Section 4 - Unit 9: Genetic Diversity

Question 1

Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another (3 marks)

Answer 1

• Deletion causes frame shift/alters base sequence
• Changes sequence of amino acids (from this point)
• Substitution alters one codon/one amino acid

Question 2

What is meant by genetic diversity? (1 mark)

Answer 2

• Differences in base sequence of DNA/alleles

Question 3

Describe and explain the appearance of a chromosome during meiosis (3 marks)

Answer 3

• Chromosome is formed of two chromatids
• Because DNA replication has occurred
• Sister chromatids held together by centromere

Question 4

Name two ways in which meiosis produces genetic variation

Answer 4

• Independent segregation of homologous chromosomes

- Crossing over

Question 5

Explain why fewer species of a plant would lead to fewer species of an animal that depends on it (3 marks)

Answer 5

• Fewer habitats
• So lower diversity of insects
• So fewer food sources

Question 6

Suggest how you could obtain data to give a more precise value for the index of diversity of a habitat (2 marks)

Answer 6

• Take more samples and find mean

- Randomised sampling

Question 7

What is the difference between species richness and an index of diversity (1 mark)

Answer 7

• Species richness only measures a number of different species (does not measure the number of individuals)

Question 8

Suggest how you could decide where to take samples of a population (2 marks)

Answer 8

• Draw grid over area

- Select squares/coordinates at random

Question 9

What two measurements are needed to calculate an index of diversity? (2 marks)

Answer 9

• Number of individuals of each species

- Total number of individuals / number of species

Question 10

Name two ways in which meiosis produces genetic variation (2 marks)

Answer 10

• Independent segregation of homologous chromosomes

- Recombination by crossing over

Question 11

Suggest a reason why populations might show very low levels of genetic diversity (1 mark)

Answer 11

• Inbreeding

Question 12

Explain why an error that occurs during DNA replication is likely to be more damaging than an error that occurs during transcription? (4)

Answer 12

• DNA replication means errors may be inherited
• So the whole organism may be permanently affected
• Errors in transcription only affect specific cells
• So whole organism isn’t affected

Question 13

Suggest using your knowledge why mules are sterile (5)

Answer 13

• Mules have 63 chromosomes
• Gametes are produced via meiosis
• With an odd number of chromosomes, precise pairing
• Of homologous chromosomes is impossible
• This prevents meiosis and gamete production so the mules are sterile

Question 14

Define selection (1)

Answer 14

Process by which organisms that are better adapted to their environment survive and breed

Question 15

Explain 3 differences between directional and stabilising selection (6)

Answer 15

• Favours phenotypes at one extreme of a population vs favours phenotypes around the mean of a population
• Changes the characteristics of a population vs preserves the characteristics of a population
• Curve remains same shape but mean shifts to left or right vs curve becomes narrower and higher but mean does not change

Question 16

The principles of natural selection in the evolution of populations (4)

Answer 16

• Random mutation can result in new alleles of a gene.
• Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success
• The advantageous allele is inherited by members of the next generation
• As a result, over many generations, the new allele increases in frequency in the population

Question 17

Directional selection (1)

Answer 17

• When the environmental conditions change and the phenotypes best suited to the new conditions are more likely to survive

Question 18

Example of directional selection (3)

Answer 18

• Antibiotic Resistance
• A single bacteria will have had a mutation meaning that it wouldn’t have been killed by an antibiotic
• Therefore, it was able to grow and populate, and the population moved to have greater penicillin resistance.

Question 19

Stabilising selection (3)

Answer 19

• The phenotypes with successful characteristics are preserved and those of greater diversity are reduced
• If the environment stays the same then the individuals closest to the mean are favoured because they have the alleles that have given them the survival advantage
• The furthest from the mean are selected against

Question 20

Example of stabilising selection (2)

Answer 20

• New born babies weights

- Those that have a birth weight of around 3kg are more likely to survive than those at the extremes

Question 21

Explain why courtship behaviour is important for species (4)

Answer 21

• Recognise members of their own species
• Synchronise mating
• Form a pair bond
• Become able to breed

Question 22

State four ways in which you could investigate the genetic diversity between species (4)

Answer 22

• The frequency of measurable or observable characteristics
• The base sequence of DNA
• The base sequence of mRNA
• The amino acid sequence of the proteins encoded by DNA and mRNA

Question 23

Describe the process of crossing over and explain how it increases genetic diversity (4 marks)

Answer 23

• Homologous pairs of chromosomes associate
• Chiasma(ta) form
• (Equal) lengths of chromatids/alleles are exchanged
• New combinations of alleles

Question 24

Explain why scientists can use protein structure to investigate the evolutionary relationships between
different species (2 marks)

Answer 24

• Closer the amino acid sequence, the closer the relationship
• Protein structure related to base sequence

Question 25

Explain why comparing the base sequence of genes provides more evolutionary information than comparing the structure of proteins (2 marks)

Answer 25

• Base triplets longer / more bases than amino acids

- Introns also available