Monohybrid inheritance and genetic diagrams

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Monohybrid inheritance and genetic diagramsSpec 3.23

In short

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. It is shown with a genetic diagram such as a Punnett square, giving the parent genotypes, gametes, and offspring genotypes and phenotypes. Crossing two heterozygous tall pea plants, Tt × Tt, gives 1 TT : 2 Tt : 1 tt, so 3 tall : 1 short.

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. You show it using a genetic diagram, such as a Punnett square.

Choose letters for the alleles: a capital letter for dominant, lower case for recessive.
Write the genotypes of the parents.
Write the possible gametes from each parent. Each gamete has one allele.
Combine the gametes in a Punnett square to show the possible genotypes of the offspring.
Write the phenotype for each genotype.

Crossing two heterozygous pea plants

Tall (T) is dominant to short (t). Two heterozygous tall plants (Tt) are crossed. Show the possible offspring.

Parent genotypes: Tt and Tt.
Gametes: T or t from each parent.
Punnett square: the four boxes are TT, Tt, Tt and tt.
Phenotypes: TT, Tt and Tt are tall; tt is short.

Answer: Offspring genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short.

Punnett square for Tt × Tt: gametes T and t from each parent give offspring TT, Tt, Tt (tall) and tt (short). — Tt × Tt gives 3 tall : 1 short (1 TT : 2 Tt : 1 tt).

Exam tip:

Always write the gametes in circles or in a labelled row and column, and say which letter is dominant. Marks are given for gametes, offspring genotypes and phenotypes.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the alleles an organism has for a gene, written as letters such as TT, Tt or tt. A phenotype is the characteristic that is expressed and can be observed, such as tall or short. A heterozygous Tt plant has the dominant phenotype, because the dominant allele is expressed with only one copy.

What is a Punnett square used for in genetics?

A Punnett square is a genetic diagram used to show the possible offspring of a cross. The gametes from each parent are written along the top and side, then combined to give the offspring genotypes, phenotypes and ratios. For example, Tt × Tt gives 1 TT : 2 Tt : 1 tt, or 3 tall : 1 short.

How do you know if a characteristic is dominant or recessive from a family pedigree?

Find two parents with the same phenotype who have a child with the other phenotype. If two parents without the characteristic have an affected child, it is recessive and both parents are heterozygous carriers. If two parents with the characteristic have an unaffected child, it is dominant and both parents are heterozygous.

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