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Course: Science — Third Preparatory
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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

Common mistakes to avoid

Common mistakes to avoid

Comparison aspectMain ideaKeywords
Confusing the genotype with the phenotype.
  • "Brown eyes" is a phenotype
  • "Bb" is a genotype. Always say
  • one you are describing
BrowneyesAlwaysdescribing
Thinking a dominant allele is "stronger" or more common.
  • Dominance only describes
  • allele shows in a heterozygote. A dominant allele can be rare in a population
Dominancedescribesshowsheterozygote
Writing gametes with two alleles.
  • A gamete carries only one allele of each gene. For Yy the gametes are Y and y
  • never "Yy". For RrYy each gamete carries one allele of each gene: RY
  • rY or ry
carriesgeneneverRrYy
Forgetting that ratios are probabilities.
  • A 3 : 1 ratio does not mean that a family of four children must have exactly three dominant and one recessive child. Each child has a 75% chance of showing the dominant trait
  • independent of the previous child
doesmeanfamilyfour
Mixing up carriers and patients.
  • A carrier (Nn) is healthy but can pass on the disease allele
  • a patient (nn) shows the disease
healthypassdiseaseallele
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