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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Memorise genetics definitions precisely. Practise monohybrid crosses with Punnett squares until you can do them quickly. Know sex determination (XX/XY). Extended: codominance and sex linkage.
Topic 17 is the genetics topic: definitions (gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive), monohybrid crosses using Punnett squares, and sex determination. Extended content adds codominance and sex linkage.
Priority 1: Definitions
These must be precise: gene (section of DNA coding for a protein), allele (version of a gene), genotype (combination of alleles, e.g. Aa), phenotype (observable characteristic), dominant (expressed in heterozygous), recessive (only expressed when homozygous).
Priority 2: Genetic crosses
Be able to draw a genetic diagram or Punnett square for a monohybrid cross. Show parental phenotypes, parental genotypes, gametes, offspring genotypes, offspring phenotypes, and ratio. The standard ratio from Aa × Aa is 3 dominant: 1 recessive.
Priority 3: Sex determination
Humans: XX = female, XY = male. The Y chromosome carries the gene for male development. Punnett square of XX × XY gives 1:1 ratio of male to female offspring.
Frequently asked questions
What genetics is Extended only?
Codominance (e.g. Blood groups), sex linkage (e.g. Colour blindness), and pedigree diagrams.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.