Monohybrid inheritance

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Monohybrid inheritanceSpec 3.14

In short

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. It can be shown using a genetic diagram, a Punnett square or a family pedigree. Crossing two heterozygous parents, such as Tt × Tt, gives the genotypes 1 TT : 2 Tt : 1 tt, so the phenotype ratio is 3 dominant : 1 recessive.

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. You can show it using a genetic diagram, a Punnett square or a family pedigree.

Choose letters for the alleles: a capital 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: top row (gametes from parent 1) T and t; side column (gametes from parent 2) T and t. 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).

Family pedigrees

A family pedigree (family tree) shows who in a family has a characteristic. Squares are usually males and circles are females. Shaded shapes show people with the characteristic. A horizontal line joins the parents and a vertical line leads down to their children.

If two parents who do not have a characteristic have a child who does, the characteristic is recessive. Both parents must be carriers, heterozygous with one copy of the recessive allele.
If two parents who both have a characteristic have a child who does not, the characteristic is dominant. Both parents must be heterozygous.

Three-generation family pedigree with squares for males and circles for females: an affected grandmother (rr) and her carrier son (Rr) and his unaffected carrier partner (Rr) have an affected daughter (rr) and two unaffected children (RR or Rr). — Two unaffected parents with an affected daughter: the characteristic is recessive and both parents are carriers (Rr).

Exam tip:

In a pedigree question, start by finding two parents with the same phenotype and a child with the other phenotype. That tells you which allele is dominant.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the alleles an organism has for a gene, such as Tt. A phenotype is the characteristics that are expressed and can be observed, such as being tall. A heterozygous plant, Tt, has the phenotype of the dominant allele, so it is tall even though it also carries the allele for short.

What is a Punnett square used for in genetics?

A Punnett square is used to show the possible offspring of a genetic cross. The gametes from each parent are written along two sides, then combined in the boxes to give the possible genotypes. Counting the boxes gives the ratio, probability or percentage of each genotype and phenotype in the offspring.

What is the difference between homozygous and heterozygous?

Homozygous means having two identical alleles of a gene, such as TT or tt. Heterozygous means having two different alleles of a gene, such as Tt. A heterozygous organism shows the dominant characteristic, because a dominant allele is expressed in the phenotype even when only one copy is present.

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