Cambridge IGCSE Biology 0610: Inheritance – Study Guide

This guide teaches topic 17, Inheritance, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. It covers sections 17.1 Chromosomes, genes and proteins, 17.2 Mitosis, 17.3 Meiosis and 17.4 Monohybrid inheritance. Core candidates need the Core statements in 17.1 and 17.4. Sections 17.2 and 17.3 are entirely Supplement, so they are marked Extended only, along with the Supplement statements in 17.1 and 17.4.

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What this topic covers

Section	What you must be able to do	Tier
17.1 (1–4)	Link chromosomes, DNA and genes; define gene and allele; explain how sex is inherited using X and Y	Core
17.1 (5–12)	Link base sequence to amino acid sequence and protein shape; outline how a protein is made; explain gene expression; describe haploid and diploid nuclei	Extended only
17.2	Describe mitosis, its roles, and stem cells	Extended only
17.3	Describe meiosis as a reduction division that makes gametes	Extended only
17.4 (1–12)	Use the key terms; interpret pedigrees; use genetic diagrams and Punnett squares for 1 : 1 and 3 : 1 crosses	Core
17.4 (13–18)	Test crosses, codominance, ABO blood groups, sex linkage and red-green colour blindness	Extended only

Core candidates sit Paper 1 and Paper 3, which are set on Core content only. Extended candidates sit Paper 2 and Paper 4, set on Core and Supplement content.

17.1 Chromosomes, genes and proteins

Core

• Chromosomes are made of DNA. DNA carries genetic information in the form of genes.
• A gene is a length of DNA that codes for a protein.
• An allele is an alternative form of a gene. For example, one gene controls a feature, and different alleles of that gene give different versions of it.

Inheritance of sex. Human females have two X chromosomes (XX). Males have one X and one Y (XY). Every egg carries an X. Half of the sperm carry an X and half carry a Y. The sperm that fertilises the egg decides the sex of the child.

Worked example 1: sex inheritance

Parents' phenotypes:   female        male
Parents' genotypes:      XX           XY
Gametes:               X  X         X  Y

            X       X
     X     XX      XX
     Y     XY      XY

Offspring: 2 XX (female) : 2 XY (male) = 1 : 1

So each child has a 1 in 2 (50%) chance of being male. Each fertilisation is a separate event.

Extended only

• The sequence of bases in a gene decides the sequence of amino acids used to make a specific protein. You do not need details of nucleotide structure.
• Different amino acid sequences give protein molecules different shapes. Shape decides what a protein does, for example which substrate fits an enzyme’s active site.
• DNA controls how a cell works by controlling which proteins it makes. These include enzymes, membrane carriers and receptors for neurotransmitters.

How a protein is made (the syllabus limits you to these points; no details of transcription or translation):

1. The gene coding for the protein stays in the nucleus.
2. Messenger RNA (mRNA) is made in the nucleus as a copy of the gene.
3. The mRNA moves out to the cytoplasm.
4. The mRNA passes through ribosomes.
5. The ribosome assembles amino acids into a protein molecule.
6. The order of amino acids is set by the sequence of bases in the mRNA.

Gene expression. Most body cells in an organism contain the same genes. But many genes in a given cell are not expressed, because the cell only makes the specific proteins it needs. For example, a cell in the stomach lining expresses the gene for a protease. A muscle cell in the same person carries that gene too, but leaves it switched off and instead makes the proteins a muscle cell needs.

Haploid and diploid.

• A haploid nucleus contains a single set of chromosomes (gametes).
• A diploid nucleus contains two sets of chromosomes. There is a pair of each type of chromosome.
• A human diploid cell has 23 pairs (46 chromosomes). A human gamete has 23.

17.2 Mitosis (Extended only)

• Mitosis is nuclear division giving rise to genetically identical cells. You do not need the stages.
• Exact replication of the chromosomes happens before mitosis.
• During mitosis the copies of the chromosomes separate, so each daughter cell keeps the same chromosome number as the parent cell.
• Roles of mitosis: growth, repair of damaged tissues, replacement of cells and asexual reproduction.

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions. Link this to 17.1: the daughter cells keep all the genes, but each type of specialised cell expresses a different set of them.

17.3 Meiosis (Extended only)

• Meiosis is involved in the production of gametes.
• It is a reduction division: the chromosome number is halved from diploid to haploid.
• The cells it produces are genetically different from each other. You do not need the stages.

Halving matters because fertilisation joins two haploid gametes. The zygote is then diploid again, and the chromosome number stays constant from one generation to the next.

Feature	Mitosis	Meiosis
Daughter cells	Genetically identical	Genetically different
Chromosome number	Kept the same	Halved (diploid to haploid)
Used for	Growth, repair, replacement, asexual reproduction	Making gametes

17.4 Monohybrid inheritance

Core terms

• Inheritance: the transmission of genetic information from generation to generation.
• Genotype: the genetic make-up of an organism, in terms of the alleles present (for example Gg).
• Phenotype: the observable features of an organism (for example green body).
• Homozygous: two identical alleles of a particular gene (GG or gg). Two identical homozygous individuals that breed together are pure-breeding.
• Heterozygous: two different alleles of a particular gene (Gg). A heterozygous individual is not pure-breeding.
• Dominant allele: expressed if it is present in the genotype (capital letter).
• Recessive allele: only expressed when there is no dominant allele of the gene present (lower-case letter).

Setting out a genetic diagram

Each line can earn credit, so always write all of them:

1. Parents’ phenotypes
2. Parents’ genotypes
3. Gametes (circle them)
4. Punnett square or lines showing fertilisation
5. Offspring genotypes
6. Offspring phenotypes and the ratio

Worked example 2: a 1 : 1 cross and a 3 : 1 cross

In a species of beetle, the allele for green body (G) is dominant to the allele for brown body (g).

(a) A heterozygous green beetle is crossed with a brown beetle.

Phenotypes:   green    x   brown
Genotypes:     Gg          gg
Gametes:      G  g        g  g

          G      g
   g     Gg     gg
   g     Gg     gg

Offspring: 2 Gg (green) : 2 gg (brown)  ->  1 green : 1 brown

(b) Two heterozygous green beetles are crossed and produce 160 offspring.

          G      g
   G     GG     Gg
   g     Gg     gg

Genotypes: 1 GG : 2 Gg : 1 gg
Phenotypes: 3 green : 1 brown

The Punnett square shows three different genotypes. Expected numbers: 3/4 × 160 = 120 green and 1/4 × 160 = 40 brown. Real results will be close to, not exactly, these numbers, because fertilisation is random.

Interpreting pedigree diagrams

In a pedigree, squares are usually males, circles are females, and shaded symbols show the characteristic. A key is always given, so read it first.

Worked example 3. Two parents without a condition have a daughter who has it. What does this tell you?

• The daughter must have received an allele for the condition from each parent, yet neither parent shows it. So the allele is recessive.
• Both parents must be heterozygous (Aa), where a is the recessive allele.
• For any further child, Aa × Aa gives a 1 in 4 chance of aa (affected).

The key test: if two unaffected parents have an affected child, the condition is recessive.

Extended only: test cross

An organism showing the dominant phenotype could be homozygous or heterozygous. To find out, cross it with a homozygous recessive individual.

Worked example 4. A green beetle of unknown genotype is crossed with a brown beetle (gg).

• If the green beetle is GG, all the gametes carry G, so all offspring are green (Gg).
• If it is Gg, half the gametes carry g, so you expect 1 green : 1 brown.
• One brown offspring is enough to show the unknown parent is Gg. All-green offspring point to GG, and more offspring make that conclusion more reliable.

Extended only: codominance and ABO blood groups

Codominance is when both alleles in a heterozygous organism contribute to the phenotype. Write codominant alleles as a capital letter with superscripts, because neither is recessive.

Worked example 5: roan cattle. In some cattle, Cᴿ gives red hairs and Cᵂ gives white hairs. CᴿCᵂ cattle have a mix of red and white hairs (roan).

Parents:   CᴿCᵂ (roan)  x  CᴿCᵂ (roan)
Gametes:   Cᴿ  Cᵂ           Cᴿ  Cᵂ
Offspring: CᴿCᴿ : CᴿCᵂ : CᵂCᵂ
           red  : roan : white  =  1 : 2 : 1

ABO blood groups. There are three alleles: Iᴬ, Iᴮ and Iᵒ. Iᴬ and Iᴮ are codominant, and both are dominant to Iᵒ.

Genotype	Phenotype
IᴬIᴬ or IᴬIᵒ	A
IᴮIᴮ or IᴮIᵒ	B
IᴬIᴮ	AB
IᵒIᵒ	O

Worked example 6. A man with group A (IᴬIᵒ) and a woman with group B (IᴮIᵒ) have children.

          Iᴬ       Iᵒ
   Iᴮ    IᴬIᴮ     IᴮIᵒ
   Iᵒ    IᴬIᵒ     IᵒIᵒ

Phenotypes: AB : B : A : O = 1 : 1 : 1 : 1

Each child has a 1 in 4 chance of each blood group, so two parents with groups A and B can have a child with group O.

Extended only: sex linkage

A sex-linked characteristic is one where the gene is located on a sex chromosome. This makes the characteristic more common in one sex than the other. Red-green colour blindness is the syllabus example. The gene is on the X chromosome. Xᴮ gives normal colour vision and Xᵇ gives colour blindness. The Y chromosome carries no allele of this gene.

A male has only one X, so a single Xᵇ makes him colour-blind. A female needs two copies (XᵇXᵇ). That is why more males than females are colour-blind.

Worked example 7. A woman with normal vision who carries the allele (XᴮXᵇ) has children with a colour-blind man (XᵇY).

           Xᴮ       Xᵇ
   Xᵇ    XᴮXᵇ     XᵇXᵇ
   Y     XᴮY      XᵇY

XᴮXᵇ  normal-vision female (carrier)
XᵇXᵇ  colour-blind female
XᴮY   normal-vision male
XᵇY   colour-blind male
Ratio 1 : 1 : 1 : 1; chance a child is colour-blind = 2/4 = 1/2

Always write sex-linked genotypes with the X and Y shown. Writing “Bb” loses the mark.

Common errors

• Writing “a gene is a characteristic”. A gene is a length of DNA that codes for a protein.
• Mixing up allele (a form of a gene) and genotype (the alleles an organism has).
• Showing gametes with two alleles, such as “Gg”. Each gamete carries one allele of each gene.
• Giving a ratio for genotypes when the question asks for phenotypes, or the reverse.
• Saying specialised cells have lost genes. They have the same genes; different genes are expressed.
• Saying mitosis halves the chromosome number. Only meiosis does.
• Writing the blood group alleles as A, B and O. Use Iᴬ, Iᴮ and Iᵒ.
• Leaving out the Y chromosome in a sex-linked cross, so the sons’ phenotypes cannot be worked out.

Next steps

• Condensed recall and a self-test: Inheritance revision notes
• Exam-style questions with answers: Inheritance practice questions
• The next topic: Variation and selection study guide
• Find your gaps across the whole course: Core diagnostic or Extended diagnostic
• Tick off every statement on the 0610 checklist and return to the course hub

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge University Press & Assessment. Topic 17 Inheritance, sections 17.1–17.4.