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S5 Biology: Mendelian Genetics (Paper 2)
S5 Biology: Mendelian Genetics (Paper 2)
The second paper drills probability: counting Punnett squares, multiplying chances for one and two gene pairs, the nine to three to three to one ratio, and the two thirds heterozygous trick in pedigrees.
Direct answer Multiply probabilities across gene pairs; Tt by Tt gives tt one quarter of the time; a dihybrid self cross gives nine to three to three to one; two thirds of tall offspring are Tt; assortment is the opposite of linkage.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S5 Biology: Mendelian Genetics |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationIn a Tt by Tt cross, what is the probability of a dwarf offspring?
- FoundationIn a Tt by tt cross, what is the probability of a tt offspring?
- FoundationHow many squares does a monohybrid Punnett grid have, and how many are homozygous?
- IntermediateIn a TtGg by TtGg cross, what is the probability of ttgg?
- IntermediateIf that cross gives 640 plants, about how many are ttgg?
- IntermediateWhat is the phenotypic ratio of a TtGg by TtGg cross?
- ChallengeAmong the tall offspring of a Tt by Tt cross, what fraction is Tt?
- ChallengeTwo Tt plants produce three dwarf offspring in a row. What is the probability?
- ChallengeIn a TtGg by ttgg cross, what is the probability of a TtGg offspring?
- Error redoA student says independent assortment means two gene pairs are always inherited together. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | One quarter | Only the tt square is dwarf, one of the four possible outcomes. |
| 2 | One half | Tt passes on t half the time and tt always does, so half the offspring are tt. |
| 3 | Four squares, two of them homozygous, TT and tt | The four squares are TT, Tt, Tt and tt, so homozygotes take up half. |
| 4 | One sixteenth | The chance of tt is one quarter and of gg also one quarter, and multiplying gives one sixteenth. |
| 5 | About 40 | Six hundred and forty divided by sixteen gives 40, subject to random variation. |
| 6 | Nine to three to three to one | Each character gives three to one, and combining them produces nine to three to three to one. |
| 7 | Two thirds | Tall plants cover one TT and two Tt squares, so two thirds are heterozygous, which matters in pedigree risk sums. |
| 8 | One sixty-fourth | Each dwarf has a chance of one quarter, and cubing that gives one sixty-fourth. |
| 9 | One quarter | The chance of Tt is one half and of Gg also one half, so the product is one quarter. |
| 10 | Independent assortment means random combination of different pairs; being inherited together is linkage | Only genes on the same chromosome tend to travel together, which is linkage, the opposite of independent assortment. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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