Population Genetics and Evolution

Question 1

Define Population

Answer 1

Local group of a species among which mating can occur.

Question 2

State the Hardy-Weinberg Equilibrium Law - 1908

Answer 2

• If these assumptions are met:
  1. Infinitely large population
  2. Panmictic (random mating)
  3. No selection (equal reproduction by all genotypes)
  4. No migration
  5. No mutation
• Then: Allele and genotypic frequencies will arrive at and remain at equilibrium frequencies after one generation of random mating if all assumptions are met.
• Equation: p^2 AA + 2pq Aa +q^2 aa

Question 3

What genotypic frequencies are expected if a population is consistent with Hardy-Weinberg equilibrium frequencies?

Answer 3

p^2 AA + 2pq Aa +q^2 aa

Question 4

List the conditions required for Hardy-Weinberg equilibrium

Answer 4

1. Infinitely large population
2. Panmictic (random mating)
3. No selection (equal reproduction by all genotypes)
4. No migration
5. No mutation

Question 5

Be able to calculate allele and genotype frequencies from population data
o Note: You will not be able to use a calculator, but genotype and allele frequencies will come out evenly to not more than 2 significant figures.
o Note: If you need to take the square root of a number to answer the question, the square root will come out evenly to 1 significant figure.
o Eg. Calculate the frequency of the M allele given the frequencies of the genotypes as
f(MM) = 0.6, f(MN) = 0.3 and f(NN) = 0.1
o Be able to calculate the expected number (and frequency) of heterozygotes in a population if the population is assumed to be at Hardy-Weinberg equilibrium
o Eg. If 1 in 10,000 individuals in the population are genotype tt, what is the frequency of heterozygous individuals in the population?
o Be able to apply population genetics information to pedigree analysis

Answer 5

Example:
Look at tongue rolling:
- 84 rollers (RR and Rr)
- 16 nonrollers (rr)
- Let p = f(R) and q = f(r)
If we assume Hardy-Weinberg equilibrium, then we assume the population genotypic array is
p^2 AA + 2pq Aa +q^2 aa
q^2 = f(rr) = 16/100 therefore, q = f(r allele) = 0.4 (16/100=0.16, sq. rt. of 0.16=0.4)
p = f(R allele) = 1 - q = 0.6
Frequency of carriers (Rr) = 2pq = 2(0.6)(0.4) = 0.48
In our population of 100 students, 48 are expected to be carriers.

Question 6

What is the source of new alleles?

Answer 6

Mutation

Question 7

What is the effect of consanguineous matings on genotypic frequencies in a population?

Answer 7

(relatives marry) - increases homozygosity

• See more recessive characteristics - many times these are detrimental.
• Increase homoqygousity and decrease heterozygousity.
• Less than 1% of marriages in the USA are first cousin marriages, but 33% of PKU cases and 17% of albinism cases are from these marriages.

Question 8

Define: Genetic Drift and describe their effects on allele and genotypic frequencies

Answer 8

Genetic drift - Random fluctuations of allele frequencies from generation to generation in small populations. Small population size causes genetic drift.
Can cause homozygosity of specific recessive alleles in a population.

Question 9

Define: Founder Effect and describe their effects on allele and genotypic frequencies

Answer 9

Founder effect - Allele frequencies established by chance in a population started by a small number of individuals. Sampling error in establishing new populations.
Ex. Glass family - founder effect with clinodactyly (dominant trait).

Question 10

Define: Selection and describe their effects on allele and genotypic frequencies

Answer 10

Selection - Unequal reproduction by different genotypes.

Question 11

What are the 3 types of selection and what changes are brought about in a population with each type of selection?

Answer 11

- Directional Selection
Favors one extreme or the other.
Population mean increases or decreases depending on which extreme is favored.
- Stabilizing Selection
Heterozygotes favored.
Decreases variance.
Leads to polymorphisms (both alleles remain in population).
- Disruptive Selection
Advantage for both extremes.
Leads toward bimodal population.

Question 12

Define: Species

Answer 12

Species:
Defined by reproductive isolation such that members of one species can interbreed and produce fertile progeny, but they cannot produce fertile progeny with members of a different species.

Question 13

Define: Evolution

Answer 13

Evolution:
Change in genetic composition of population including:
- emergence of new species
- divergence of 2 species
- extinction of species
Involves variation, heredity, and selection.
If variation is not heritable, then it cannot be passed to progeny.
Selection works on entire organism’s phenotype, so many loci as well as environmental factors are important.
Study diversity that exists in a population and between populations and the factors that can cause diversity.

Question 14

Define: Haplotype

Answer 14

Haplotype:
Allele combinations at several genes along length of chromosome. (ex. AbC and ABC would be 2 haplotypes of the genes along the chromosome).
The more alike the haplotypes are, the more recently 2 groups shared a common ancestor.

Question 15

Define: Race

Answer 15

Race:
A breeding population characterized by allele frequencies that are different from other populations of the same species.
Racial features allowed for increased survival in different environments.

Question 16

Define: Molecular Clock

Answer 16

Molecular Clock:
Applies mutation rate and mutation accumulation to a timeline to estimate when populations most recently shared ancestors.

Question 17

What types of DNA sequences are typically evaluated to determine evolutionary relationships?

Answer 17

Comparing DNA or protein sequences and chromosome banding patterns to discern evolutionary relationships.
The more alike the haplotypes are, the more recently 2 groups shared a common ancestor.

Question 18

Where is the human species thought to have originated?

Answer 18

Africa

Question 19

What type of chromosome aberration separates human and chimpanzee chromosomes? (Think about how many chromosomes we have and how many chimps have)

Answer 19

Banding patterns show that human chromosome #2 could be derived from a Robertsonian translocation between 2 smaller ancestral chromosomes.
We have 46, chimps have 48.

Question 20

In which population would you expect to see the most genetic diversity and why? (Asian, European, South American, African, Polynesian)

Answer 20

African: more time to accumulate differences if been around longer.

Question 21

Discuss the effects of selection in humans with regards to sickle cell anemia

Answer 21

Higher frequency of sickle cell allele in Africa where malaria is more common due to selection against both homozygous types.
Balanced polymorphism: Heterozygote is the most fit genotype so both alleles are maintained in population.

Question 22

Discuss the effects of selection in humans with regards to lactase production

Answer 22

• Individuals with low lactase levels develop gas, cramps, nausea, etc. after eating lactose containing foods.
• Often see increase in lactose intolerance with age.
• Adult mammals originally intolerant.
• Selection for tolerance as dairy products introduced into diet (advantage to be able to use as nutrition source).
• Turkey and Eastern Russia seem to be original region for lactose tolerance.

Question 23

Discuss the effects of selection in humans with regards to cystic fibrosis

Answer 23

Cystic Fibrosis and resistance to diarrheal diseases:
- Cystic Fibrosis homozygotes lack functioning chloride channels so fluids accumulate.
- Thought that about 6% of people of European origin carry allele.
Cholera Resistance:
- Cholera causes diarrhea by opening channels and not allowing them to close - severe dehydration.
- Cystic Fibrosis heterozygotes seem to have some protection since have fewer functioning channels.
- However, timing of cholera and CF origin don’t agree - seems that CF was around a long time before cholera so thought that initial increase in CF allele was from typhoid resistance.
Typhoid resistance:
- Salmonella typhi invades gastrointestinal cells in normal individuals by attaching to the normal cfr protein to enter cell.
- Mutant proteins do not seem to be able to attach to the bacteria - prevents infection.