7.1.5Genetics, Populations, Evolution and Ecosystems (A Level only)

Investigating Genetic Ratios — AQA A Level Biology 7402 Revision Notes & Topical Questions

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7.1.5 Investigating Genetic Ratios — AQA A Level Biology 7402 notes: Predict and test genetic ratios in breeding experiments, including autosomal, sex-linked, codominant and epistatic inheritance, and explain why observed ratios differ from expected ones.Genetic crosses give expe…

expected ratios
Drosophila melanogaster
Exam-style question 1
Exam-style question 2
Exam-style question 3
Monohybrid cross

Section 1 of 9Why study ratios?

Genetic crosses give expected ratios of phenotypes in the offspring, based on Mendel's laws: alleles segregate when gametes are formed, and, for genes on different chromosomes, alleles assort independently.

Breeders and geneticists compare the ratios they actually observe with the predicted values to find out how a characteristic is inherited.

3:1

Typical ratios are a 3:1 ratio of dominant to recessive phenotypes from a monohybrid cross between two heterozygotes, a 1:1 ratio from a test cross,

and a 9:3:3:1 ratio in a dihybrid cross between two double heterozygotes.

Deviations from these ratios provide clues about linkage, codominance, epistasis or lethal alleles.

Section 2 of 9Organisms used and method

Organisms used for genetic crosses have a short life cycle, produce many offspring and are easy to keep. Drosophila melanogaster (fruit fly) has a generation time of about 10 to 14 days, and its phenotypes, such as eye colour and wing form, are easy to see.

Organisms used and method

Other examples are the garden pea, maize (grain colour), and mice or Brassica (rapid-cycling brassica) seeds.

A typical investigation is:

Choose parents of known phenotype, usually pure-breeding (homozygous).

Cross them to produce the F₁ generation, then cross the F₁ with each other (or test cross) to produce the F₂.

Anaesthetise the flies (or collect seeds), count the offspring of each phenotype in the F₂, taking care to record all of them and avoiding bias.

Calculate the observed ratio and compare with the predicted ratio using a chi-squared test (next lesson).

Ethical issues include humane treatment and anaesthesia of animals.

Section 3 of 9Predicting ratios: a worked sequence

To predict the ratio:

Define symbols for alleles, for example R (round) and r (wrinkled).

Write the parental genotypes and the gametes each can produce.

Use a Punnett square to combine gametes.

Read off genotype and phenotype ratios, considering dominance.

For example, Rr × Rr gives 1 RR : 2 Rr : 1 rr, a phenotype ratio of 3 round : 1 wrinkled. Rr × rr gives 1 Rr : 1 rr, so 1:1. For a sex-linked gene on the X chromosome, such as white eyes in Drosophila, a heterozygous red-eyed female (X^RX^r) crossed with a red-eyed male (X^RY) produces all red-eyed females and half red- and half white-eyed males, a ratio different from that of an autosomal gene.

Section 4 of 9Modified ratios

Situation	Cross	Result
Codominance (e.g. human blood group alleles I^A and I^B)	Heterozygote × heterozygote	1 : 2 : 1 phenotype ratio
Sex linkage	X^RX^r × X^RY	Ratio differs between sexes
Autosomal linkage	Genes on the same chromosome	Parental types more frequent than expected 9:3:3:1
Epistasis	Dihybrid, one gene masks the other	Ratios such as 9:3:4 or 9:7
Lethal allele	Homozygous genotype dies	2:1 ratio

Epistasis occurs when one gene influences the expression of another, for instance when the product of one gene is needed to produce a pigment from the product of another. In a dihybrid cross a 9:3:4 ratio can show recessive epistasis.

Section 5 of 9Why observed results differ

Observed numbers rarely match the predicted ratio exactly.

Fertilisation is random,

so by chance a small sample will give a ratio different from the expected one;
- the larger the sample, the closer the results come to the predicted ratio.

Differences may also arise from linkage, selective survival of certain genotypes (some phenotypes may be less viable), incorrect assumptions about dominance, errors in classification of phenotypes,

or sex linkage.

A statistical test is used to decide if the difference is due to chance.

If it is, the results are consistent with the predicted ratio; if not, the hypothesis about the mode of inheritance should be rejected and another suggested.

Section 6 of 9Interpreting breeding data

Given numbers of offspring, simplify the ratio by dividing by the smallest number.

Interpreting breeding data

For example, 612 round : 188 wrinkled is close to 3.3 : 1, which is close to 3:1, consistent with a monohybrid cross between heterozygotes.

For a test cross, a 1:1 ratio shows that the unknown parent is heterozygous, whereas all dominant offspring suggest the parent is homozygous dominant.

Working out the genotype of the parents from the offspring ratios is a favourite exam question: first identify the dominant allele from a cross of two parents with the same phenotype producing offspring with a different phenotype,

then deduce genotypes.

Section 7 of 9Exam-style questions

In Drosophila, grey body (G) is dominant to black body (g). Two heterozygous flies are crossed and give 400 offspring. State the expected numbers of each phenotype and explain why the observed numbers may differ. [4]4 marks
A cross between a red-eyed female and a white-eyed male gives offspring that are all red-eyed. The F₁ are crossed. Explain the expected F₂ if the gene is X-linked, with red dominant. [4]4 marks
A cross between two heterozygous plants gives 9 : 3 : 4. Suggest an explanation. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Monohybrid cross	Cross involving one gene with two alleles
F1 / F2	First and second filial generations
Test cross	Cross of an unknown genotype with a homozygous recessive
Sex linkage	Inheritance of a gene located on a sex chromosome
Codominance	Both alleles are expressed in the heterozygote
Epistasis	Interaction in which one gene affects the expression of another
Autosomal linkage	Genes on the same chromosome inherited together

Section 9 of 9Exam technique

Expecting the exact predicted numbers in a small sample; chance variation is normal.

Forgetting that sex-linked genes give different ratios in males and females.

Writing that a test cross uses a heterozygous parent; the tester is homozygous recessive.

Thinking that a 9:3:3:1 ratio will result when genes are linked.

Confusing genotype ratio with phenotype ratio.

Last updated: 18 October 2026

Section 1 of 9Why study ratios?

Section 2 of 9Organisms used and method

Section 3 of 9Predicting ratios: a worked sequence

Section 4 of 9Modified ratios

Section 5 of 9Why observed results differ

Section 6 of 9Interpreting breeding data

Section 7 of 9Exam-style questions

In Drosophila, grey body (G) is dominant to black body (g). Two heterozygous flies are crossed and give 400 offspring. State the expected numbers of each phenotype and explain why the observed numbers may differ. [4]4 marks

A cross between a red-eyed female and a white-eyed male gives offspring that are all red-eyed. The F1 are crossed. Explain the expected F2 if the gene is X-linked, with red dominant. [4]4 marks

A cross between two heterozygous plants gives 9 : 3 : 4. Suggest an explanation. [2]2 marks

Section 8 of 9Key terms

Section 9 of 9Exam technique

Marked practiceQuizzes for this lesson

A cross between a red-eyed female and a white-eyed male gives offspring that are all red-eyed. The F₁ are crossed. Explain the expected F₂ if the gene is X-linked, with red dominant. [4]4 marks