The basics
The human genome comprises the approximately 3.2 billion base pairs in 23 chromosome pairs that make up the DNA within the cell nuclei, as well as some DNA contained in the cell mitochondria. Between 1-2 per cent of DNA encodes for 20,000 to 30,000 genes, with each gene making an average of three proteins, including enzymes. The remaining DNA has a role in regulating the genes and genome, such as turning a gene on or off at a particular time.9,10
About 99.9 per cent of DNA is shared by all humans: it is the 0.1 per cent that differs which is of particular relevance for genomic medicine. A large proportion of the differences do not appear to cause harm, but people with the same genetic changes linked to a condition do not necessarily have similar symptoms or may even not have any symptoms.7
An allele is one of two or more variant forms of a gene. Alleles can be dominant or recessive and contribute to the phenotype, the observable traits of the individual. Some traits are mostly defined by the collective alleles or genotype, but other traits may have a largely environmental influence.