Cambridge IGCSE Biology 0610. Topic 17: Inheritance

Inheritance, Genes & Punnett Squares

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Inheritance questions combine accurate terminology with processes such as mitosis, meiosis and genetic crosses. Students must distinguish genes, alleles, genotypes and phenotypes before applying them in Punnett squares.

How Cambridge tests this topic

Genetics is where method marks are everything: examiners want a defined key of symbols, parental genotypes, gametes circled, a complete Punnett square and the ratio stated. Even one wrong answer can still earn most marks if the working is shown. The losses come from confusing genotype with phenotype, undefined symbols, and forgetting that ratios are probabilities, not guarantees. Master the layout and the vocabulary and this is one of the most predictable scoring topics.

Focused lessons

Break the topic into manageable skills

Use these guides when one concept, calculation or question pattern needs more attention than the full-topic overview.

Mitosis and Meiosis: How Each Type of Cell Division Works

Supplement · Cambridge 0610 Topics 17.2–17.3

Chromosomes, Genes and Alleles: The Basics of Inheritance

Core and Supplement · Cambridge 0610 Topic 17.1

Monohybrid Crosses and Punnett Squares: Predicting Offspring

Core and Supplement · Cambridge 0610 Topic 17.2

Co-dominance: When Both Alleles Are Expressed

Supplement · Cambridge 0610 Topic 17.3

Sex Determination and Sex-Linked Inheritance

Supplement · Cambridge 0610 Topic 17.4

Genetic Diagrams and Ratios: Interpreting Expected Outcomes

Core and Supplement · Cambridge 0610 Topic 17.5

DNA Structure and Protein Synthesis Overview

Core and Supplement · Cambridge 0610 Topic 17.6

Genetic Engineering and Cloning: Modifying and Copying Organisms

Core and Supplement · Cambridge 0610 Topic 17.7

The vocabulary examiners insist on

Genetics marks live in precise definitions. Learn these exactly:

Gene: a length of DNA that codes for a protein / a unit of inheritance.
Allele: a different/alternative version (form) of a gene.
Genotype: the genetic makeup / the alleles present (e.g. Bb).
Phenotype: the observable features/characteristics (e.g. Brown eyes).
Dominant: an allele expressed even if only one copy is present.
Recessive: an allele only expressed when two copies are present (homozygous).
Homozygous: two identical alleles (BB or bb); heterozygous: two different alleles (Bb).

The most common confusion is genotype versus phenotype. Genotype is the letters, phenotype is the appearance. When a question asks for the genotype, give the letters; when it asks for the phenotype, describe the feature. Define a chromosome as a thread of DNA made up of genes.

Drawing a monohybrid cross: the layout that scores

Follow this exact structure every time, because each line is a mark:

Define your symbols/key: e.g. 'Let B = allele for brown eyes (dominant), b = allele for blue eyes (recessive).'
Write the parental genotypes and phenotypes.
Show the gametes (circle them). Each gamete carries one allele.
Draw the Punnett square and fill in the offspring genotypes.
State the offspring genotype and phenotype ratio.

For Bb × Bb:

	B	b
B	BB	Bb
b	Bb	bb

This gives genotype 1 BB: 2 Bb: 1 bb and phenotype 3 brown: 1 blue. Always define the key: an undefined symbol can cost the first mark even if the square is perfect. Show every step so method marks survive a slip.

Ratios, probabilities and predicted outcomes

Examiners test whether you understand that a genetic ratio is a probability, not a guarantee. A 3:1 ratio means each offspring has a 3 in 4 (75%) chance of the dominant phenotype, not that exactly three of every four will show it. If asked for the chance of a particular outcome, express it as a fraction, ratio or percentage and state it clearly.

A test cross determines whether an organism showing the dominant phenotype is homozygous (BB) or heterozygous (Bb): cross it with a homozygous recessive (bb). If any offspring show the recessive phenotype, the unknown parent must be heterozygous. The exam mark is explaining the reasoning, not just the cross. When sample sizes are small, actual results may not match the predicted ratio exactly. Examiners reward the answer 'fertilisation is random / small sample size' for this. Punnett squares connect to meiosis, which produces genetically varied gametes.

Mitosis, meiosis and cell division (Extended)

Distinguish the two divisions carefully. Mitosis produces two genetically identical daughter cells with the same number of chromosomes as the parent (diploid); it is used for growth, repair and asexual reproduction. Meiosis (Extended only) produces four genetically different cells with half the chromosome number (haploid gametes), and is a reduction division.

The key marks: mitosis = identical, same chromosome number; meiosis = varied, halved chromosome number. (Extended only) Meiosis is the source of genetic variation in gametes, and fertilisation restores the full chromosome number. Students often say meiosis 'makes identical cells' (that is mitosis). Keep them separate. A diploid cell has chromosomes in pairs; a haploid gamete has one of each. This underpins why offspring vary, linking to variation and natural selection.

Co-dominance and sex linkage (Extended)

(Extended only) Co-dominance is when both alleles in a heterozygote are expressed equally: neither is recessive. The classic example is the ABO blood group, where I^A and I^B are co-dominant (giving group AB), and both are dominant to I^O. When writing co-dominant crosses, use capital-letter alleles with superscripts rather than upper/lower case.

Sex determination: females are XX, males are XY; a cross shows a 1:1 ratio of male to female offspring. Sex linkage refers to genes carried on the X chromosome; because males have only one X, a recessive allele (e.g. For red-green colour blindness or haemophilia) is expressed in males more often. The exam skill is showing the carrier mother (X^HX^h) cross and explaining why sons are more likely affected. State the alleles as superscripts on the X chromosome. To practise these crosses with a specialist, book a trial class.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

Allele: An alternative form of a gene, found at the same position on a chromosome. For example, the gene for seed shape has a round allele and a wrinkled allele.
Base pair: The specific pairing of nitrogenous bases in DNA held together by hydrogen bonds: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This complementary base pairing ensures accurate replication of DNA.
Carrier: An individual who is heterozygous for a recessive genetic condition, carrying one copy of the recessive allele but not showing the condition because the dominant allele masks its effect. Carriers can pass the recessive allele to their offspring.
Chromosome: A thread-like structure made of DNA and protein, found in the nucleus of a cell. Chromosomes carry genes in a specific order along their length. Humans have 46 chromosomes (23 pairs), including one pair of sex chromosomes (XX in females, XY in males).
Co-dominance: A pattern of inheritance in which both alleles of a gene are expressed equally in the phenotype of a heterozygote, rather than one being dominant over the other. For example, in sickle cell disease, a heterozygote (HbA HbS) produces both normal and sickle-shaped red blood cells.
Cystic fibrosis: A genetic disorder caused by a recessive allele that results in the production of thick, sticky mucus in the lungs, pancreas and other organs. The thick mucus blocks airways, making breathing difficult, and blocks pancreatic ducts, reducing enzyme secretion for digestion.
Diploid: A cell or organism that has two complete sets of chromosomes, one set inherited from each parent. In humans, diploid cells contain 46 chromosomes (23 pairs). All body cells are diploid, while gametes are haploid.
DNA: Deoxyribonucleic acid, a double-stranded molecule in the shape of a double helix found in chromosomes. DNA carries the genetic code as a sequence of bases (A, T, C, G) that determines the order of amino acids in proteins and therefore controls cell activities.
Dominant allele: An allele that is expressed in the phenotype when only one copy is present (in the heterozygous condition).
Gene: A length of DNA that codes for a protein (the unit of inheritance passed from parent to offspring).
Genotype: The genetic make-up of an organism. The combination of alleles it carries for a characteristic, written as letters (e.g. RR, Rr or rr).
Haploid: A cell that has one complete set of chromosomes, half the diploid number. In humans, haploid cells (gametes) contain 23 chromosomes. Haploid cells are produced by meiosis and restore the diploid number when they fuse at fertilisation.
Heterozygous: Having two different alleles of a particular gene.
Homozygous: Having two identical alleles of a particular gene.
Meiosis: Reduction division in which the chromosome number is halved, producing four genetically different gametes from one parent cell.
Mitosis: Nuclear division that produces two genetically identical daughter cells with the same number of chromosomes as the parent cell. Used for growth, repair of damaged tissue, replacement of cells, and asexual reproduction.
Phenotype: The observable features of an organism that result from its genotype and its environment. For example, round seeds or brown eyes.
Protein synthesis: The process by which the sequence of bases in DNA is used to determine the sequence of amino acids in a protein. DNA is transcribed into mRNA in the nucleus, and the mRNA moves to ribosomes in the cytoplasm where it is translated into a chain of amino acids.
Punnett square: A diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross. The alleles of one parent are placed along the top and the other parent's alleles along the side, and the boxes show all possible offspring combinations.
Recessive allele: An allele that is only expressed in the phenotype when two copies are present (in the homozygous condition).
Sex linkage: A pattern of inheritance where a gene is located on the X chromosome and therefore the trait is more commonly expressed in males. Males have only one X chromosome (XY), so a single recessive allele on the X chromosome will be expressed since there is no second X to carry a dominant allele.
Sickle cell anaemia: A genetic blood disorder caused by a mutation in the gene for haemoglobin, resulting in abnormal sickle-shaped red blood cells. These rigid, crescent-shaped cells can block capillaries, reducing oxygen delivery to tissues and causing pain and fatigue.

Browse the full IGCSE Biology glossary when a related term needs checking.

Try a worked question (6 marks)

In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t). Two heterozygous tall plants are crossed. Using a genetic diagram, determine the expected ratio of tall to short offspring, and explain why the actual numbers from one cross might not exactly match this ratio.

Show the answer and marking guidance

key defined: T = tall (dominant), t = short (recessive); parental genotypes both Tt (× Tt); gametes T and t from each parent; correct Punnett square giving TT, Tt, Tt, tt; phenotype ratio 3 tall: 1 short; actual numbers may differ because fertilisation is random / the sample size is small;

Each semicolon separates a distinct answer point. This is original practice content, not an official Cambridge past-paper question.

Common mix-ups to avoid

Use these as a correction checklist after attempting a question.

Confusing genotype (the alleles, e.g. Tt) with phenotype (the appearance, e.g. Tall).
Not defining the symbol key, which can cost a mark even with a correct square.
Stating a ratio as a certainty rather than a probability/chance.
Saying meiosis produces genetically identical cells (that is mitosis).
Forgetting to halve the chromosome number for meiosis/gametes.
Using upper/lower case letters for co-dominant alleles instead of capital letters with superscripts.

How to structure the method

For a 6-mark genetics question, the marks come from a complete, labelled genetic diagram, not prose. Always: define the key ('Let T =... Dominant, t =... Recessive'), give parental genotypes and phenotypes, circle the gametes, draw the full Punnett square, then state both the genotype and phenotype ratios. Finish with a probability statement using a stem like 'There is a 3 in 4 (75%) chance each offspring is tall.' If asked why real results differ, write 'fertilisation is random and the sample size is small.' Showing every step protects method marks even if one box is wrong.

Review the full extended-response method →

Malaysia and syllabus context

Genetics is a guaranteed high-mark topic in both May/June and Oct/Nov 0610 papers, and Punnett squares are reliably worth several marks. Malaysian students taking SPM Biology will know monohybrid crosses, but 0610 Extended adds co-dominance and sex linkage, which SPM treats more lightly. Focus revision there. Examiners in 0610 are strict about defining the key, a step often skipped in school practice here.

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