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IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 3 (10 Questions)
IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 3 (10 Questions)
Paper 3 concentrates on ratios and statistical judgement: deriving phenotype numbers from 3:1 and 1:1 crosses, testing observed counts with chi-square, and handling incomplete dominance and test crosses.
Quick answer Two heterozygotes give about three hundred tall and one hundred dwarf plants out of four hundred. Tt crossed with tt gives one to one. Observed counts of 288 and 112 give chi-square 1.92, below 3.84, so the 3:1 hypothesis stands, and incomplete dominance splits F2 one to two to one.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | 25 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: IB Biology: Genetics (Monohybrid Crosses) – Tutor Lesson Plan |
| Level | IB SL |
02 Questions (10 items)
- FoundationWhat is the phenotype ratio of Tt × Tt? Out of 400 offspring, how many are tall and how many dwarf?
- FoundationWhat are the genotypes and phenotypes of the offspring of TT × tt?
- FoundationWhat is a test cross?
- FoundationHow do genotype and phenotype differ?
- IntermediateTt × tt gives a 1:1 ratio. Out of 240 offspring, how many tall and how many dwarf are expected?
- IntermediateTwo heterozygotes produce 320 offspring. How many tall and dwarf are expected?
- IntermediateThe expected ratio is 300 to 100 and the observed numbers are 288 and 112. Find chi-square and the conclusion at the five percent level.
- ChallengeRed RR crossed with white WW gives all pink offspring, a case of incomplete dominance. What phenotypes appear in 400 F2 plants?
- ChallengeA tall plant crossed with another tall plant produces some dwarf offspring. Out of 40 offspring, how many dwarf plants are expected?
- Error redoCommon error: a chi-square value above 3.84 is taken as proof that the genetic hypothesis is correct. What is the correct reading?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 3:1; 300 tall and 100 dwarf | Tall plants with at least one T make three quarters; dwarf plants make one quarter. |
| 2 | All are Tt and all are tall | Two homozygotes crossed give only heterozygous offspring. |
| 3 | Crossing an individual of unknown genotype with a homozygous recessive individual | If any recessive offspring appear, the parent must carry the recessive allele. |
| 4 | Genotype is the combination of alleles; phenotype is the observable characteristic | One phenotype can come from different genotypes, such as TT and Tt. |
| 5 | 120 each | Half are heterozygous and tall, half are homozygous recessive and dwarf. |
| 6 | 240 tall and 80 dwarf | Three quarters of three hundred and twenty is two hundred and forty. |
| 7 | 1.92; below 3.84, so the 3:1 hypothesis is not rejected | One hundred and forty-four over three hundred is zero point four eight, and over one hundred it is one point four four. |
| 8 | 100 red, 200 pink and 100 white, a 1:2:1 ratio | Heterozygotes show the intermediate colour, so F2 splits one to two to one. |
| 9 | 10 | Dwarf offspring show that both parents are heterozygous, so one quarter are dwarf. |
| 10 | A value above 3.84 rejects the hypothesis; a value below it only fails to reject | A statistical test never proves a hypothesis; it only asks whether the data contradict it. |
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.
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.
