10. Evolution of resistance

Question 1

What are the effects of only having one sickle cell gene (HBS gene)?

Answer 1

• decreased growth
• delayed sexual maturity
• impaired mental function
• impaired infection resistance
• increased chance of stroke, kidney failure…

Question 2

What happens if you have 2 HBS genes?

Answer 2

Usually die at birth

Question 3

What happens to HBS gene in areas with malaria?

Answer 3

• heterozygotes with HBS gene survive malaria
• HBA/HBA people with most likely die from malaria
• so the HBS gene is passed on from the heterozygotes

Question 4

What is polygenic variation?

Answer 4

Many genes combine to determine a characteristic

Question 5

What are the requirements for natural selection to occur?

Answer 5

• individuals vary in how they cope with env

* the relevant characteristics are heritable

Question 6

What happens if the requirements for natural selection are met?

Answer 6

Those that cope well produce more offspring that survive to reproduce

Question 7

Why do pesticides have to concentrated when sprayed?

Answer 7

• it becomes diluted over time

e.g. after combining with dust, water, etc it loses strength

Question 8

What is incomplete dominance

Answer 8

• most gene alleles have incomplete dominance - neither of the alleles is dominant
• shows intermediate characteristics

Question 9

What is Co-dominance

Answer 9

• both of the allele characteristics are shown

Question 10

What happens when resistance is common amongst a mosquito population?

Answer 10

• selection time increases

* zone of selection widens

Question 11

How is selection for resistance minimised when pesticides are sprayed?

Answer 11

• repeat regularly to keep dosage high
• dont delay until pest returns
• use chemicals with rapid decay
• prevent unnecessary use
• prevent dispersion of pests
• minimise edge effects

Question 12

Why are pesticides with rapid decay used?

Answer 12

• minimise the selection time during decay

* not enough time for the resistance to spread to next generations

Question 13

How are edge effects minimised?

Answer 13

• large area could be sprayed at once

* if sprayed in small areas, pests can move to areas that have not been sprayed or where the spray has decayed

Question 14

how do we know that new mutations always have bad side effects?

Answer 14

• otherwise R alleles would be common already
• instead, under normal conditions R allele is selected against
• it is only when the pesticide is introduced that R allele has selective advantage

Question 15

Why must the use of pesticide/drug be stopped as soon as the resistance is detected?

Answer 15

• if pesticide is used after resistance becomes normal, mutant R allele must be retained
• selection acts on combinations of R with other to modify bad side effects
• results in resistance with reduced side effects

this process involved polygenic variation and gene combinations

Question 16

Why must we switch to unrelated chemicals if resistance is present?

Answer 16

To prevent cross-resistance
• toxins target key insect physiology
• many toxins act in similar ways
• if similar toxins are use, pests are already partly resistant - resistance builds rapidly

Question 17

What is cross-resistance

Answer 17

Cross-resistance is the tolerance to a usually toxic substance as a result of exposure to a similarly acting substance.