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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Text lesson

Unit 3 — Variation of Traits in Living Organisms

Science — Third Preparatory

Unit 3 — Variation of Traits in Living Organisms

1.9 Genetic (inherited) diseases

1.9 Genetic (inherited) diseases

  • Sometimes a gene carries a mutation (a change) that causes a disease.
  • Such a disease is a genetic disease; it can appear from birth or later in life, and it can be passed to the next generation.
  • In many inherited diseases the disease-causing allele is recessive. Let N = normal allele and n = disease allele:
Key points
NNhealthy, not a carrier.
Nnhealthy-looking, but a carrier: has one copy of the disease allele but shows no symptoms; can pass it on.
nnhas the disease (two disease alleles, one from each parent).

Worked example 6: two carrier parents

  • Both parents are Nn (carriers). Punnett square:
Comparison aspect N n
N NN Nn
n Nn nn
Comparison aspectMain ideaKeywords
Healthy, not carrier (NN)25%
Healthy carriers (Nn)50%
Affected (nn)
  • 25% So each child of two carriers has a 1 in 4 chance of the disease
  • even though both parents look healthy. This answers "Why can a child be affected though both parents appear healthy?"
childcarrierschancedisease

Worked example 7: one carrier, one normal

  1. Nn × NN → 50% NN and 50% Nn. No child is affected
  2. but half are carriers.

Worked example 8: carrier × affected

  1. Nn × nn → 50% Nn (carrier) and 50% nn (affected).
  2. Examples of recessive inherited diseases:
Key points
Thalassaemia (Mediterranean anaemia)affects the production of haemoglobin and causes anaemia of varying severity.
Galactosaemiathe body cannot break down the sugar galactose; babies suffer vomiting, yellowing of skin and eyes, and poor growth.
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