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IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 1 (10 Questions)
IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 1 (10 Questions)
Paper 1 concentrates on monohybrid crosses: gametes, Punnett squares, the three to one and one to one ratios, test crosses for unknown genotypes and the first step of a chi-square test.
Quick answer A cross of two heterozygotes gives three to one and a test cross gives one to one. Two hundred seeds give one hundred and fifty tall, four hundred give three hundred, and a near one to one result identifies a hidden recessive allele.
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)
- FoundationTwo heterozygous tall pea plants are crossed. What is the phenotypic ratio of the offspring?
- FoundationIn a cross of a heterozygote with a homozygous recessive, what proportion of the offspring is tall?
- FoundationA cross of two heterozygotes yields two hundred seeds. On a three to one expectation, how many are tall and how many short?
- FoundationA homozygous tall plant is crossed with a homozygous short plant. Give the genotype and phenotype of the first filial generation.
- IntermediateA test cross of a heterozygote gives one hundred offspring. How many are expected to be tall and short?
- IntermediateTwo tall plants produce some short offspring. What are the parental genotypes and what share is short?
- IntermediateTwo heterozygotes give four hundred seeds that all germinate. Give the expected numbers and the genotype of the short plants.
- ChallengeA tall plant crossed with a short plant gives sixty tall and fifty-eight short offspring. What is the likely genotype of the tall parent and why?
- ChallengeThree hundred tall and one hundred short plants are observed. Compute chi-square against a three to one expectation.
- Error redoA student writes a phenotypic ratio of one to two to one for a heterozygous cross. Give the correct ratio and the mistake.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | tall to short is three to one, so three quarters are tall | Half the gametes carry T, so three of the four boxes are tall. |
| 2 | half tall and half short | The recessive parent gives only t, so half the offspring receive T. |
| 3 | one hundred and fifty tall and fifty short | Three quarters of two hundred is one hundred and fifty. |
| 4 | every offspring is heterozygous and tall, with no short plants | Each parent gives one gamete type, so all offspring are heterozygous and tall. |
| 5 | fifty tall and fifty short | A test cross with a recessive parent separates a heterozygote from a homozygote. |
| 6 | both are heterozygous and one quarter are short | A short plant needs two recessive alleles, so both parents must carry one. |
| 7 | three hundred tall and one hundred short, all the short ones homozygous recessive | Three quarters of four hundred is three hundred and every short plant is homozygous recessive. |
| 8 | heterozygous, because the near one to one ratio rules out a homozygous tall parent | A homozygous tall parent would give only tall offspring, so the short ones prove a hidden allele. |
| 9 | chi-square is zero | The expected numbers equal the observed ones, so every deviation is zero. |
| 10 | the phenotypic ratio is three to one; one to two to one describes the genotypes | Homozygous and heterozygous tall look the same, so the phenotypes merge into three to one. |
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.
