantage
 The high frequency of genetic disorders in some
populations is the result of selection that often
confers increased fitness on heterozygotes
• A single sickle-cell allele confers resistance to
malaria
• A single Tay-Sachs allele confers resistance to
tuberculosis
Genetics in Society:
Lactose Intolerance and Culture
 The enzyme lactase converts lactose (milk
sugar) into glucose and galactose
• Lactase production slows or stops after childhood
 Some populations have a gene for adult lactose
metabolism (LA)
• The cultural practice of keeping dairy herds was a
selective factor that provided an advantage for
the LA genotype
Keep In Mind
 Survival and differential reproduction are the
basis of natural selection
Genetic Variation
in Human Populations
 The biological concept of race changed from an
emphasis on phenotypic differences to an
emphasis on genotypic differences
 Mutation introduces genetic variation
 Natural selection and drift are the primary
mechanisms that spread alleles through local
population groups
How Can We Measure
Gene Flow Between Populations?
 Gene flow between populations is used to
reconstruct the origin and history of populations
 Example: Gene flow into the American black
population from Europeans
• West African populations have blood group FY*O
• Europeans have blood groups FY*A and FY*B
• In northern US cities, about 20% of genes in the
black population are derived from Europeans
Are There Human Races?
 Studies of variations in proteins, microsatellites,
and nuclear genes show more genetic variation
within populations than between populations
 Conclusion: There is no clear genetic basis for
dividing our species into races