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

Case A: A Marker in the Blood

Certain small villages in Venezuela have a higher frequency of Huntington disease (HD)

Nancy Wexler collected information and created pedigrees (~19,000 people)

Collected blood samples to test DNA

Identified DNA marker in people with HD

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-Sachswithin 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

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 p2 (p X p = p2)• Aa or aA 2pq (pq X qp)• aa 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

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 = 0.00043. 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

Spotlight on Society: Heritage of African Tribe Revealed

Lemba people, in southern Africa

Oral tradition of ancestor Jews who left Middle East thousands of years ago

Culture very similar to Orthodox Judaism

ID pattern of molecular markers on Y chromosome, Cohen Model Haplotype