This guide covers IGCSE Biology topic 17: inheritance. You will revise chromosomes, genes, DNA (structure and role), genetic diagrams (monohybrid cross), dominant and recessive alleles, and phenotype vs genotype.

Key concepts to revise

• Chromosomes – In nucleus; carry genes; come in pairs (diploid); one from each parent.
• Gene – Section of DNA; codes for a characteristic; different forms = alleles (e.g. T and t).
• Dominant / Recessive – Dominant allele expressed if present (T); recessive only if two copies (tt).
• Genetic diagram – Punnett square; parental genotypes (e.g. Tt × Tt); offspring genotypes and phenotypes; ratio (e.g. 3 : 1).

Exam tips and command words

• State – Where genes are found (on chromosomes); what a gene codes for (a characteristic/protein).
• Draw a genetic diagram – Show parental genotypes, gametes, offspring genotypes and phenotypes; give ratio.
• Calculate – Probability of offspring having a genotype or phenotype (e.g. 1/4, 3/4).
• Define – Genotype (alleles present); phenotype (visible characteristic); dominant; recessive.

Common mistakes

• Confusing genotype (e.g. Tt) with phenotype (e.g. tall).
• Not showing gametes in the diagram (e.g. T and t from Tt parent).
• Writing ratio as “3 tall : 1 short” without stating it is phenotype ratio.

Revision checklist

• Define chromosome, gene, allele, genotype, phenotype, dominant, recessive.
• Draw a monohybrid cross (e.g. Tt × Tt) and state offspring genotype and phenotype ratios.
• State where genes and DNA are found and the role of DNA (codes for proteins).
• Calculate probability of offspring having a given characteristic from a genetic cross.

Next steps

Book a free trial with an IGCSE Biology tutor to practise genetic diagrams and inheritance questions, or explore Tutopiya’s learning portal for more revision resources.

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