We are going to start with some concepts that you might already know, but are important to understand the following sections.

The units of heredity in living organisms are the genes. All genes are located on chromosomes in the nucleus of each cell, and each contains the information ("recipe") for a different protein. This information is written in the DNA as long sequences consisting of the four bases, A (Adenosine), C (Cytosine), G (Guanidine) and T (Thymine). With help of the "genetic code", the cell can translate this information and produce the encoded molecule.

Each individual of the same species has the same set of genes (genome). The human genome, for example, is thought to consist of around 25,000 genes. However, humans do not all look alike. This is because there are tiny differences in the sequences of the same genes between individuals. The alternative forms or "flavours" of the same gene are called alleles. They are the reason why two people have for example a different size or hair colour or tulips have a different coloured heads. This means that, while the common set of genes defines a species, allelic differences make every single human, plant or other organism an individual. Differences in alleles are also important in malaria research because they are the reason why some mosquitoes of the same Anopheles species survive insecticide treatment while others die. This resistance is an important factor in the fight against malaria and that is why we want to explain you the appearance of new alleles in this module.