3. MOLECULAR EVOLUTION

Question 1

Define Natural Selection

Answer 1

• Natural selection refers to the effects of selection pressures & other factors on the frequency of heritable changes such as genetic variants

Question 2

Define fitness in terms of evolution

Answer 2

• Fitness refers to the ability to reproduce

- Genetic variants that increase fitness will be selected for & vice versa

Question 3

What four factors can affect genetic variation?

Answer 3

1. SELECTION
2. MUTATION/GENE FLOW
3. MIGRATION
4. GENETIC DRIFT

Question 4

What is Selection?

Answer 4

• Selection refers to genetic variants that confer a positive advantage being selected & passed on to the offspring & vice versa
• The frequency of genetic variants increases in the population

Question 5

What is a mutation?

Answer 5

• A mutation refers to a change or variation which arises in the genome

Question 6

How can migration lead to admixture?

Answer 6

• Migration involves the movement of one population into another, which introduces new genetic variants into the gene pool which is known as admixture
• In the original population, these genetic variants may have been neutral but could be disease causing when introduced into a new population

Question 7

What is genetic drift?

Answer 7

• Genetic drift refers to the random change in the frequency of genetic variants over time (it tends to decrease)
• All genetic variants can undergo genetic drift but neutral variants are more likely to undergo genetic drift as they don’t confer any negatives or positives
• Another reason is that we don’t pass on all our genetic variants to offspring due to recombination events

Question 8

• What is sequence conservation?

Answer 8

• Sequence conservation - some sequences are conserved as they are resistant to changes or mutations. This is because changes in these regions could be detrimental to function e.g developmental genes.

Question 9

What are the three levels of sequence conservation?

Answer 9

1. HIGH = coding regions (exons)
2. INETRMEDIATE = Promoter, 3’UTR , 5’UTR
3. LOW = introns, 3rd base of codon

Question 10

• What are two benefits of sequence conservation?

Answer 10

1. Create evolutionary profiles for genes & gene families

2. Identify important gene regions

Question 11

• What is Phylogenetics?

Answer 11

• Phylogenetics is the study of evolutionary relationships between species, organisms etc.

Question 12

What are phylogenetic trees?

Answer 12

• Phylogenetic trees aim to show the relatedness between species
• Close = more related
• Distant = less related
• Distance could be due to time or evolutionary pressures

Question 13

What is gene duplication & how does it occur?

Answer 13

• Gene duplication involves copying or duplicating a gene. It occurs due to unequal crossing over, where there’s recombination of similar but not identical sequences
• The duplicated gene will evolve a new function that is beneficial to the original or it will act as a pseudogene
• The original gene will continue to carry out it’s original function

Question 14

What are the two clusters of globin genes?

Answer 14

• there are 2 alpha & 2 beta gene clusters
• alpha = chromsome 16, 3 normal genes, 3 pseudogenes
• beta = chromosome 11, 5 normal gene, 1 pseudogene

Question 15

*How does globin gene expression change with time?

Answer 15

HbF is more common before birth but HbA is more common after birth. Beta levels increase before birth, gamma levels fall & alpha remains high

Question 16

How did the globin gene clusters evolve?

Answer 16

• Globin gene clusters evolved through gene duplications & mutations

Question 17

What are pseudogenes?

Answer 17

• Pseuodgenes are genes with no function, they can accumulate mutations which prevent them from carrying out their function.
• Pseudogenes reduce the risk of mutations occurring in more important genes.

Question 18

*What changes in genes cause Sickle cell disease?

Answer 18

• A single base change from GAG to GTG causes a substitution from Glutamic acid to Valine at position 6 in the beta globin gene
• Sickle cell disease is an autosomal reccessive disorder meaning that two copies of the gene are needed
• HbS is the Hb that arises as a result of this base change

Question 19

*What are the three main symptoms of Sickle Cell disease?

Answer 19

1. Anaemia - jaundice, fatigue
2. Increased frequency of infection due to spleen damage
3. Acute pain episodes due to oxygen crisis

Question 20

Why did natural selection allow the SCD gene to eb passed on?

Answer 20

• Two copies of the SCD gene would lead to sickle cell disease & decrease fitness individuals would not live long enough to have children.
• BUT, one copy of the SCD gene provides immunity from malaria which is known as HETEROZYGOTE ADVANTAGE
• SCD is common in areas where malaria is prevalent, natural selection would have removed this variant but it confers the advantage of malaria resistance