Polygenic features and phenotypic variation

A single-gene inheritance model follows the alleles of one gene. It is useful for explaining some inherited characteristics, but most phenotypic features are the result of multiple genes, rather than single-gene inheritance. This is called polygenic inheritance: ‘poly’ means many.

With several genes contributing to a characteristic, individuals can inherit many different combinations of alleles. These combinations help produce a wide range of phenotypes.

Eye colour is an example. Several genes influence it, so a simple model with one allele for brown eyes and one for blue eyes does not fully explain its inheritance. Green and hazel eyes are also possible. Height is another polygenic feature: there is not just one gene that determines whether a person is tall or short.

Genetic variation: mutation and sexual reproduction

Genetic variation means differences in genetic material between individuals. Two causes are mutation and sexual reproduction, but they contribute in different ways.

A mutation is a change in genetic material. A mutation in a gene can produce a new allele, adding to genetic variation. It may influence a characteristic, although a change in DNA does not necessarily produce an observable change in phenotype.

Sexual reproduction brings together genetic material from two parents. Meiosis, the cell division that produces gametes, creates gametes with different combinations of alleles. Gametes are sex cells, such as sperm and egg cells.

At fertilisation, a male gamete fuses with a female gamete to form a zygote, the first cell of the offspring. Which gametes fuse is random, so offspring receive different combinations of alleles. This explains why siblings can differ even though they have the same biological parents. Sexual reproduction reshuffles existing alleles; mutation can introduce new ones.

Environmental variation and acquired characteristics

An organism’s environment includes the conditions and experiences that affect it during its life. Diet, temperature, light, water availability and injuries can all influence phenotype.

A scar caused by an injury is an acquired characteristic. It developed during the person’s lifetime rather than being inherited through their alleles. Their children do not inherit that scar. Similarly, the accent someone develops depends on the speech they hear, not on an inherited allele for that accent.

Environmental variation also occurs in plants. Differences in water supply or light can affect growth and the condition of leaves. Plants with similar genetic makeup can therefore look different if they grow in different conditions.

Genes and environment can act together

Genetic and environmental influences are not always alternatives. Many characteristics are affected by both.

Height provides a useful example. A child inherits alleles from both parents that influence growth, but nutrition during childhood also affects their eventual height. Poor nutrition can limit growth even when the child has inherited alleles associated with being tall.

The important distinction is between what an organism inherits and the conditions it experiences. Several genes can influence a feature, while the environment affects how that feature develops. Knowing the genotype alone is therefore not always enough to predict the phenotype.

Keep the distinctions clear

• Genotype: combination of alleles.
• Phenotype: observable characteristics, influenced by genotype and environment.
• Acquired characteristics: develop through environmental influences; scars are not inherited.
• Both genes and environment: height depends on inherited alleles and factors such as childhood nutrition.