Your physical characteristics are inherited from your parents and so are many genes that influence your health. Your genes partly determine how your body ages and whether you are predisposed to certain diseases. You cannot alter your genes, but medical intervention may help to prevent diseases to which you are prone. Genetic diagnosis and expert counselling may enable you to control your risk of disease by adapting your lifestyle.
The genes that you inherit from your parents form the basis of your physical and mental characteristics. Half of your genes come from your mother and the other half from your father, via the egg and sperm cells respectively. Each child receives a different mix of the genes provided by each parent. These different gene combinations account for the marked differences in appearance, health, and personality among most siblings. The genes you inherit determine many aspects of your appearance, such as the colour of your eyes and hair. They direct the striking bodily changes that are brought about by growth and aging, noticeably during infancy and puberty. Genes also affect your body chemistry, which may influence your risk of disease.
At conception, the fertilized egg, or zygote, is a single cell that contains all the information necessary to make a new human being. This information is unique to each individual and is carried in genes, which are sections of tightly coiled DNA strands. DNA is found in the nuclei of cells and contains the instructions that control the physical characteristics, growth, development, and functioning of an individual. As the fertilized egg divides, the information is duplicated so that a copy exists in every cell of the growing baby’s body.
Humans have about 20,000–25,000 pairs of genes but only certain ones are active in cells, depending on the cell’s specialized function; for example, different sets of genes are active in brain and liver cells. The genes direct the activity of cells, usually by controlling the production of specific proteins. Many of these proteins are involved in the formation of body structures or in regulating the body’s activities. For example, some genes are responsible for the production of proteins that cause cells to form tissues, such as skin, hair, and muscle; others give rise to proteins that control processes in the body, such as enzymes that regulate particular chemical reactions, or immunoglobulins, which fight infectious organisms.
Some genes, called control genes, produce proteins that influence certain other genes by switching those genes “on” or “off”. Control genes regulate the processes of growth and development, such as the limits of cell growth. They also determine the specialization of cells. However, if control genes malfunction, the cells multiply out of control. This is one of the mechanisms by which cancers develop. In cell division, during the copying of the genetic material, a fault occasionally occurs, leading to a mutation or change in the gene. This altered gene is then passed on to the new cells each time the cell divides. Disorders that result from such mutated genes are known as genetic disorders.
Altered genes may cause an organ to develop or function abnormally, and particular genes are known to cause rare genetic diseases. If a genetic disease runs in your family, you may decide to have tests to find out if you have inherited the altered gene. More common disorders, such as heart disease, have also been shown to have a genetic component. Screening tests to detect these diseases early on are particularly important if your family history suggests that you are at risk.
As you age and pass through distinct life stages, your body’s needs and your health concerns change accordingly. For example, infants need high-energy diets to promote rapid growth, whereas elderly people require less energy and eat proportionately less.
Young adults are most at risk from accidents, but later in life people are increasingly susceptible to degenerative disease as the functioning of their body organs and major body systems gradually declines. Natural healing processes also tend to become less efficient as the immune system’s resistance to disease is reduced with age. The body’s systems tend to age at different rates and are affected by genetic and lifestyle factors that vary from one person to the next.
Although life expectancy has risen dramatically in the UK over the past century, due to improved nutrition, health care, and sanitation, your personal good health throughout life and during old age is inevitably the result of the genes you have inherited and the way you live your life.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.