Cambridge IGCSE Biology 0610 · Topic 17
Inheritance
Every question in this chapter is asking some version of the same thing: what genetic information does an organism carry, and what happens to it when that organism divides or reproduces? Chromosomes, genes and alleles describe what is being carried. Mitosis and meiosis describe what happens to it during division. Monohybrid crosses, pedigrees, codominance and sex linkage describe what happens to it between generations. Hold that shape in your head and the eighteen syllabus points stop feeling like eighteen separate facts to memorise.
Cambridge groups this material into four subtopics — 17.1 Chromosomes, genes and proteins, 17.2 Mitosis, 17.3 Meiosis, and 17.4 Monohybrid inheritance — and the syllabus is unusually specific about what it does not want. Detailed nucleotide structure, the mechanics of transcription and translation, and the named stages of mitosis and meiosis are all explicitly excluded. That matters for how you revise: the marks in this chapter sit on precise use of a fairly small set of terms — gene, allele, genotype, phenotype, dominant, recessive, homozygous, haploid, diploid — not on molecular detail you were never asked to learn in the first place.
Core candidates need the vocabulary in chromosomes, genes and alleles, sex determination, genotype and phenotype, homozygous and heterozygous, dominant and recessive, pedigree diagrams, and genetic diagrams giving 1:1 and 3:1 ratios. Supplement candidates add protein synthesis and gene expression, haploid and diploid nuclei, the mechanics of mitosis and meiosis, stem cells, test crosses, codominance with ABO blood groups, and sex-linked inheritance. The pages below follow that same Core-then-Supplement shape, so you can stop at the end of the Core material if that is what your tier requires.
What’s in this chapter
| Page | Level | What it covers |
|---|---|---|
| Chromosomes, genes and alleles | Core + Supplement | Why a chromosome, a gene and an allele are three different levels of the same thing, and how X and Y chromosomes decide a child’s sex. |
| Protein synthesis and gene expression | Supplement | How a base sequence becomes a protein shape, why the gene itself never leaves the nucleus, and why not every cell uses every gene it carries. |
| Mitosis | Supplement | The nuclear division that keeps chromosome number and genetic information the same — and why growth, repair and stem cells all depend on it. |
| Meiosis | Supplement | The reduction division that halves the chromosome number and makes every gamete genetically different from the next. |
| Monohybrid inheritance | Core | Genotype versus phenotype, homozygous versus heterozygous, and exactly what “dominant” does and doesn’t mean. |
| Punnett squares, genetic diagrams and test crosses | Core + Supplement | The seven-step method for turning parental genotypes into a predicted ratio, plus how a test cross unmasks a hidden recessive allele. |
| Pedigree diagrams | Core | How to read a family tree of a characteristic and work out which genotypes the evidence actually forces, rather than guessing. |
| Codominance and ABO blood groups | Supplement | What it means for neither allele to be masked, and how three alleles produce four ABO blood-group phenotypes. |
| Sex-linked inheritance | Supplement | Why a recessive characteristic carried on the X chromosome, such as red-green colour blindness, turns up far more often in males. |
Core or Supplement?
Two pages here are Core only: monohybrid inheritance and pedigree diagrams, plus the Core half of chromosomes, genes and alleles (the definitions and sex determination) and the Core half of Punnett squares (the 1:1 and 3:1 ratio method itself). Everything past that — protein synthesis, mitosis, meiosis, haploid and diploid nuclei, test crosses, codominance and sex linkage — is Supplement only, required for the Extended paper and not for Core-only candidates. If you are not sure which tier you are sitting, check before spending time on the Supplement pages, since none of the Core reasoning method depends on them.
One habit is worth building before you go further: reason every genetic cross the same way, every time — parental genotype, then gametes, then offspring genotype, then phenotype, then ratio. That sequence is introduced properly on the Punnett squares page, but it is the method the whole second half of this chapter is built on, from a simple 3:1 cross through to a sex-linked diagram with codominant alleles on the X chromosome.