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AQA GCSE Biology

Genetics and inheritance — AQA GCSE Biology revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 5 minutes to read.

The basics

  • A gene is a section of DNA coding for a protein; a chromosome carries many genes.
  • Alleles are versions of a gene. Dominant alleles show with one copy; recessive need two.
  • Genotype = alleles you have. Phenotype = characteristics you show.
  • Sex chromosomes: XX female, XY male.

Meiosis and crosses

  • Meiosis makes four genetically different gametes, each with half the chromosomes.
  • Use a Punnett square to predict ratios, e.g. Bb × Bb gives 3 : 1 dominant to recessive.

Key terms

Allele
A different version of the same gene.
Homozygous
Two identical alleles for a gene.
Heterozygous
Two different alleles for a gene.
Gamete
A sex cell with half the normal chromosome number.
Dominant allele
An allele that is expressed even if only one copy is present.
Recessive allele
An allele only expressed when two copies are present.

Common exam traps

  • Writing probabilities as certainties — a 3:1 ratio is a probability, not a guarantee.
  • Using the same letter in upper and lower case that look alike (e.g. Ss vs ss) — write clearly.
  • Confusing genotype with phenotype in the answer line.

Practice questions with answers

  1. 1. Two heterozygous parents (Bb × Bb) have a child. What is the probability of a recessive phenotype?

    • • 0%
    • • 25%
    • • 50%
    • • 75%

    Answer: 25%

    Only bb shows the recessive phenotype — 1 in 4.

  2. 2. Define phenotype.

    Answer: The physical characteristics an organism shows, resulting from its genotype and environment.

    Genotype = alleles; phenotype = what you see.

  3. 3. How many cells does meiosis produce from one parent cell?

    • • 1
    • • 2
    • • 4
    • • 8

    Answer: 4

    Four genetically different gametes with half the chromosomes.

  4. 4. Give the sex chromosomes of a human male.

    Answer: XY

    Females are XX.

  5. 5. A gene for eye colour has two alleles, B (brown, dominant) and b (blue, recessive). What genotype shows a brown-eyed phenotype?

    • • bb only
    • • BB or Bb
    • • Bb only
    • • BB only

    Answer: BB or Bb

    Only one dominant allele is needed to show the dominant phenotype.

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