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IGCSEBiology
Trial Class

Original practice · 5 questions · 16 marks · about 18 minutes

Monohybrid Crosses Practice Questions

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

These questions test your ability to carry out genetic crosses involving one gene, predict ratios, and use correct terminology.

Before you open the answers

  • Attempt every question without notes.
  • Show working for calculations and write complete biological statements.
  • Use the suggested time as a guide, not a strict limit on the first attempt.

These are original practice questions. They do not reproduce an official Cambridge paper or official mark scheme.

  1. Question 1

    Paper 4 Extended 4 marks Difficulty 3/5

    In mice, brown fur (B) is dominant over white fur (b). Cross a heterozygous brown mouse with a white mouse. Show the genetic diagram and state the expected phenotype ratio of the offspring.

    Show model answer and marking guidance
    1. Parents: Bb × bb.
    2. Gametes: B or b from Bb; b from bb.
    3. Offspring genotypes: Bb, Bb, bb, bb (or 50% Bb, 50% bb).
    4. Phenotype ratio: 1 brown: 1 white (50% brown, 50% white).

  2. Question 2

    Paper 4 Both 3 marks Difficulty 2/5

    Define the following terms: (a) genotype, (b) phenotype, (c) homozygous.

    Show model answer and marking guidance
    1. (a) Genotype: the genetic makeup / combination of alleles for a particular trait (e.g. Bb).
    2. (b) Phenotype: the observable characteristic / physical appearance (e.g. Brown fur).
    3. (c) Homozygous: having two identical alleles for a gene (e.g. BB or bb).

  3. Question 3

    Paper 4 Extended 4 marks Difficulty 4/5

    Two heterozygous tall pea plants (Tt) are crossed. Draw a Punnett square and state the expected genotype and phenotype ratios.

    Show model answer and marking guidance
    1. Punnett square: TT, Tt, Tt, tt.
    2. Genotype ratio: 1 TT: 2 Tt: 1 tt.
    3. Phenotype ratio: 3 tall: 1 short.
    4. This is the classic 3:1 monohybrid ratio.

  4. Question 4

    Paper 2 Core 2 marks Difficulty 1/5

    Define the term "allele".

    Show model answer and marking guidance
    1. An allele is a version / form of a gene.
    2. Different alleles of a gene code for different versions of a characteristic.

  5. Question 5

    Paper 4 Extended 3 marks Difficulty 4/5

    A plant breeder crosses two red-flowered plants and some offspring have white flowers. Explain the genotypes of the parents.

    Show model answer and marking guidance
    1. Both parents must be heterozygous (e.g. Rr × Rr).
    2. Red (R) is dominant; white (r) is recessive.
    3. Some offspring inherit rr: showing the recessive white phenotype.

Mark the set consistently

Award a point only when the idea is clearly present in the written answer. Correct any missed point in a complete sentence, then test the same skill on a different question several days later.

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