Basic Concepts in Genetics

Living organisms are made up of cells. Within each cell there are genes, which hold the biological information that is essential for the growth of new cells. Genes are like instruction manuals and contain codes that determine biological characteristics, such as the size, shape or colour of particular features (e.g. skin, hair, eyes, and limbs).

Stores of biological information

Genes are made from molecules of deoxyribonucleic acid (DNA), which join together to form a double helix shape. Genes string together inside cells to form chromosomes, which are huge stores of biological information.

An organism’s genes are passed down from their parents through inheritance. An organism does not inherit all of their parents’ genes. Instead, they inherit some from their mother and some from their father. Because the process of inheritance is random, the appearance of offspring cannot be predicted.

DNA is a molecule in the form of a double helix © GCT

Variation among individuals

The instructions within a chromosome vary between individual organisms of the same species. For example, all Galapagos giant tortoises will have slightly different genes (unless they are identical twins!) and will therefore look slightly different. This is known as genetic variation.

Variation can be either continuous or discontinuous. Height is an example of continuous variation, as an individual can be any height between the minimum and maximum height for that species. Gender is an example of discontinuous variation, as an individual can either be one of two categories (male or female). Continuous variation can be measured on a scale, whereas discontinuous variation can be grouped into clearly-defined categories.

As well as being inherited, variation can also be due to environmental factors. Environmental variation occurs when a habitat affects the physical characteristics of an individual. For example, the weight of Galapagos giant tortoises varies according to the availability and accessibility of food. They can get very large due to the lack of predators on the Islands.

© Caroline Pannell