Principles of genetics

Genetics is the study of genes, hereditary and genetic variation that exists amongst living organisms. This section aims to introduce you to the basic terminology that describes the primary principles of genetics.

Inheritance terminology


Genetic makeup of a cell or an organism.


Interaction of an organism’s genotype with the environment. It is an expression of an organism’s genotype in its structural, biochemical, physiological and behavioural characteristics.

Homozyous genotype (pure breeding)

Identical alleles of a gene are present on both homologous chromosomes.

Heterozygous genotype

Cell contains two different alleles of a gene on a homologous chromosomes.

Hemizygous genotype

Only once copy of a chromosome is present Eg- X linked gene found on a X chromosome in males.


A genetically determined characteristic.

Dominant phenotype

A phenotype that is fully expressed in the heterozygote phenotype (in complete dominance).

The dominant trait is given a capital letter, it is not necessarily the most common trait in the population.

Recessive phenotype

A phenotype corresponding to the expression of two recessive alleles in the homozygote or is not expressed in the heterozygote (in complete dominance).

The recessive trait is given a lower case letter.


A heterozygous organism which carries a an allele that is not expressed in the individual’s phenotype (in complete dominance) and is not expressed in the phenotype but can pass it on to the offspring.

Note that traits are dominant or recessive. Genes or alleles are never dominant or recessive.

See also