Cystic Fibrosis (CF)

• CF is a disorder of cell membranes ($\larr$ could be asked in an exam).

• Caused by a single gene with two alleles:

  • C = normal cell membrane function (dominant).

  • c = defective cell membrane (recessive).

• To have CF → must be homozygous recessive (cc).

• Carrier = heterozygous (Cc) → no CF, but carry defective allele and offspring could potentially have CF.

Inheritance of CF

• If one parent = carrier (Cc), one parent = unaffected (CC):

  • Gametes: Male (Cc), Female (CC).

    	C	C
    C	CC	CC
    c	Cc	Cc

  • Punnett square outcome:

    • 50% CC → unaffected, not carrier.

    • 50% Cc → carriers (no symptoms).

  • Ratio: 1 : 1 (unaffected : carrier).

    
    

• If both parents = carriers (Cc × Cc):

  • Gametes: C, c (from each parent).

    	C	c
    C	CC	Cc
    c	Cc	cc

  • Punnett square outcome:

    • 25% CC → unaffected.

    • 50% Cc → carriers.

    • 25% cc → have cystic fibrosis.

  • Ratio: 1 : 2 : 1 (unaffected : carrier : affected).

Key Exam Points

• Probabilities only → ratios show likelihood, not guarantees.

• Define clearly:

  • Carrier = heterozygous with one defective allele.

  • Affected = homozygous recessive.

  • Not affected = homozygous dominant.

• Be able to:

  • Draw and complete a Punnett square.

  • Express outcomes as percentages or ratios.

✅ Exam tip: Always label gametes, genotypes, and phenotypes separately in a Punnett square answer — examiners often award marks for each stage.