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IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 4 (10 Questions)
IB Biology: Genetics (Monohybrid Crosses) – Practice Paper 4 (10 Questions)
Paper 4 works backwards from ratios to parents: Punnett squares give expected counts, a real three to one family is used to infer genotypes, then incomplete dominance and small sample probability follow.
Quick answer A Tt by Tt cross of eight hundred plants gives six hundred tall and two hundred dwarf, a family of six hundred and fifty to two hundred and ten points to heterozygous parents, and four tall offspring in a row happen with probability eighty-one over two hundred and fifty-six.
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/HL |
02 Questions (10 items)
- FoundationA Tt by Tt cross produces eight hundred offspring. How many tall and dwarf plants are expected?
- FoundationA Tt by tt test cross yields two hundred and forty offspring. How many tall and dwarf are expected?
- FoundationWhat phenotypes appear among the offspring of a TT by Tt cross?
- FoundationWhat is the difference between genotype and phenotype?
- IntermediateA family shows six hundred and fifty tall and two hundred and ten dwarf plants. What are the most likely parental genotypes?
- IntermediateIf four hundred and eighty tall and one hundred and sixty dwarf are expected and the actual counts match exactly, what is chi squared?
- IntermediateA tall plant is test crossed and gives sixty tall and fifty-eight dwarf offspring. What is its genotype?
- ChallengeWith incomplete dominance, an Rr by Rr cross gives six hundred offspring. How many of each phenotype?
- ChallengeTwo Tt tall plants are crossed and only four offspring are sampled. What is the probability that all four are tall?
- Error redoCommon error: the offspring of Tt by Tt are given a one to two to one phenotypic ratio. Which ratio is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Six hundred tall and two hundred dwarf | The phenotypic ratio is three to one, giving six hundred and two hundred. |
| 2 | One hundred and twenty of each | The ratio is one to one, so half of two hundred and forty is one hundred and twenty. |
| 3 | All tall, half TT and half Tt | Both parents carry the dominant allele, so no dwarf plant appears. |
| 4 | Genotype is the allele combination while phenotype is the observable trait | TT and Tt look the same yet carry different allele combinations. |
| 5 | Tt by Tt | Six hundred and fifty over two hundred and ten is about three point one, close to three to one. |
| 6 | Zero | Each squared deviation is zero, so the total is zero. |
| 7 | Tt | A near one to one ratio means the plant carries a single dominant allele. |
| 8 | One hundred and fifty, three hundred and one hundred and fifty | The ratio is one to two to one, matching the genotypic ratio. |
| 9 | Eighty-one over two hundred and fifty-six, about 31.6 percent | Three quarters to the fourth power, since each offspring is independent. |
| 10 | The phenotypic ratio is three to one, while one to two to one describes genotypes | Because T is fully dominant, TT and Tt look identical. |
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.
