Cambridge IGCSE Biology 0610: Inheritance – Revision Notes

For full explanations and worked genetic diagrams, use the Inheritance study guide. These notes are for the final weeks.

They cover topic 17 of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028: 17.1 Chromosomes, genes and proteins, 17.2 Mitosis, 17.3 Meiosis and 17.4 Monohybrid inheritance. Core candidates need 17.1 statements 1–4 and 17.4 statements 1–12. Everything labelled Extended only is Supplement content, which includes all of 17.2 and 17.3.

Links: course hub · checklist · practice questions · Core diagnostic · Extended diagnostic

Definitions to learn word for word

Term	Definition	Tier
Gene	A length of DNA that codes for a protein	Core
Allele	An alternative form of a gene	Core
Inheritance	The transmission of genetic information from generation to generation	Core
Genotype	The genetic make-up of an organism in terms of the alleles present	Core
Phenotype	The observable features of an organism	Core
Homozygous	Having two identical alleles of a particular gene	Core
Heterozygous	Having two different alleles of a particular gene	Core
Dominant allele	An allele that is expressed if it is present in the genotype	Core
Recessive allele	An allele that is only expressed when there is no dominant allele of the gene present in the genotype	Core
Haploid nucleus	A nucleus containing a single set of chromosomes	Extended only
Diploid nucleus	A nucleus containing two sets of chromosomes	Extended only
Mitosis	Nuclear division giving rise to genetically identical cells	Extended only
Meiosis	A reduction division in which the chromosome number is halved from diploid to haploid, giving genetically different cells	Extended only
Stem cell	An unspecialised cell that divides by mitosis to produce daughter cells that can become specialised	Extended only
Codominance	Both alleles in a heterozygous organism contribute to the phenotype	Extended only
Sex-linked characteristic	A feature whose gene is on a sex chromosome, making it more common in one sex	Extended only

Also learn (Core): two identical homozygous individuals that breed together are pure-breeding; a heterozygous individual is not pure-breeding.

17.1 Chromosomes, genes and proteins

Core

• Chromosomes are made of DNA; DNA carries genetic information as genes.
• Sex: female XX, male XY. Eggs all carry X; sperm are half X, half Y. Chance of a boy = 1/2 at every birth.

Extended only

• Base sequence in the gene → amino acid sequence → protein shape → protein function.
• DNA controls cell function by controlling protein production: enzymes, membrane carriers, receptors for neurotransmitters.
• Protein synthesis chain: gene stays in nucleus → mRNA copy made in nucleus → mRNA moves to cytoplasm → passes through ribosome → ribosome joins amino acids → order set by mRNA base sequence.
• Most body cells have the same genes; many genes are not expressed because each cell makes only the proteins it needs.
• Human diploid cell: 23 pairs (46). Human gamete: 23.

17.2 Mitosis and 17.3 Meiosis (Extended only)

	Mitosis	Meiosis
Result	Genetically identical cells	Genetically different cells
Chromosome number	Maintained	Halved: diploid → haploid
Before division	Exact replication of chromosomes	(not required)
During division	Copies of chromosomes separate	(stages not required)
Roles	Growth, repair of damaged tissues, replacement of cells, asexual reproduction	Production of gametes

Stem cells divide by mitosis; their daughter cells can then become specialised.

17.4 Genetic diagrams: method in steps

Method: any monohybrid cross

1. Choose a letter; capital = dominant, lower case = recessive. Give a key.
2. Write both parents’ phenotypes and genotypes.
3. Write the gametes, one allele each, circled.
4. Draw a Punnett square.
5. List offspring genotypes, then phenotypes.
6. Give the ratio or probability asked for, in the form asked for.

Cross	Genotypes	Phenotype ratio	Tier
Aa × aa	1 Aa : 1 aa	1 : 1	Core
Aa × Aa	1 AA : 2 Aa : 1 aa	3 : 1	Core
AA × any	all show dominant	all dominant	Core
Codominant C¹C² × C¹C²	1 : 2 : 1	1 : 2 : 1	Extended only
IᴬIᵒ × IᴮIᵒ	IᴬIᴮ, IᴬIᵒ, IᴮIᵒ, IᵒIᵒ	AB : A : B : O = 1 : 1 : 1 : 1	Extended only

Expected numbers: multiply the fraction by the total. A 3 : 1 cross with 300 offspring gives 225 : 75.

Pedigree diagrams (Core)

Method

1. Read the key (usually squares male, circles female, shaded = has the feature).
2. Look for two unaffected parents with an affected child. If you find one, the feature is recessive and both parents are heterozygous.
3. An affected person with a recessive feature is always homozygous recessive.
4. Work out other genotypes from parents and children.

Test cross (Extended only)

Cross the dominant-phenotype organism with a homozygous recessive. All offspring dominant → unknown was probably homozygous dominant. Any recessive offspring → unknown was heterozygous (expect 1 : 1).

ABO blood groups (Extended only)

• Alleles Iᴬ, Iᴮ, Iᵒ. Iᴬ and Iᴮ are codominant; both are dominant to Iᵒ.
• Group A: IᴬIᴬ or IᴬIᵒ. Group B: IᴮIᴮ or IᴮIᵒ. Group AB: IᴬIᴮ only. Group O: IᵒIᵒ only.

Sex linkage (Extended only)

• Red-green colour blindness: gene on the X chromosome. Xᴮ normal, Xᵇ colour-blind. No allele on Y.
• Males: XᴮY normal, XᵇY colour-blind. Females: XᴮXᴮ normal, XᴮXᵇ normal carrier, XᵇXᵇ colour-blind.
• More common in males because one recessive allele on their single X is always expressed.
• A son always gets his X from his mother. A father passes his X to every daughter.

Must-know distinctions

• Gene vs allele: the gene is the length of DNA for a feature; alleles are its alternative forms.
• Genotype vs phenotype: letters vs what you can observe.
• Homozygous vs heterozygous: identical vs different alleles of one gene.
• Dominant vs codominant: a dominant allele masks a recessive one; codominant alleles both show.
• Mitosis vs meiosis: identical and same number vs different and halved.
• Haploid vs diploid: one set vs two sets.
• Ratio vs probability: 3 : 1 means a 1/4 (25%) chance of the recessive phenotype, not 1/3.

Quick self-test

1. State what chromosomes are made of.
2. A heterozygous tall plant (Tt) is crossed with a dwarf plant (tt). There are 84 offspring. How many are expected to be dwarf?
3. Two plants heterozygous for flower colour (Rr) are crossed and produce 200 seeds. How many are expected to show the recessive phenotype?
4. State the number of chromosomes in a human body cell and in a human sperm cell. (Extended)
5. Where in the cell is mRNA made, and where does it go? (Extended)
6. State four roles of mitosis. (Extended)
7. A person with blood group A (IᴬIᴬ) has children with a person with blood group O. State the genotype and phenotype of all their children. (Extended)
8. A person with blood group AB has children with a person with blood group O. State the possible blood groups of the children and the ratio. (Extended)
9. A colour-blind woman has children with a man with normal colour vision. State the phenotypes of their sons and of their daughters. (Extended)
10. Two pure-breeding pea plants, one with purple flowers and one with white flowers, are crossed. All the offspring have purple flowers. State which allele is dominant and the genotype of the offspring.
11. Explain why a muscle cell does not make insulin even though it contains the insulin gene. (Extended)

Answers

1. DNA.
2. Tt × tt gives 1 Tt : 1 tt, so half are dwarf: 42.
3. Rr × Rr gives 1 RR : 2 Rr : 1 rr, so one quarter: 50.
4. Body cell 46 (23 pairs); sperm 23.
5. Made in the nucleus; moves to the cytoplasm, where it passes through ribosomes.
6. Growth, repair of damaged tissues, replacement of cells, asexual reproduction.
7. All IᴬIᵒ, all group A.
8. IᴬIᴮ × IᵒIᵒ gives IᴬIᵒ and IᴮIᵒ: group A and group B, 1 : 1. No child has AB or O.
9. XᵇXᵇ × XᴮY: all sons XᵇY, colour-blind; all daughters XᴮXᵇ, normal vision (carriers).
10. Purple is dominant. Offspring are all heterozygous (for example Pp).
11. The gene is not expressed in a muscle cell; each cell makes only the proteins it needs.

Where marks are usually lost

• Defining a gene as “a characteristic” or “a section of a chromosome” without saying it codes for a protein.
• Writing gametes as pairs of alleles, or forgetting to show the gametes line at all.
• Giving “3 : 1” when the question asks for the probability or percentage of one phenotype.
• Stating that a heterozygous organism is pure-breeding.
• In a pedigree, calling a feature dominant because many people have it. Look for unaffected parents with an affected child.
• Using A, B and O as allele symbols instead of Iᴬ, Iᴮ and Iᵒ, or saying Iᴬ is dominant to Iᴮ.
• Writing sex-linked genotypes without X and Y, or putting an allele on the Y chromosome.
• Saying mitosis produces gametes in humans, or that meiosis gives identical cells.
• Describing mRNA as “the gene leaving the nucleus”. The gene stays in the nucleus; mRNA is a copy.

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.