Curriculum
Science — Third Preparatory
Science — Third Preparatory
0/8
Science — Third Preparatory 2026/2027
0/4
-
Unit 1 — Chemical Reactions and Their Environmental Impacts
Preview -
Unit 2 — Electricity and Magnetism
Preview
-
Unit 3 — Variation of Traits in Living Organisms
Preview
- 1.1 Why are family members alike but not identical?
- 1.2 How are traits passed on?
- 1.3 Alleles, dominance and recessiveness
- 1.4 Mendel and the discovery of the laws of heredity
- 1.5 Mendel's first law (law of segregation)
- 1.6 The Punnett square
- 1.7 Mendel's second law (law of independent assortment)
- 1.8 Genetic diversity
- 1.9 Genetic (inherited) diseases
- 1.10 Marriage between close relatives
- 1.11 Summary of Lesson 1
- 2.1 Populations
- 2.2 Population growth
- 2.3 Factors affecting population growth
- 2.4 Natural selection
- 2.5 Artificial selection
- 2.6 Natural versus artificial selection
- 2.7 Techniques used in artificial selection
- 2.8 Summary of Lesson 2
- Extra Study Section: Common Mistakes and More Practice
-
Unit 4 — Dating Planet Earth
Preview
Unit 1 - Force, Motion, Acceleration and Physical Quantities
0/1
Text lesson
Unit 3 — Variation of Traits in Living Organisms
Unit 3 — Variation of Traits in Living Organisms
1.6 The Punnett square
1.6 The Punnett square
- A Punnett square is a tool for predicting the possible genotypes and phenotypes of the offspring of a particular cross.
- How to build one:
- Work out the gametes each parent can make; the number of boxes depends on the number of gamete types.
- Draw a grid of rows and columns.
- Write the gametes of one parent across the top, and the gametes of the other parent down the left side.
- Fill each box by combining the allele above with the allele at the side.
- Count genotypes and phenotypes and give the ratio or percentage.
Worked example 1: purple × white (Check your understanding)
| Comparison aspect | Main idea | Keywords |
|---|---|---|
| • | Purple (P) is dominant over white (p). Cross pure purple PP × white pp | Purpledominantoverwhite |
| Gametes |
| purple |
| F₁ × F₁ | Pp × Pp |
| Comparison aspect | P | p |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
F₂→1 PP : 2 Pp : 1 pp, i.e. 3 purple : 1 white (75% purple, 25% white).
Worked example 2: a heterozygous axial plant × a terminal plant
- A = axial (dominant), a = terminal. Cross Aa × aa.
- A and a (from Aa); a only (from aa).
| Comparison aspect | a | a |
|---|---|---|
| A | Aa | Aa |
| a | aa | aa |
Offspring→2 Aa : 2 aa → 50% axial (Aa), 50% terminal (aa); phenotype ratio 1 : 1. Such a cross (with a homozygous recessive) is useful for finding out whether a dominant-looking plant is pure or hybrid.
Worked example 3: unknown purity
- A tall pea plant (T = tall, t = short) is crossed with a short plant.
- Half of the offspring are short.
- What is the genotype of the tall parent?
- Short offspring are tt, so each parent gave a t; so the tall parent must be Tt (heterozygous).
- If the tall parent had been TT, all offspring would have been tall.
Unit 3 — Variation of Traits in Living Organisms · 7 / 22