Key genetic terms

Inheritance · Monohybrid inheritance · note 1 of 5

Spec 17.4.1, 17.4.2, 17.4.3, 17.4.4, 17.4.5, 17.4.6, 17.4.7, 17.4.8, 17.4.9Download all Monohybrid inheritance notes (PDF)Download all notes (PDF)8 pages

Key genetic termsSpec 17.4.1, 17.4.2, 17.4.3, 17.4.4, 17.4.5, 17.4.6, 17.4.7, 17.4.8, 17.4.9

In short

Genotype is the genetic make-up of an organism in terms of the alleles present, while phenotype is its observable features. Homozygous means two identical alleles, such as TT or tt, and heterozygous means two different alleles, Tt. A dominant allele is expressed if present; a recessive allele is only expressed when no dominant allele is present.

Inheritance is the transmission of genetic information from generation to generation. Parents pass genes to their offspring in their gametes, so offspring have features like their parents.

Inheritance: The transmission of genetic information from generation to generation.
Genotype: The genetic make-up of an organism and in terms of the alleles present.
Phenotype: The observable features of an organism.
Homozygous: Having two identical alleles of a particular gene. Example: TT or tt.
Heterozygous: Having two different alleles of a particular gene. Example: Tt.
Dominant allele: An allele that is expressed if it is present in the genotype.
Recessive allele: An allele that is only expressed when there is no dominant allele of the gene present in the genotype.

Dominant alleles are written as capital letters (T) and recessive alleles as lower case letters (t). A heterozygous individual (Tt) shows the dominant feature, because the dominant allele is expressed.

Pure-breeding

Two identical homozygous individuals that breed together will be pure-breeding. For example, TT × TT can only give TT offspring, and tt × tt can only give tt offspring, so every generation looks the same.

A heterozygous individual will not be pure-breeding. A Tt parent makes two types of gamete (T and t), so its offspring can have different genotypes and different phenotypes.

Exam tip:

Genotype is the alleles, such as Tt. Phenotype is what you can see, such as tall. Do not swap them, and do not say a recessive allele is 'weaker'.

Common mistake:

Heterozygous and homozygous describe genotypes only. Do not use them for phenotypes.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the genetic make-up of an organism in terms of the alleles present, such as Tt. A phenotype is the observable features of an organism, such as being tall. A heterozygous Tt pea plant has the tall phenotype, because the dominant allele T is expressed even though a recessive allele is present.

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 of one parent go along the top and the other along the side, and the boxes show the possible offspring genotypes. From these you work out the phenotypes and the ratio, such as 3 : 1 or 1 : 1.

How do you find out if an organism is homozygous or heterozygous?

Supplement You use a test cross: cross the organism showing the dominant phenotype with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous. If all the offspring show the dominant phenotype, it is probably homozygous dominant. A large number of offspring gives a more reliable result.

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