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Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Chapter 19
Population Genetics
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Populations as Genetic Reservoirs
• Population–A group of interbreeding organisms
belonging to a single species• Gene pool
–Set of genetic information carried by themembers of a sexually reproducingpopulation
• Allelic frequency–Frequency of an allele is present in the
population
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Calculating Allelic Frequencies
Population Genotype54 with Blood Type M MM26 with Blood Type MN MN20 with Blood Type N NN
Total 100 individuals = 200 alleles
Freq. of M = 2(54)+26/200 = .67Freq. of N = 2(20) +26/200 = .33
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Allelic Frequencies
• Dominant and codominant alleles canbe measured directly
• Recessive allelic frequencies cannotbe measured directly
• Mathematical formulas such asHardy-Weinberg can be used todetermine allelic frequencies
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Using the Hardy-Weinberg Law
Allele Frequencies
p = frequency of all dominant allelesin population
q = frequency of all recessive allelesin population
p + q = 1.0
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Calculating Allelic and GenotypicFrequencies
Fig. 19.4
p + q = 1
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Genotype FrequenciesFor gene with 2 alleles:
p2 = frequency of homozygous dominant individuals in population
q2 = frequency of homozygousrecessive individualsin population
2pq = frequency of heterozygousindividuals in population
p2 + 2pq + q2 = 1.0
Using the Hardy-Weinberg Law
Allele Frequencies
p = frequency of all dominant allelesin population
q = frequency of all recessive allelesin population
p + q = 1.0
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Calculating Frequency of Alleles andGenotypes
Fig. 19.5
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Assumptions of Hardy-Weinberg
• Large population• No selection; all genotypes survive
and reproduce equally• Random mating• No mutation or migration
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Frequency of Heterozygous Traits
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Calculating the Probability of Havingan Affected Child
Probability themother is
heterozygousX
Probability thefather is
heterozygousX ¼=
For CF, 1/2000 are homozygous recessive.So, cc (genotype) frequency is 1/2,000 or 0.0005 = q2
√q2 = q = 0.022p = 1 - q = 0.9782pq = heterozygote frequency = 2 X 0.978 X 0.022 = 0.043 = 1 in 23.
1/23 X 1/23 X 1/4 = 1/2,116
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Mutations Generate New Alleles
• Mutation alone hasminimal impact on thegenetic variability inthe population
• Drift, migration, andselection determinethe frequency ofalleles in thepopulation
Fig. 19.6
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Changing Allelic Frequencies in aPopulation
• Genetic drift– Random fluctuations in allelic frequencies from
generation to generation in a small population• Founder effects
– Allelic frequencies due to change in apopulation started by a small number ofindividuals
• Natural selection– Unequal reproductive success that is a result of
differences in fitness
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Natural Selection and Frequency ofGenetic Disorders
Examples• Lactose intolerance• Duchenne muscular dystrophy• Sickle cell anemia• Tay-Sachs disease
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Distribution of Sickle Cell Anemiaand Malaria
Fig. 19.9
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Measuring Genetic Diversity
Duffy blood group alleles• FY*A, FY*B, and FY*O• Frequency of FY*O in West Africans close
to 100%• Frequency of FY*O in Europeans close to
0%• Measure the frequency of FY*A and FY*B
in U.S. black population to estimategenetic mixing between populations
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Frequency of FY*A
Fig. 19.10
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Are There Races?
• Most genetic variation is present withinpopulations
• Minimal variation among populations,including those classified as different racialgroups
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
GeneticVariation
Fig. 19.11
The variationwithin apopulation isgreater thanthe variationbetweenpopulations
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Homo sapiens
• Combination of anthropology,paleontology, archaeology, and geneticsused to study the dispersal of humanpopulations
• Evidence suggests North and SouthAmerica were populated by migrationsduring the last 15,000 or 30,000 years
Chapter 19 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Appearance and Spread ofHomo sapiens
Fig. 19.12
