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A2 · Practice questions · Genes, proteins and phenotype

Gene, protein, phenotype.

Six original Cambridge-style questions linking a change in a gene to a change in a protein and a visible phenotype.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Follow the chain.

01
Explain
[4 marks]

Explain how a mutation in the TYR gene can result in albinism.

  • A change in the base sequence of TYR changes the sequence of codons / triplets. ✓
  • This changes the amino acid sequence (primary structure) of tyrosinase, so it folds differently and the active site changes shape. ✓
  • The enzyme is non-functional, so tyrosine is not converted to melanin. ✓
  • Without melanin the skin, hair and eyes lack pigment (albinism); only seen in homozygous recessive individuals. ✓
02
Explain
[4 marks]

Explain how a single base substitution in the HBB gene leads to the symptoms of sickle cell anaemia.

  • The substitution changes one codon so that valine replaces glutamic acid in β-globin. ✓
  • Valine is non-polar / hydrophobic, so at low oxygen concentration HbS molecules stick together and form long fibres. ✓
  • Red blood cells become sickle shaped, carry less oxygen and are destroyed more quickly (anaemia). ✓
  • Sickle cells block capillaries, reducing blood flow and causing pain and organ damage. ✓
03
Recall
[2 marks]

State the protein coded for by the F8 gene and explain why haemophilia is described as sex-linked.

  • F8 codes for factor VIII, a blood-clotting protein. ✓
  • The F8 gene is on the X chromosome (no equivalent locus on the Y), so the inheritance pattern differs between males and females. ✓
04
Explain
[3 marks]

Explain how the allele causing Huntington's disease differs from the normal HTT allele and why the disease is dominant.

  • The mutant allele has an increased number of CAG repeats (more than about 36, compared with fewer than about 27). ✓
  • Huntingtin therefore contains a long chain of glutamine, so it misfolds and forms clumps that kill neurones in the brain. ✓
  • The faulty protein causes damage even when a normal allele is present, so one copy causes the disease. ✓
05
Explain
[4 marks]

Explain the role of the Le gene in determining the height of pea plants.

  • The Le gene codes for an enzyme in the gibberellin synthesis pathway that converts inactive GA20 to active GA1. ✓
  • The dominant allele Le codes for a functional enzyme, so LeLe and Lele plants make GA1. ✓
  • GA1 causes stem elongation (cell division and elongation in the internodes), so these plants are tall. ✓
  • The recessive allele le codes for a non-functional enzyme (one amino acid different), so lele plants make little GA1 and are dwarf. ✓
06
Data
[3 marks]

Dwarf lele pea plants and tall LeLe plants were grown; half of each group was sprayed with GA1. Mean heights after 4 weeks: LeLe unsprayed 62 cm, LeLe sprayed 66 cm, lele unsprayed 18 cm, lele sprayed 58 cm. Explain these results.

  • lele plants are short because they cannot make enough active GA1. ✓
  • Spraying lele plants with GA1 increases height greatly (18 to 58 cm), showing they can respond to gibberellin; only its synthesis is faulty. ✓
  • LeLe plants already make GA1, so extra GA1 has only a small effect (62 to 66 cm). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.