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IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 2 (10 Questions)
IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 2 (10 Questions)
Paper two starts from test crosses and codominance: convert one to one and three to one ratios into expected counts, test them with chi-squared, compute joint probabilities for several offspring, then handle ABO blood groups and the one to two to one ratio of incomplete dominance.
Quick answer Three to one means three quarters against one quarter, and a test cross uses tt to reveal 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)
- FoundationA tall pea plant of genotype Tt is crossed with a short one, tt. Give the expected phenotypic ratio and the numbers in two hundred forty offspring.
- FoundationHow does codominance differ from complete dominance? Give an example.
- FoundationA parent of genotype I-A i marries one of genotype I-B i. Which blood groups can the children have, and in what ratio?
- FoundationWhat do genotype and phenotype mean? Illustrate with tall pea plants.
- IntermediateA Tt by Tt cross yields four hundred offspring, two hundred eighty-eight tall and one hundred twelve short. Compute chi-squared and judge the three to one expectation.
- IntermediateTwo heterozygotes are crossed. What is the probability that all four offspring are short?
- IntermediateHow does a test cross distinguish a TT from a Tt tall plant?
- ChallengeA red and a white flower cross gives all pink F1 under incomplete dominance. What colours are expected among four hundred F2 plants?
- ChallengeThe father is I-A i and the mother I-B i. What is the probability of an O child, and of an AB child?
- Error redoA student reports three hundred tall and one hundred short from a three to one cross with only two hundred offspring. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | one to one, that is one hundred twenty tall and one hundred twenty short | Half the offspring are Tt and half tt, so each group is one hundred twenty. |
| 2 | in codominance both alleles are fully expressed, as in blood group AB, while complete dominance hides the recessive trait | Group AB red cells carry both A and B antigens at once. |
| 3 | A, B, AB and O, each one quarter | The four combinations each take one quarter, so one hundred of four hundred. |
| 4 | the genotype is the allele pair such as TT or Tt, the phenotype is the visible trait such as tall | TT and Tt look the same, so appearance alone cannot tell them apart. |
| 5 | chi-squared is one point nine two, below the critical value of three point eight four, so three to one is accepted | Against expected counts of three hundred and one hundred the value is one point nine two. |
| 6 | one in two hundred fifty-six, about zero point three nine percent | Each offspring has a one in four chance, and the fourth power gives one in two hundred fifty-six. |
| 7 | cross it with tt: all tall offspring means TT, a one to one split means Tt | The tt parent supplies only t, so any short offspring reveal a hidden t. |
| 8 | one hundred red, two hundred pink and one hundred white, a one to two to one ratio | Heterozygote selfing gives one to two to one, so one hundred, two hundred and one hundred. |
| 9 | one quarter for group O and one quarter for group AB | Two i alleles give O and the two dominant alleles give AB, each one quarter. |
| 10 | the ratio must be scaled to the total, so two hundred offspring give one hundred fifty and fifty | Three to one means three quarters and one quarter of whatever total is observed. |
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.
