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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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Unit 3 — Variation of Traits in Living Organisms

Science — Third Preparatory

Unit 3 — Variation of Traits in Living Organisms

1.7 Mendel’s second law (law of independent assortment)

1.7 Mendel’s second law (law of independent assortment)

  • When two purebred individuals that differ in two or more pairs of contrasting traits are crossed, each pair of traits is inherited independently of the others.
  • Analysed separately, each trait appears in F₂ in the ratio 3 : 1.

The dihybrid cross: seed shape and seed colour

Comparison aspectMain ideaKeywords
•
  • R = round (dominant)
  • r = wrinkled
  • Y = yellow (dominant)
  • y = green
rounddominantwrinkledyellow
•P: pure round yellow (RRYY) × wrinkled green (rryy)pureroundyellowRRYY
GametesRY and ry
F₁
  • all RrYy
  • round yellow (100%) — both dominant traits appear
RrYyroundyellowboth
F₁ × F₁
  • RrYy × RrYy. Each parent produces four types of gametes with equal probability: RY
  • ry. Fusion gives 16 combinations
RrYyparentproducesfour
Comparison aspect RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
  • Count of phenotypes in F₂ (out of 16):

round yellow (both dominant): 9

  1. round green (first dominant, second recessive): 3
  2. wrinkled yellow (first recessive, second dominant): 3

wrinkled green (both recessive): 1

Comparison aspectMain ideaKeywords
Phenotype ratio 93 : 3 : 1
Check each trait alone
  • round = 9 + 3 = 12
  • wrinkled = 3 + 1 = 4 → 12 : 4 = 3 : 1. Yellow = 9 + 3 = 12
  • green = 3 + 1 = 4 → 3 : 1. So shape is inherited independently of colour
roundwrinkledYellowgreen

Worked example 4 (check your understanding)

  • T = tall, t = short; G = green pod, g = yellow pod.
  • Two plants heterozygous for both traits (TtGg × TtGg) are crossed.
  • What fraction are tall with green pods?
  • From the 9 : 3 : 3 : 1 pattern, tall green = 9/16 (56.25%).
  • You can also multiply independent probabilities: P(tall) = 3/4 and P(green) = 3/4, so 3/4 × 3/4 = 9/16.

Worked example 5: a test cross in humans

  • Curly hair (H) is dominant over straight (h); brown eyes (B) dominant over blue (b).
  • A man with curly hair and brown eyes (HhBb) marries a woman with straight hair and blue eyes (hhbb).
  • Man’s gametes HB, Hb, hB, hb.
  • Woman’s gametes only hb.
Comparison aspect HB Hb hB hb
hb HhBb Hhbb hhBb hhbb
Phenotypes→curly brown : curly blue : straight brown : straight blue = 1 : 1 : 1 : 1 (25% each).
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