AQA A Level Biology
Topic 7
Genetics, populations,
evolution and ecosystems
Model answer notes by @biologywitholivia (updated for 2025)
Topic Understand Memorise Practise
7.1 Inheritance
7.2 Populations
7.3 Evolution may lead to speciation
7.4 Populations in ecosystems
Required practical 12
These notes are for PERSONAL USE ONLY. Redistribution,
reproduction, or sale of any portion of this material is prohibited.
For enquiries, please email
,More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
7.1 Inheritance
What is meant by the term genotype?
Genetic constitution of an organism
What is meant by the term phenotype?
The expression of this genetic constitution (genotype)
and its interaction with the environment
What are alleles and how do they arise?
Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)
How many alleles of a gene can be found in diploid organisms?
● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population
Describe the different types of alleles
Dominant allele Always expressed (shown in the phenotype)
Recessive allele Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)
Codominant alleles Both alleles expressed / contribute to phenotype (if inherited together)
What is meant by the terms homozygous and heterozygous?
Homozygous Alleles at a specific locus (on each homologous chromosome) are the same
Heterozygous Alleles at a specific locus (on each homologous chromosome) are different
What do monohybrid and dihybrid crosses show?
● Monohybrid cross - inheritance of one phenotypic characteristic coded for by a single gene
● Dihybrid cross - inheritance of two phenotypic characteristics coded for by two different genes
2
, More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
Worked example: monohybrid cross (basic)
The dominant allele for the grey gene Parental phenotypes Non-grey Grey
(G) in horses results in a coat colour
Parental genotypes gg Gg
turning progressively grey.
Gamete genotypes g (and g) G and g
The recessive allele (g) results in the
normal coat colour being maintained Genetic diagram
(non-grey phenotype).
A non-grey female is crossed with a
heterozygous male.
Offspring genotypes Gg, gg
Draw a genetic diagram to show the
expected ratio of phenotypes in the Offspring phenotypes Grey, non-grey
offspring.
Ratio 1:1
Worked example: monohybrid cross with multiple alleles
In one snail species, shell colour is Parental phenotypes Pink-shelled Pink-shelled
controlled by a gene with 3 alleles. The
Parental genotypes CPCY CPCY
shell may be brown, pink or yellow. The
allele for brown (CB) is dominant to the Gamete genotypes CP and CY CP and CY
other two alleles. The allele for pink (CP)
Genetic diagram
is dominant to the allele for yellow (CY).
A cross between two pink-shelled
snails produced only pink-shelled and
yellow-shelled snails.
Offspring genotypes CPCP, CPCY - pink-shelled
Use a genetic diagram to explain why. and phenotypes CYCY - yellow-shelled
Worked example: monohybrid cross with codominance and multiple alleles
The inheritance of the ABO blood Parental phenotypes Blood group A Blood group B
groups in humans is controlled by
Parental genotypes IAIO IBIO
three alleles of a single gene, IA, IB and
IO. The alleles IA and IB are codominant, Gamete genotypes IA and IO IB and IO
and the allele IO is recessive to IA and
Genetic diagram
recessive to IB.
Two heterozygous parents plan to
have a child. One has blood group A
and the other has blood group B.
Offspring genotypes IAIB, IAIO, IBIO, IOIO
Use a genetic diagram to show all the
possible genotypes and the ratio of Offspring phenotypes AB, A, B, O
phenotypes expected in their offspring.
Ratio 1:1:1:1
3
Topic 7
Genetics, populations,
evolution and ecosystems
Model answer notes by @biologywitholivia (updated for 2025)
Topic Understand Memorise Practise
7.1 Inheritance
7.2 Populations
7.3 Evolution may lead to speciation
7.4 Populations in ecosystems
Required practical 12
These notes are for PERSONAL USE ONLY. Redistribution,
reproduction, or sale of any portion of this material is prohibited.
For enquiries, please email
,More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
7.1 Inheritance
What is meant by the term genotype?
Genetic constitution of an organism
What is meant by the term phenotype?
The expression of this genetic constitution (genotype)
and its interaction with the environment
What are alleles and how do they arise?
Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)
How many alleles of a gene can be found in diploid organisms?
● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population
Describe the different types of alleles
Dominant allele Always expressed (shown in the phenotype)
Recessive allele Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)
Codominant alleles Both alleles expressed / contribute to phenotype (if inherited together)
What is meant by the terms homozygous and heterozygous?
Homozygous Alleles at a specific locus (on each homologous chromosome) are the same
Heterozygous Alleles at a specific locus (on each homologous chromosome) are different
What do monohybrid and dihybrid crosses show?
● Monohybrid cross - inheritance of one phenotypic characteristic coded for by a single gene
● Dihybrid cross - inheritance of two phenotypic characteristics coded for by two different genes
2
, More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
Worked example: monohybrid cross (basic)
The dominant allele for the grey gene Parental phenotypes Non-grey Grey
(G) in horses results in a coat colour
Parental genotypes gg Gg
turning progressively grey.
Gamete genotypes g (and g) G and g
The recessive allele (g) results in the
normal coat colour being maintained Genetic diagram
(non-grey phenotype).
A non-grey female is crossed with a
heterozygous male.
Offspring genotypes Gg, gg
Draw a genetic diagram to show the
expected ratio of phenotypes in the Offspring phenotypes Grey, non-grey
offspring.
Ratio 1:1
Worked example: monohybrid cross with multiple alleles
In one snail species, shell colour is Parental phenotypes Pink-shelled Pink-shelled
controlled by a gene with 3 alleles. The
Parental genotypes CPCY CPCY
shell may be brown, pink or yellow. The
allele for brown (CB) is dominant to the Gamete genotypes CP and CY CP and CY
other two alleles. The allele for pink (CP)
Genetic diagram
is dominant to the allele for yellow (CY).
A cross between two pink-shelled
snails produced only pink-shelled and
yellow-shelled snails.
Offspring genotypes CPCP, CPCY - pink-shelled
Use a genetic diagram to explain why. and phenotypes CYCY - yellow-shelled
Worked example: monohybrid cross with codominance and multiple alleles
The inheritance of the ABO blood Parental phenotypes Blood group A Blood group B
groups in humans is controlled by
Parental genotypes IAIO IBIO
three alleles of a single gene, IA, IB and
IO. The alleles IA and IB are codominant, Gamete genotypes IA and IO IB and IO
and the allele IO is recessive to IA and
Genetic diagram
recessive to IB.
Two heterozygous parents plan to
have a child. One has blood group A
and the other has blood group B.
Offspring genotypes IAIB, IAIO, IBIO, IOIO
Use a genetic diagram to show all the
possible genotypes and the ratio of Offspring phenotypes AB, A, B, O
phenotypes expected in their offspring.
Ratio 1:1:1:1
3
