5.2 Genes and chromosomes

Genes are commonly referred to as the ‘blueprints’ for life – the basic units of heredity, which means the passing on of physical and behavioural traits from one generation to the next. Differences in, for example, our eye colour or hair structure are due to differences in genes we inherit from our parents. Genes are small sections of very long molecular structures called deoxyribonucleic acid (DNA). DNA has a precise sequence of units, with a section of these units together constituting a gene.

Each gene contains the instructions for making a specific protein which in turn instructs our cells and tissues how to interact, grow or respond to damage and diseases. For example, there is a gene containing instructions for making the hormone insulin, a substance with an important role in regulating our blood sugar level. While we each have a gene that codes for insulin, the precise sequence of units within that gene can vary between individuals. Such differences, known as DNA variants, may cause differences in the way a protein functions

Genes are organised into 23 distinctive pairs of structures called chromosomes, carried within the cells of our body, and visible down a microscope (Fig. 10). Of each pair of chromosomes, one is inherited from the mother and one is inherited from the father. The first 22 pairs of chromosomes look the same down the microscope for men and women. The last pair are the sex chromosomes. The body cells of males have one copy each of the X chromosome and the (much smaller) Y chromosome, while the cells of females typically have two X chromosomes.

In humans, the 23 chromosome pairs hold tens of thousands of genes that together are known as the human genome.

Each of us inherits one member of each chromosome pair from each of our parents – but before they are passed on during sexual reproduction, material within each of these chromosome pairs crosses over during the formation of egg or sperm, part of a process known as meiosis (Fig. 11). Natural breakages occur on each paired chromosome, shown here at the white line two thirds of the way down, and a section of genetic material is exchanged such that novel combinations are formed. Each egg or sperm inherits just one of these paired chromosomes which includes a novel combination of material from each parental chromosome.

The result is that offspring inherit combinations of each of their parents' chromosomes, and can therefore also exhibit characteristics of each parent, and of earlier generations (Fig. 12).

During egg and sperm formation, and during breakage and rejoining of the parental chromosomes, additional changes to the genetic sequence may occur. Such ‘mutations’ give rise to new DNA variants which may contain altered instructions for protein development. This may in turn trigger differences in the way the brain and nervous system develop, which are then passed on to further generations