The roles of genes in determining the phenotype

Genes and Phenotype

A gene is a unit of heredity transferred from parents to offspring, carrying information on how to build and maintain cells.
Phenotype represents the visible or detectable expression of the information contained in the genes. This expression can range from physical characteristics like height and eye colour to physiological and even behavioural traits.
While a gene carries the blueprint, the way it manifests as a characteristic — or phenotype — can be influenced by other factors.

An allele is a form of a gene, usually distinguished from other alleles by its different effects on the phenotype.
If two identical alleles are present (for a trait), the individual is homozygous and the resultant trait is straightforward. If two different alleles are present, the individual is heterozygous and the trait’s expression can be more complex.
Traits determined by a single gene with two alleles can be dominant (expressed when at least one allele is present) or recessive (expressed only when two copies of the allele are present).
Co-dominance exists when alleles have equally strong effects and both phenotypes are expressed (e.g., AB blood type in humans).

Multiple genes can interact to influence a single phenotype - this is called polygenic inheritance. This can result in a wide range of possible phenotypes (e.g., human skin colour or height).
A gene’s expression can be influenced by another gene’s presence or absence, a phenomenon known as epistasis.

Phenotype is not determined by genes alone, the environment can play a significant role, resulting in a range of phenotypes from the same genetic basis - this is known as phenotypic plasticity.
An individual’s unique combination of genes, the genotype, interacts with their environment to create the phenotype.
Genes can be switched on or off depending on environmental conditions, through a process called gene expression regulation.
This gene-environment interaction is also the basis for the study of epigenetics, which focuses on how environmental factors can change the way genes are expressed, and therefore influence the phenotype.