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AQASpec 4.6Foundation & Higher~35 min

Inheritance & Variation

Biology · Topic revision workspace

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Topic overview

What you need to know

Inheritance & Variation is a key part of GCSE Biology. Connect structures to their functions and explain biological processes as clear causal sequences. A version of a gene. Organisms inherit two alleles for each gene — one from each parent. An allele that is always expressed in the phenotype, even if only one copy is present. Shown with a capital letter. An allele that is only expressed when two copies are present (homozygous recessive). Shown with a lowercase letter.

Exam tip

Use precise scientific vocabulary and make each link in an explanation explicit.

Revision notes

Core ideas

Secure these ideas first. Say each definition in your own words, then connect it to the topic overview.

  • Allele: A version of a gene. Organisms inherit two alleles for each gene — one from each parent.
  • Dominant Allele: An allele that is always expressed in the phenotype, even if only one copy is present. Shown with a capital letter.
  • Recessive Allele: An allele that is only expressed when two copies are present (homozygous recessive). Shown with a lowercase letter.
  • Genotype: The combination of alleles an organism has for a particular gene. E.g. BB, Bb, or bb.

Revision notes

Apply it in the exam

The exam will rarely ask for an isolated definition. Practise selecting the right idea and using it as part of a complete explanation or method.

  • Phenotype: The observable physical characteristics of an organism, determined by genotype and environment.
  • Heterozygous: Having two different alleles for a gene, e.g. Bb.
  • Homozygous: Having two identical alleles for a gene, e.g. BB (homozygous dominant) or bb (homozygous recessive).
  • Punnett Square: A diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

Core knowledge

Key facts for Inheritance & Variation

1

Allele

A version of a gene. Organisms inherit two alleles for each gene — one from each parent.

2

Dominant Allele

An allele that is always expressed in the phenotype, even if only one copy is present. Shown with a capital letter.

3

Recessive Allele

An allele that is only expressed when two copies are present (homozygous recessive). Shown with a lowercase letter.

4

Genotype

The combination of alleles an organism has for a particular gene. E.g. BB, Bb, or bb.

5

Phenotype

The observable physical characteristics of an organism, determined by genotype and environment.

6

Heterozygous

Having two different alleles for a gene, e.g. Bb.

7

Homozygous

Having two identical alleles for a gene, e.g. BB (homozygous dominant) or bb (homozygous recessive).

8

Punnett Square

A diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

Active recall

Close the notes and answer these

  1. 1.Without looking, explain allele and give one example or consequence.
  2. 2.Without looking, explain dominant allele and give one example or consequence.
  3. 3.Without looking, explain recessive allele and give one example or consequence.
  4. 4.Without looking, explain genotype and give one example or consequence.

Content reviewed 23 July 2026 against the current linked specification.

Finished learning?

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