IB Biology: Mendelian Genetics & Inheritance FAQ

You need Mendel's law of segregation and law of independent assortment, monohybrid and dihybrid crosses, genotype/phenotype ratios, test crosses, codominance, multiple alleles, sex-linked inheritance and pedigree analysis. According to the IB Biology guide (first exams 2025), this sits under Topic A3 (Heredity) at SL and HL, with HL adding linkage and gene interaction.

Genotype is the actual allele combination an organism carries (e.g. Ff); phenotype is the observable trait that results (e.g. tall). Examiners frequently ask you to state both from a cross, and losing the distinction — writing 'tall' when asked for genotype — is one of the commonest silly marks dropped in Paper 2.

Set up a 4x4 Punnett square using the gametes produced by independent assortment (each parent makes four gamete types from two heterozygous genes), then read off the classic 9:3:3:1 phenotype ratio. HL students must also recognise when linked genes break this ratio.

In codominance, both alleles are fully expressed simultaneously — the classic IB example is ABO blood groups, where a person with genotype IAIB shows both A and B antigens. Incomplete dominance instead produces a blended intermediate phenotype, like pink flowers from red x white parents.

Work through it systematically: check if affected individuals appear in every generation (dominant) or skip generations (recessive), then check if the trait affects males and females equally (autosomal) or mostly one sex (sex-linked). The IB expects you to justify your conclusion with a specific individual, not just state the pattern.

Autosomal genes sit on chromosomes 1–22 and affect both sexes equally; sex-linked genes sit on the X (or rarely Y) chromosome, so inheritance patterns differ by sex — colour blindness and haemophilia are the IB's standard X-linked recessive examples. Males need only one recessive allele to show an X-linked trait, since they carry a single X.

Yes — monohybrid crosses, dihybrid crosses, codominance, multiple alleles, sex linkage and pedigrees are core content for both SL and HL under Topic A3 (Heredity). HL students additionally cover gene linkage, recombination via crossing over, and more complex non-Mendelian ratios that arise from linked genes.

Expect 'state' (genotype/phenotype ratios), 'determine' (a cross outcome or probability), 'deduce' (mode of inheritance from a pedigree), and 'explain' (why a ratio occurred, referencing meiosis). 'Deduce' questions carry the most marks and need a reasoned chain, not just a stated answer.

The chi-squared test compares observed offspring counts against the expected Mendelian ratio (e.g. 3:1 or 9:3:3:1) to check if any deviation is due to chance or a real biological cause like linkage. If your calculated chi-squared value exceeds the critical value at the given degrees of freedom, you reject the null hypothesis.

The biggest one, in my experience marking mocks, is inconsistent allele letters — mixing capital and lowercase forms of different letters instead of the same letter (Tt, not At). Second is forgetting to show full working in a Punnett square when a question asks you to 'determine' a ratio; stating the answer alone loses method marks.

Drill Punnett squares until the notation is automatic, then practise past-paper pedigree questions specifically — that's where most marks separate a 6 from a 7. I tell every student I teach: don't just get the ratio right, show the gamete-formation step, because that's often a separate mark on the scheme.

Past papers are non-negotiable — nothing replicates the exact phrasing IB examiners use for 'deduce' and 'determine' questions. Beyond that, a structured topical question bank that isolates A3 (Heredity) questions from the rest of the syllabus lets you drill the notation until it's automatic, rather than getting only one or two genetics questions per full past paper.

Free resources (past papers, the IB's own subject guide) cover the basics but won't isolate weak topics like genetics for targeted practice. Paid self-study platforms with topical worksheets and mock papers typically run in the tens of pounds/dollars per subject per year — a reasonable investment if your child is consistently dropping marks on one syllabus area rather than needing broad revision.

Both cover Mendelian crosses, codominance, sex-linkage and pedigrees at a similar depth, but IB Biology folds genetics into a broader 'Heredity' topic (A3) that connects explicitly to meiosis and evolution, whereas A-Level tends to separate genetics and evolution into distinct modules. HL IB adds linkage and recombination, roughly matching A-Level's more mathematical treatment of linked genes.

Not at all — genetics is a self-contained skill built on notation and logical deduction, distinct from the data-analysis and experimental-design skills tested elsewhere in the course. A student who finds Punnett squares fiddly can still do very well in ecology, physiology or biochemistry topics; it's worth targeted practice on this one area rather than worrying about the whole subject.