What is meant by a recessive allele ?

(1 mark)

Only expressed in the phenotype whenhomozygous / NOT expressed when heterozygous.

Explain what is meant by codominant alleles

(1 mark)

p² + 2pq + q² = 1.0

Where p is the DOMINANT allele

q is the recessive allele

And 1.0 is all the alleles in the population (100%)

What does the Hardy - Weinberg principle predict?

(3 marks)

1. The proportion/frequency of alleles

2. Will stay constant from 1 generation to the next

3. Providing there's : NO mutation / NO selection / the population's large / pop. genetically isolated / mating is random

• Fusion of gametes is random
• Small sample was used
• Gametes are not produced in equal numbers
• Offspring ratios arise by chance
  

Explain how speciation occurs.

(5 marks)

1. Geographical Isolation

2. Separate gene pools / NO interbreeding

3. Variation due to Mutation

4. Different environmental conditions

5. Selection for differing characteristics/alleles

6. Differential reproductive success

7. Leads to change in allele frequency

8. Occurs over a long period of time

• Change in environmental conditions = change in phenotype needed for survival
• individuals to the left/right of the mean will possess alleles that are advantageous & better suit them to the new environment.
• phenotypes at 1 extreme are selected for
  
• They're more likely to survive and reproduce - contribute more alleles to the next generation
  

• Environmental conditions remain stable
  
• Individuals with phenotypes closest to the mean are favoured
• More likely to survive, reproduce and contribute more alleles to the next generation
• eliminates the phenotypes at the extremes
• Mean remains the same but there's fewer individuals at the extremes - spread is smaller