Transcript q 2

Genetics and Populations
Chapter 14
Central Points
 Genetic conditions can be very common in a
specific community
 Huntington disease affects large numbers in two
villages in Venezuela
 Traits can vary from one population to another
 Calculations can determine frequency of an allele
 Population genetics used in DNA forensics
14.1 Why Study Populations?
 Small isolated populations often have a high
frequency of one or more genetic disorders
 Pedigrees, blood and tissue samples used to
identify, map, and isolate genes responsible
 Pedigrees trace HD mutation to one woman
 Founder effect
Repeated DNA Triplet Causes Huntington
Disease (HD)
 Normally, 10–35 copies of CAG repeat, < 27 CAG
repeats do not get HD
 27–35 copies do not get HD, but children at risk
 36–40 copies may or may not get HD
 > 40 repeats almost always get HD
 Increase of number of repeats each generation
14.2 Other Genetic Disorders
 Geographic distribution of sickle cell anemia and
malaria
 Link between sickle cell anemia and malaria
 Malaria affects > 500 million people worldwide
and kills > 3 million people/year
 Caused by parasite, infects red blood cells
Frequency of Sickle Cell
Malaria and Sickle Cell Anemia
 Spread by mosquitoes
 Carriers of sickle cell anemia (heterozygotes)
and affected individuals (homozygotes),
resistant to infection by malaria parasite
 Membrane of red blood cells altered, very
difficult for parasite to enter cells
The Anopheles Mosquito
14.3 Specific Genetic Traits
 Carrier frequency: Some populations have
higher frequency of carriers of recessive traits
 Differences among different populations
 Two carriers from a high-risk population have a
child, increased chance of genetic disorder
Frequencies of Carriers of Tay-Sachs
within Populations
14.4 Environmental Conditions Affect
Frequency of Genetic Traits in Populations?
 Cystic fibrosis (CF) common in some populations
but nearly absent in others
 CF affects glands that produce mucus, digestive
enzymes, and sweat, causing far-reaching effects
 Most individuals with CF develop obstructive lung
disease and infections, leading to premature death
Cystic Fibrosis Centers
CF and Typhoid Fever
 Previously, affected individuals usually died before
having children
 Some evidence that heterozygotes more resistant
to typhoid fever
 Caused by a bacterium that infects cells of
intestinal lining
 In mice, carriers of CF injected with typhoid fever,
intestinal cells infected by fewer bacteria
14.5 Frequency of Alleles in a Population
 Genetic disorder caused by recessive allele
 Cannot directly count those who carry allele in
population (cannot ID heterozygote, Cc)
 Hardy and Weinberg developed formula, measures
numbers of alleles and genotypes in a population
Hardy-Weinberg Law to Study Genes in
Populations
 p represent A and q represent a
 Only possible genotypes in a population
• AA
• Aa or aA
• aa
p2 (p X p = p2)
2pq (pq X qp)
q2 (q X q = q2)
 Sum of three genotypes must equal 100%
 Therefore, p2 + 2pq + q2 = 1
How Can We Use the Hardy-Weinberg Law?
 Frequency of alleles and genotypes population
provides information
 Risk factors for having child affected with genetic
disorder
 To determine if populations are evolving
 Used to calculate frequency of:
• Disease-causing alleles in these populations
• Heterozygotes in population
Calculation of Allele Frequencies and
Heterozygote Frequencies

Frequency of CF allele (c) in population:
1. Calculate number of people who have CF
(genotype cc):
2. Frequency of CF = 1/2500 = 0.0004
3. Per Hardy-Weinberg law, cc = q2 (q = frequency
of CF)
4. Therefore genotype cc = q2 = 0.0004.
5. q = square root of 0.0004 = 0.02

Therefore 2% of alleles in population are
mutant CF allele (c)
Animation: Allele and genotype
frequencies
Other Uses of Hardy-Weinberg Equation
 No malaria in U.S., but individuals with West
African ancestry carry sickle cell gene
 Frequency of children with sickle cell anemia:
• Use Hardy-Weinberg to calculate frequency of
carriers (Ss)
• ~8% or 1/12 African Americans with West African
ancestry
 Some areas of West Africa: 20–40% of
population are carriers (Ss) of sickle cell gene
Animation: The Hardy-Weinberg equation
14.6 Legal and Ethical Issues (1)
 Construction and use of DNA databases
 Who should be forced to provide sample?
 Who has the authority to order sample?
 Should DNA profiles of those found innocent
remain in database?
 What crimes should be included in database?
14.6 Legal and Ethical Issues (2)
 What about private information unrelated to
crimes
 What about privacy rights?
 Does DNA data show the criminal justice system
is racially or ethnically biased?