Genetics, Populations, Evolution and Ecosystems

Question 1

Explain why male rabbits cannot be roan when the gene controlling fur colour is on the X chromosome
(2 marks)

Answer 1

Male rabbits only inherit 1 X chromosome and both alleles must be inherited.
The other alleles are codominant.
Only female rabbits inherit 2 alleles for fur colour.

Question 2

Explain why the actual number observed may be different to the expected number
(3 marks)

Answer 2

Offspring phenotypes are a probability
Gametes are not produced in exactly equal numbers
Fertilisation of gametes is random
The sample size is small

Question 3

State and explain which statistical test should be used to determine whether there is a significant difference between the observed and expects results
(2 marks)

Answer 3

Chi squared test
The data collected is categorical

Question 4

What assumptions are made from the Hardy Weinberg Principle?
(3 marks)

Answer 4

• no mutations arise
• the populate is isolated
• there is no selection
• large population
• mating is random

Question 5

State 3 reasons why calculations using the Hardy-Weinberg principle may be inaccurate
(3 marks)

Answer 5

• random mutations can occur within the gene so there may be more alleles
• individuals within a population may move into or out of an area so the true number of individuals with a particular phenotype will be different
• individuals tend to mate with members who have desirable characteristics so mating isn’t random
• population size may be small so this can lead to genetic drift
• some characteristics are more/less favourable so this leads to natural selection

Question 6

Explain how crossing over can contribute to genetic variation
(3 marks)

Answer 6

Sections of chromatids are crossed over, which results in alleles being exchanged. This creates new combinations of alleles that may not have existed before.

Question 7

Suggest and explain how speciation may occur as a result of deforestation
(4 marks)

Answer 7

ALLOPATRIC SPECIATION
There is isolation of two populations.
Different environmental conditions exist for each separated group
Favourable alleles are passed onto offspring
Gene pools are separated

Question 8

Explain how populations can become geographically isolate and how it can lead to speciation
(4 marks)

Answer 8

The presence of a physical barrier.
Variation within the populations will already exist due to random mutations in the DNA.
Different selection pressures will exist in the local populations.
These different adaptations results in reproductive isolation.
Only those alleles for desirable characteristics are passed onto offspring.
Frequency of desirable alleles increases in a population over time.

Question 9

How can genetic variation arise?
(2 marks)

Answer 9

• random mutations within genes
• crossing over in meiosis when alleles are exchanged and new combinations of chromosomes are formed
• random fertilisation of gametes which produces new combinations of alleles

Question 10

How is a community of land plants formed in a small lake over may years?
(4 marks)

Answer 10

The microscopic plants are the pioneer species which colonise the lake initially.
When they decompose the nitrates are released creating a soil layer.
This makes the lake less hostile so more complex species can grow.
Over time the submerged plants die out because they are outcompeted for light.
This is an increase in diversity as more niches are created.

Question 11

Suggest how deforestation might affect the process of succession in the lake
(2 marks)

Answer 11

There may be leaching which increases the nutrient content in the lake.
More soil may be bought into the lake which increases the soil layer.