3.2Reproduction and Inheritance

Inheritance — Edexcel Modular - OL Revision Notes & Topical Questions

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Lesson summary

Inheritance Edexcel O Level Modular Biology notes: understand that the genome is the entire DNA of an organism and that a gene is a section of a molecule of DNA that codes for a specific proteinGenes on the same chromosome tend to be inherited together.Mitochondria come from the …

Inheritance Edexcel O Level Modular Biology: what to do next
Incomplete dominance
Codominance
Multiple alleles
Sex-linked
Linked genes

Section 1 of 28Process & sequence

Count recombinant offspring.

Divide by total offspring.

× 100 = recombination frequency.

1% = 1 map unit (centimorgan).

Genes far apart

More crossing over

Higher recombination frequency

Section 2 of 28Keep going

Inheritance Edexcel O Level Modular Biology: what to do next

These Inheritance Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Inheritance Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Inheritance Edexcel O Level Modular Biology point you were unsure about before moving on.

Practice: Inheritance Edexcel O Level Modular Biology practice questions

Previous lesson: 3.1 Reproduction — Notes

Next lesson: 4.1 The organism in the environment — Notes

Course: Edexcel Modular - OL — all units

Official syllabus: Pearson Edexcel International GCSEs

Section 3 of 28Genetic terms

The observable characteristics of an organism are its phenotype.

Genetic terms

Genotype

The combination of alleles an organism has for a particular gene is its genotype (the term can also refer to the complete set of genetic material).

Allele

An allele is one version of a gene, represented by a single letter such as B or b.

You inherit two alleles of each gene, one from each parent; they can be the same (homozygous) or different (heterozygous).

Term	Definition	Example (PTC tasting)
Homozygous	Two identical alleles	TT or tt
Heterozygous	Two different alleles	Tt
Genotype	Combination of alleles	TT, Tt, tt
Phenotype	Observable characteristic	Taster or non-taster

PTC itself is not found in food but tastes like bitter compounds in vegetables such as cabbage, kale and Brussels sprouts; tasters often find these foods more bitter than non-tasters. The allele for tasting is dominant (T) and non-tasting is recessive (t): a heterozygous Tt person is a taster.

Section 4 of 28Codominance and blood groups

Usually one allele is dominant over another.

Codominance and blood groups

Codominance

On occasion, both alleles are expressed in the phenotype; this is codominance.

Inheritance of the ABO blood group is an example, controlled by a gene with three alleles (not the usual two): I^A, I^B and I^O.

I^A and I^B are codominant and I^O is recessive.

Blood group	Possible genotypes
A	I^AI^A or I^AI^O
B	I^BI^B or I^BI^O
AB	I^AI^B
O	I^OI^O

Worked examplea parent with group A has the genotype I^AI^O, and the other parent group B (I^BI^O).

Codominance and blood groups

We know these are not homozygous as they can produce a child with blood group O, which must have inherited an I^O allele from each parent.

The Punnett square shows the possible children: AB, A, B or O, each with 25% probability.

Section 5 of 28Polygenic inheritance

Most characteristics are the result of multiple genes interacting rather than a single gene.

Polygenic

Characteristics controlled by more than one gene are polygenic (poly = many), and their inheritance is polygenic inheritance.

Examples: height, skin colour and body mass, which show continuous variation and are also affected by the environment.

You are not expected to explain polygenic inheritance with a genetic diagram; you just need to know that many characteristics are controlled by groups of genes.

Section 6 of 28Practising genotype and phenotype questions

Questiona plant with red flowers (RR) is crossed with a plant with white flowers (rr), and the offspring all have pink flowers.

This is incomplete dominance,

where the heterozygote shows a blend; do not confuse with codominance,
where both alleles show clearly (for example roan cattle with red and white hairs,
or AB blood).

Always check the wording of the question to decide which applies.

For blood group questions, write the genotypes with the I^A, I^B and I^O notation, and use a Punnett square.

A person with group AB cannot have a child with group O, which is sometimes used to test whether a man could be the father.

Human characteristics

some, such as ability to roll the tongue or PTC tasting, are close to single-gene traits;
- others, such as height, depend on many genes and the environment, and show continuous variation with a normal distribution in the population.

Section 7 of 28Compare

Non-Mendelian inheritance
Pattern	Example	Heterozygote / cross result	Ratio	Deviation from Mendel	Note
Incomplete dominance	Snapdragon colour	Pink	1 : 2 : 1	Blended	—
Codominance	AB blood, roan cattle	Both traits	1 : 2 : 1	Both shown	—
Sex-linked	Hemophilia, colour blindness	Carrier females	Depends on sex	More males affected	X-linked
Linked genes	Genes on same chromosome	Parental types more common	Not 9:3:3:1	Recombination frequency	Map units
Polygenic	Skin colour, height	Continuous range	Bell curve	Many genes	—
Mitochondrial	Some muscle diseases	All children of affected mother	—	Maternal only	—

Section 8 of 28Worked examples

Define genotype and phenotype. [2]2 marks
A person is homozygous recessive for PTC tasting. What is their genotype and phenotype? [2]2 marks
Explain what is meant by codominance, using blood groups. [2]2 marks
Give an example of a polygenic characteristic. [1]1 mark

Section 9 of 28Monohybrid crosses

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. A Punnett square shows the possible combinations of alleles in the offspring. The height of pea plants is controlled by a single gene with two alleles: tall (T, dominant) and short (t, recessive).

Parents: pure-breeding tall (TT) x pure-breeding short (tt).

Gametes: all T from one, all t from the other.

F1 offspring: all Tt (heterozygous), all tall.

Cross two F1 plants (Tt x Tt): gametes T or t from each parent.

F2 offspring: TT, Tt, Tt, tt = 3 tall : 1 short.

The short plant shows the recessive phenotype, so it must be homozygous (tt). A cross between a heterozygous plant and a short plant (Tt x tt) gives 1 tall : 1 short. Probabilities are read from the square: 3 of 4 = 75% tall, 1 of 4 = 25% short.

Section 10 of 28Pedigree diagrams

Family pedigree diagrams trace the pattern of inheritance of a characteristic (usually a disease) through generations.

Males are squares and females are circles; affected individuals are shaded and unaffected not.

They can be used to work out the probability that someone will inherit the characteristic.

If two unaffected parents have an affected child, the allele is recessive and the parents are carriers.

Use your own letters in a Punnett square, choosing letters whose capital and lower case look different.

Section 11 of 28Sex determination

Sex is determined by an entire chromosome pair (unlike most characteristics, determined by one gene or a few).

Sex determination

Females have two X chromosomes (XX), males have X and Y (XY).

As only a father can pass on a Y chromosome, the sperm determines the sex of the offspring.

In a Punnett square: XX x XY gives XX, XX, XY, XY: 50% male and 50% female.

Section 12 of 28Mitosis and meiosis

	Mitosis	Meiosis
Definition	Nuclear division giving genetically identical cells	Nuclear division giving genetically different cells
Chromosomes	Doubled beforehand; each new cell has two copies of each (diploid)	Halved; gametes are haploid
Uses	Growth, replacement of skin and red blood cells, asexual reproduction	Making gametes

All cells of the body (except gametes) are produced by mitosis of the zygote. Without halving during meiosis, there would be double the number of chromosomes after fertilisation. Meiosis creates genetic variation between the gametes and random fertilisation adds more. Exam hint: learn two or three differences and be specific.

Section 13 of 28Probability and pedigree analysis step by step

Identify the characteristic and decide whether the allele is dominant or recessive from the pedigree (affected children from two unaffected parents means recessive).

Assign letters, with capital for dominant and lower case for recessive.

Work out the genotypes of the known individuals, including carriers.

Draw a Punnett square for the cross in question.

Read the probability, writing it as a fraction, ratio or percentage.

Remember that probabilities apply to each pregnancy independently: if two carrier parents have an affected child, the chance of the next child being affected is still 1 in 4, not 'less'. In sex determination the 50:50 ratio is a probability, and a family may have several children of the same sex.

Section 14 of 28Worked examples

Two heterozygous tall pea plants are crossed. Draw the Punnett square and give the ratio. [4]4 marks
What is the chance of a boy? [1]1 mark
Why is a short pea plant always tt? [1]1 mark
Compare mitosis and meiosis (two points). [2]2 marks

Section 15 of 28Diploid and haploid cells

A diploid cell contains two complete sets of chromosomes (2n). The chromosomes contain the DNA needed for protein synthesis and cell function. Nearly all cells in the human body are diploid, with 23 pairs (46) of chromosomes. A haploid cell contains one complete set (n): human gametes have 23 chromosomes. Fertilisation joins two haploid gametes to make a diploid zygote.

Section 16 of 28Variation

Variation is defined as differences between individuals of the same species.

Genetic variation

There are two causes. Genetic variation arises from differences in genes: mutations, the shuffling of alleles in meiosis, and random fertilisation of gametes. Environmental variation arises from factors such as climate, diet, culture, lifestyle and accidents during lifetime; here 'environmental' simply means 'outside of the organism'.

Many characteristics, such as height and body mass, are affected by both.

Section 17 of 28Mutations

Mutations are rare, random changes in the sequence of DNA bases in a gene.

As the DNA base sequence determines the sequence of amino acids in a protein, a mutation can sometimes lead to a change in the protein that the gene codes for.

Most mutations do not alter the protein, or alter it so slightly that its function is unchanged.

A change in DNA can affect the phenotype by altering the sequence of amino acids.

There are three main ways DNA can be changed.

Type	What happens	Effect
Insertion	An extra base is added	Changes the amino acid for that group of three bases and shifts all later groups (knock-on effect)
Deletion	A base is lost	Changes that amino acid and, like an insertion, shifts the groups of three bases further on
Substitution	One base is randomly swapped for another	Changes only the one amino acid for that codon; no knock-on effect

Mutations can be neutral, harmful or beneficial.

Mutations

Example of a beneficial one: a mutation that changes a bird's feather colour,

and if the new colour makes it more attractive to mates, the mutation is more likely to be passed on.

A harmful example is sickle cell anaemia, caused by a mutation in the gene coding for haemoglobin.

Mutations happen spontaneously and continuously, but their frequency can be increased by ionising radiation and some chemicals.

Section 18 of 28Darwin's theory of evolution by natural selection

Charles Darwin proposed that evolutionary change has occurred and that natural selection is the process that has driven it. You may be given an unfamiliar example, so learn the stages:

All populations contain genetic variation, which may be caused by mutations.

Some variants result in characteristics that are advantageous: individuals are better adapted to their environment.

Those individuals are more likely to survive, compete successfully and reproduce.

Surviving individuals pass on their advantageous alleles to their offspring.

Over many generations the advantageous alleles become more common in the population.

Section 19 of 28Antibiotic resistance

Natural selection can give rise to bacterial populations that are resistant to antibiotics:

Antibiotic resistance

A random mutation produces a new allele coding for antibiotic resistance.

When the population is exposed to the antibiotic, bacteria without the resistance allele die, while those with it survive.

The survivors reproduce, passing on the resistance allele, and the resistant population grows.

This is why antibiotics must be used carefully; the more often an antibiotic is used, the stronger the selection pressure for resistance.

Section 20 of 28Worked examples

Explain the difference between diploid and haploid. [2]2 marks
A deletion mutation in a gene changes the protein more than a substitution. Explain why. [3]3 marks
Describe how a population of bacteria becomes resistant to an antibiotic. [4]4 marks
State two environmental factors that cause variation in humans. [2]2 marks

Section 21 of 28Where resistance begins

Random mutation

Bacteria in a population are not always genetically identical.

A random mutation can produce a variant that allows some bacteria to survive exposure to a particular antibiotic.
The mutation appears before or independently of the need for resistance;
- the antibiotic does not deliberately create it.

Resistance is a heritable characteristic. When a resistant bacterium reproduces, its offspring can inherit the genetic variant. This variation provides the starting point for natural selection.

Section 22 of 28Selection by the antibiotic

Selection pressure

When the population is exposed to an antibiotic, susceptible bacteria are killed or prevented from multiplying. Resistant bacteria are more likely to survive. The antibiotic is a selection pressure because it changes which variants survive and reproduce.

The survivors reproduce and pass resistance to new bacteria. Across several generations, the frequency of the resistance variant rises, so the population can become dominated by bacteria that the original antibiotic no longer controls effectively.

Section 23 of 28Why it matters

Why it matters

An infection caused by a resistant strain is harder to treat with the antibiotic to which it is resistant. A different effective antibiotic may be needed. This example shows evolution by natural selection occurring in a population over generations.

Distinguish the individual from the population: a bacterium does not choose to adapt during treatment. Resistant individuals already present have a survival advantage, and their descendants make resistance more common in the population.

Section 24 of 28Exam answer sequence

For a full explanation, write the four linked stages: variation from mutation → greater survival under antibiotic exposure → reproduction and inheritance → increased frequency of resistance over generations. Name the selection pressure and explain how it changes the population.

Check:

Check Why can a small number of resistant bacteria become a large resistant population? Answer: The susceptible bacteria are removed by the antibiotic, while resistant survivors reproduce and pass on resistance.

Section 25 of 28Key terms

Term	Meaning
Allele	A form of a gene.
Homozygous	Two identical alleles.
Heterozygous	Two different alleles.
Genotype	The combination of alleles.
Phenotype	The visible characteristic.
Codominance	Both alleles are expressed.
Polygenic	Controlled by many genes.

Term	Meaning
Punnett square	A diagram showing possible allele combinations in offspring.
Monohybrid cross	A cross involving one gene.
Pedigree	A family tree showing a trait.
F1 generation	First generation offspring.
Carrier	Heterozygous person with a recessive allele.

Term	Meaning
Diploid	Having two complete sets of chromosomes (2n).
Haploid	Having one set of chromosomes (n).
Variation	Differences between individuals of a species.
Mutation	A random change in DNA base sequence.
Natural selection	Survival and reproduction of the best adapted individuals.
Selection pressure	An environmental factor that affects survival.

Section 26 of 28Syllabus checklist

understand that the genome is the entire DNA of an organism and that a gene is a section of a molecule of DNA that codes for a specific protein

understand that the nucleus of a cell contains chromosomes on which genes are located describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases

adenine
with thymine
and cytosine
with guanine

understand how genes exist in alternative forms called alleles which give rise to differences in inherited characteristics

understand the meaning of the terms

dominant
recessive
homozygous
heterozygous
phenotype
and genotype

understand that most phenotypic features are the result of polygenic inheritance rather than single genes

describe patterns of monohybrid inheritance using a genetic diagram

understand how to interpret family pedigrees

predict probabilities of outcomes from monohybrid crosses

understand how the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male

describe the determination of the sex of offspring at fertilisation, using a genetic diagram

understand how division of a diploid cell by mitosis produces two cells that contain identical sets of chromosomes

understand that mitosis occurs during growth, repair, cloning and asexual reproduction

understand how division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes

understand how random fertilisation produces genetic variation of offspring

know that in human cells the diploid number of chromosomes is 46 and the haploid number is 23

understand that variation within a species can be genetic, environmental, or a combination of both

understand that mutation is a rare, random change in genetic material that can be inherited understand that the incidence of mutations can be increased by exposure to ionising radiation (for example, gamma rays, x-rays and ultraviolet rays) and some chemical mutagens (for example, chemicals in tobacco)

explain Darwin's theory of evolution by natural selection

understand how resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lead to infections being difficult to control

Section 27 of 28Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 28 of 28Exam technique

Thinking heterozygous means two dominant alleles.

Saying codominance is blending.

Thinking blood groups have only two alleles.

Mixing genotype and phenotype.

Writing Tt x Tt as giving 1:1.

Mixing up squares and circles in pedigrees.

Saying the mother's egg determines sex.

Saying meiosis is used for growth.

Saying mutations are caused by the need to adapt.

Saying acquired characteristics are inherited.

Saying all mutations are harmful.

Saying 'the bacteria become resistant' as individuals.

Last updated: 11 October 2026

Section 1 of 28Process & sequence

Section 2 of 28Keep going

Inheritance Edexcel O Level Modular Biology: what to do next

Section 3 of 28Genetic terms

Section 4 of 28Codominance and blood groups

Section 5 of 28Polygenic inheritance

Section 6 of 28Practising genotype and phenotype questions

Section 7 of 28Compare

Section 8 of 28Worked examples

Define genotype and phenotype. [2]2 marks

A person is homozygous recessive for PTC tasting. What is their genotype and phenotype? [2]2 marks

Explain what is meant by codominance, using blood groups. [2]2 marks

Give an example of a polygenic characteristic. [1]1 mark

Section 9 of 28Monohybrid crosses

Section 10 of 28Pedigree diagrams

Section 11 of 28Sex determination

Section 12 of 28Mitosis and meiosis

Section 13 of 28Probability and pedigree analysis step by step

Section 14 of 28Worked examples

Two heterozygous tall pea plants are crossed. Draw the Punnett square and give the ratio. [4]4 marks

What is the chance of a boy? [1]1 mark

Why is a short pea plant always tt? [1]1 mark

Compare mitosis and meiosis (two points). [2]2 marks