2.2.5Reproduction and Inheritance

Monohybrid Inheritance: Genetic Diagrams — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions

5 min read 6 parts Lesson 18 of 28

0%read

Part 1 of 5What monohybrid inheritance means

One gene

One gene Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene with two alleles.
Many real characteristics, such as height, are controlled by several genes together (polygenic inheritance).
- You only need to know that this happens; you will not be asked to draw genetic diagrams for it.

Gametes

Gametes Each parent has two alleles but passes on only one to each gamete.
- The gametes of a heterozygous parent Tt are therefore T or t, in equal numbers.

Punnett square

Punnett square A Punnett square is a grid showing every possible combination of male and female gametes.
- It lets you work out offspring genotypes, phenotypes and ratios.

Write down the genotypes of both parents, choosing a letter whose capital and lower-case forms look clearly different.

Split each parent's alleles into gametes and write the father's gametes across the top and the mother's down the side.

Fill each box by combining the allele from its column with the allele from its row.

Read off the offspring genotypes, then use the dominance information to state the phenotypes.

Count the boxes to give a ratio or a probability, such as 'a 1 in 4 chance' or '25%'.

1Parents (P)

Parents (P) Tall (T) is dominant to short (t).
- A pure-breeding tall plant TT is crossed with a short plant tt.

2F1 generation

F1 generation Every gamete from TT carries T and every gamete from tt carries t, so all offspring are Tt.
- All of them are tall.

3Cross two F1 plants

Cross two F1 plants Tt × Tt gives gametes T and t from each parent, producing TT, Tt, Tt and tt.

4F2 generation

F2 generation Genotype ratio 1 TT : 2 Tt : 1 tt.
- Phenotype ratio 3 tall : 1 short, so there is a 75% chance of a tall plant and a 25% chance of a short one.

Set-up

Set-up A short plant shows the recessive phenotype, so it must be tt. A tall plant could be TT or Tt.

Cross Tt × tt

Cross Tt × tt The gametes are T or t from the first parent and only t from the second.
- The offspring are Tt, Tt, tt and tt.

Result

Result The phenotype ratio is 1 tall : 1 short, a 50% chance of each.
- If a tall plant crossed with a short one ever produces a short offspring, the tall parent must have been heterozygous.

Term	Meaning
Monohybrid cross	A breeding cross that follows one characteristic controlled by one gene.
Punnett square	A grid used to show all possible combinations of gametes and the resulting offspring.
F1 generation	The first generation of offspring produced from a cross between the parents.
F2 generation	The offspring produced when F1 individuals breed with each other.
Pure-breeding	Homozygous for the gene, so the offspring always show the same phenotype as the parents.
Polygenic inheritance	Inheritance of a characteristic controlled by several genes.

Putting both alleles of one parent on the same side of the square; each parent's two alleles must be split into separate gametes.

Saying a 3:1 ratio will appear exactly in a small family. Punnett squares give probabilities; actual offspring numbers can differ by chance.

Giving a phenotype ratio when asked for a genotype ratio, such as writing 3:1 instead of 1:2:1.

Treating a 25% chance as 'one in every four children will' show the trait; each child has an independent 25% chance.

Forgetting that a recessive phenotype always means a homozygous recessive genotype.

Last updated: 20 October 2026