This set covers the whole of cell division and inheritance: the chromosome outcomes of mitosis and meiosis, genes and alleles, inheritance grids, genotype and phenotype, and probability. The questions run from easy to harder, and all organisms and data are invented.
Work on paper, with a pencil and a blank grid ready. Open the answer only after you have written your own. The inheritance model board and the probability tree and counting board are useful for checking a cross, and the mistake log helps you track errors you repeat.
Questions
Q1 (easy). A cell in a fictional frog has 12 chromosomes. It divides by mitosis. State the number of cells produced and the chromosome number in each.
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Mitosis gives 2 cells. Each has the same number as the parent, so 12 chromosomes, and the two cells are genetically identical.
Q2 (easy). The same frog makes gametes by meiosis from a body cell with 12 chromosomes. State the number of cells produced and the chromosome number in each.
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Meiosis gives 4 cells. The number is halved: 12 ÷ 2 = 6 chromosomes in each. The four cells are genetically different from each other.
Q3 (easy). A frog gamete with 6 chromosomes fuses with another gamete with 6 chromosomes. State the chromosome number of the zygote and explain why it matters.
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6 + 6 = 12 chromosomes. This restores the normal body cell number, so the number does not double in every generation.
Q4 (easy). Complete the sentence: “A ______ is a section of DNA that codes for a protein. Different versions of it are called ______. It is found on a ______ in the nucleus.”
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gene, alleles, chromosome.
Q5 (easy). In a fictional plant, allele T (tall) is dominant to t (short). State the phenotype of TT, Tt and tt.
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TT is tall, Tt is tall (T is dominant), tt is short.
Q6 (medium). Two heterozygous tall plants (Tt) are crossed. Draw a grid, then state the genotype ratio, the phenotype ratio and the probability that an offspring is short.
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Gametes: T and t from each parent.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotype ratio: 1 TT : 2 Tt : 1 tt. Phenotype ratio: 3 tall : 1 short. Probability of a short offspring: 1/4 (25%).
Q7 (medium). A heterozygous tall plant (Tt) is crossed with a short plant (tt). State the expected ratio of tall to short offspring and the probability of a tall offspring.
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Gametes: T and t from the first parent, only t from the second.
| t | |
|---|---|
| T | Tt |
| t | tt |
Ratio: 1 tall : 1 short. Probability of a tall offspring: 1/2 (50%).
Q8 (medium). Two tall plants are crossed and some of their offspring are short. State the genotype of both parents and explain how you know.
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A short offspring is tt, so it received a t from each parent. Both parents are tall, so each also carries T. Both parents are Tt.
Q9 (medium). A class crosses Tt × Tt and records 200 offspring: 146 tall and 54 short. How many of each were expected? Is the result consistent with the model?
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Probability tall = 3/4, so expected tall = 3/4 × 200 = 150. Expected short = 1/4 × 200 = 50. Check: 150 + 50 = 200.
The observed 146 and 54 differ from 150 and 50 by 4 in each case. That is a small difference for a sample of 200, so the result is consistent with the 3 : 1 model and with chance variation.
Q10 (harder). Two Tt plants produce 3 offspring. Calculate the probability that at least one is short.
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Probability that one offspring is tall = 3/4. All three tall: (3/4)³ = 27/64.
At least one short = 1 − 27/64 = 37/64 (about 0.58).
Check: 3³ = 27 and 4³ = 64, and 64 − 27 = 37.
Q11 (harder). A student says: “Tt × Tt gives a 1 in 4 chance of a short plant, so in a pack of 4 seedlings exactly one will be short.” Explain what is wrong and rewrite the statement correctly.
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The statement treats a probability as a certainty. Each seedling has an independent 1/4 chance of being short, so a pack of 4 could have none, one or several short seedlings. The probability of none is (3/4)⁴ = 81/256, about 32%.
Correct version: “Each seedling has a 1/4 probability of being short, so about one in four is expected to be short over a large number.”
Q12 (harder). Explain why two siblings from the same parents can look different, using meiosis and fertilisation in your answer.
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Meiosis makes gametes that are genetically different from each other, because each receives a different mixture of the parents’ chromosomes and alleles. Fertilisation is random, so any sperm may fuse with any egg. Different combinations of alleles therefore give siblings different genotypes and, in some cases, different phenotypes.
If you got these wrong
| What went wrong | Go to |
|---|---|
| Wrong number of cells or chromosomes (Q1 to Q3) | Compare chromosome outcomes of mitosis and meiosis |
| Mixed up gene, allele or chromosome (Q4) | Distinguish gene, allele and chromosome |
| Grid set up or read incorrectly (Q6, Q7) | Construct a simple inheritance grid |
| Genotype and phenotype confused, parents deduced wrongly (Q5, Q8) | Separate genotype from phenotype |
| Probability treated as certain, or sample size ignored (Q9 to Q11) | Explain probability across repeated offspring |
| Explanation lacked meiosis or random fertilisation (Q12) | Lessons one and five, then write the answer again in full sentences |
Keep a note of each error type and retest the same skill with a fresh example after a few days. If one pattern keeps returning, online one-to-one Biology tuition gives you a teacher to work through it with.