18/11/2025
🧬 MUTATIONS
🔍 So… what exactly is a mutation?
A mutation is:
➡️ A change in the base sequence of DNA,
➡️ Which may alter the structure or function of a protein,
➡️ Sometimes leading to genetic conditions.
Mutations can be inherited (germline) or occur during life (somatic).
⚡ Examples you need for IGCSE:
1️⃣ Cystic Fibrosis (CF)
Caused by a recessive allele.
Mutation affects a protein that controls movement of salt and water in cells.
Leads to thick, sticky mucus in lungs + digestive system.
Affected individuals must inherit two recessive alleles (ff).
➡️ Good exam phrase: “Caused by a recessive mutation affecting cell membranes.”
2️⃣ Albinism
Also caused by a recessive allele.
Mutation results in lack of melanin, the pigment responsible for skin, hair, and eye colour.
Affects eyesight because melanin is needed for normal eye development.
➡️ Good exam phrase: “A recessive mutation preventing melanin production.”
3️⃣ Down Syndrome (Trisomy 21)
Not caused by a gene mutation — important!
Caused by chromosome mutation: an individual has three copies of chromosome 21 instead of two.
Occurs during meiosis when chromosomes fail to separate properly (non-disjunction).
➡️ Good exam phrase: “A chromosome mutation leading to an extra chromosome 21.”
💡 Exam tips:
Use words like allele, recessive, chromosome, protein, base sequence.
Be ready to draw and interpret Punnett squares for CF and albinism
Clearly distinguish between gene and chromosome mutations — examiners test this often!
📖 Want this explained with diagrams, animations, and step-by-step practice questions?
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