L6: Population Genetics

Question 1

• What are some factors affecting allele frequencies?

Answer 1

• Mutation
• Migration
• Mating (random or assortative - inbreeding)
• Genetic drift
• Natural selection

Question 2

Using sickle cell anaemia as an example, desribe Darwin’s principle of fitness.

Answer 2

HbA/HbA = normal

HbS/HbS = severe sickle cell anaemia

HbA/HbS = mild anaemia, but with highly increased resistance to malaria.

Thus we find high prevalence of HbS alleles in areas of endemic malaria.

Question 3

Describe two examples of how humans have biologically adapted to the environment.

Answer 3

Reduced sunlight has resulted in decreased pigmentation. Lower melanin allows the limited UV to penetrate skin better, where it is required to make vitamin D.

High altitude dwellers show lowered haemoglobin and high density of blood capillaries to protect against hypoxia.

Question 4

Describe the three types of selective sweep.

Answer 4

1. Classical selective sweep
   * A rare beneficial mutation arises and rapidly increases in frequency, producing a drastic reduction in genetic variation.
2. Soft sweep
   * Change in environment confers a selective advantage to a previously neutral variant. The allele is likely to be present in several genomic backgrounds, and genetic variation is reduced but retained.
3. ​Multiple origin soft sweep
   * Multiple alleles that confer similar advantages occur on different genomic backgrounds such that no single background obtains a high frequency.

Question 5

What is positive and negative selection?

Answer 5

Positive selection increases genetic diversity e.g. through interaction of pathogens and humman immume system.

Negative selection removes deleterious alleles

Question 6

Describe the difference between negative and positive assortative mating?

Answer 6

Positive = selection of mates that are similar to you

Negative = selection of mates that are different to you

Question 7

What is the coefficient of relationship (R) and coefficient of inbreeding (F)/

Answer 7

Coefficient of relationship = degree of relatedness as measured by proportion of alleles shared by two people as a result of genetic descent.

E.g. R=1/2 for parent-child or sibling-sibling

Coefficient of inbreeding = probability that a homozygote has identical alles at a locus as a result of geneti descent.

F = 1/2 x R

E.g. If parents are cousins, R = 1/8, so F = 1/16