Genetics studies inheritance and variation. Begin by distinguishing a gene from an allele, then use examples to connect genetic information with an observable characteristic. This page brings together the existing introductory genetics notes.

How to use these resources

  1. Learn the vocabulary. A gene is a region of DNA with a biological role. Alleles are alternative versions of a gene.
  2. Separate genotype and phenotype. Genotype describes genetic information. Phenotype describes an observable characteristic influenced by genotype and, in many cases, the environment.
  3. Compare inheritance patterns. Study dominance relationships before comparing cases involving more than two alleles in a population.

Multiple alleles: individual versus population

A gene may have several allele variants across a population. At a typical autosomal locus, a diploid individual has two copies, so it carries no more than two of those variants. Do not confuse the number of variants in the population with the number carried by one individual.

Check your understanding

Explain the difference between a gene and an allele in one sentence. Then describe why a dominant allele is not necessarily the most common allele in a population: dominance describes its relationship with another allele, not its frequency.

Notes and learning resources

Recommended learning sequence

  1. Genes, alleles, genotype and phenotype
  2. Dominance relationships and genetic crosses
  3. Meiosis and the source of variation
  4. Inheritance probabilities and pedigree reasoning
  5. Testing a genetic hypothesis with observed data

Beginner route: work through the sequence in order. The lesson links above show the resources currently available; unlinked topics describe the learning route, not completed lessons.

Deeper study: Use a chi-square calculation only after stating a genetic hypothesis, the expected ratio and the test’s assumptions.

Coverage: this collection is partial. General explanations can support several courses, but curriculum alignment must be checked against the current specification. No complete exam-board coverage or endorsement is claimed.

Related pathway: Evolution · Practice centre · All biology topics