Genes and Inheritance

Genes control the growth, repair, and functions of cells. Genes are made of DNA (deoxyribonucleic acid), which is found in the nucleus of cells as structures called chromosomes. DNA provides instructions for development and growth, through building proteins and making molecules that control cellular processes. Genes are the means by which physical and some mental characteristics are passed on to children.

There are about five billion cells in the adult human body, and all of them, except red blood cells, have a set of genes made of DNA. This chemical has a double helix shape and is made of two strands of molecules joined together in the centre by a series of nucleotide bases. The DNA is coiled into structures called chromosomes that are stored in the nucleus of cells.

Gene organization

Human genes are arranged on 22 pairs of matching chromosomes, plus two sex chromosomes. One chromosome in each pair is inherited from each parent. Therefore, body cells contain two copies of genes, with the genes for the same characteristic carried on the matching chromosomes in a pair. Egg and sperm cells, called sex cells, have 23 single chromosomes so that a paired set of genes is created when a sperm cell fertilizes an egg.

There are about 20,000–25,000 pairs of genes in a human body cell. These genes provide the cell with the information that enables it to make proteins. The order of nucleotide bases along the DNA provides this information. Each body cell contains the same genes, but each tissue or organ needs to make different proteins. For this reason, a system exists to turn on genes only when they are required.

Both of the genes in a matching pair can be identical. However, some matching genes occur in slightly different forms called alleles. Some genes may have two to several hundred different alleles. These different forms account for differences between individuals.

Most of the differences that occur between genes do not affect function. For example, blue eyes work as well as brown ones. However, some genetic differences have important effects and can result in inherited disorders, such as sickle-cell disease and cystic fibrosis. Genes also play a part in disorders such as coronary artery disease and some common cancers, such as colorectal and breast cancer.

Human chromosomes

Body cells (except red blood cells) contain 46 chromosomes, organized in 23 pairs.

Copying genes

Before a cell divides into two during growth or repair, its genes are duplicated so that each new cell has a full set. When sperm and egg cells are made, the two chromosomes of a particular pair align and exchange genetic material between each other before the cell divides. This ensures that when an egg and a sperm fuse, the new child is different both from its parents and from its siblings.

Inheritance of single gene disorders

This section lists the major single gene disorders that are covered in this guide. Some disorders exist in more than one form, with different inheritance patterns. The most common pattern is indicated.

Single Gene disorders

Autosomal dominant disorders


A disorder of bone growth, resulting in short stature and abnormal proportions

Huntington’s disease

A brain disorder that causes abnormal movements and dementia in adulthood

Inherited hyperlipidaemias

Excessive levels of lipids in the blood, of which the most common form is familial hypercholesterolaemia

Marfan’s syndrome

A rare disorder that mainly affects the skeleton, heart, and eyes


A disorder in which numerous swellings develop on the covering of nerves

Polycystic kidney disease (in adults)

A disorder in which fluid-filled cysts replace normal kidney tissue


A disorder in which chemicals called porphyrins build up in the body, causing psychological and physical symptoms

Von Willebrand disease

A bleeding disorder due to a deficiency of a substance needed for blood clotting

Autosomal recessive disorders


A lack of the pigment melanin, which gives colour to the skin, hair, and eyes

Cystic fibrosis

Abnormally thick secretions, leading to digestive and respiratory problems


Inability to break down a form of sugar, leading to its accumulation in the blood


A condition in which too much iron is deposited in various organs


Deficiency of an enzyme needed to digest a component of protein-containing foods

Polycystic kidneys (in children)

A disorder in which fluid-filled cysts replace normal kidney tissue, usually apparent at birth

Retinitis pigmentosa

Progressive degeneration in the retina. It can also be inherited in an autosomal dominant and X-linked recessive manner

Sickle-cell disease

A blood disorder in which red blood cells become an abnormal shape and blood flow through vessels is impeded

Tay–Sachs disease

A severe disorder in which harmful substances build up in the brain


A blood disorder in which production of haemoglobin, the oxygen-carrying component of red blood cells, is abnormal

X-linked disorders

Colour blindness

Impaired ability to distinguish between certain colours

Fragile X syndrome

A disorder that produces severe learning disabilities and a characteristic appearance

Haemophilia and Christmas disease

Disorders in which the blood does not clot normally due to a clotting factor deficiency

Duchenne muscular dystrophy

A disorder causing progressive weakness and wasting of muscles

Structure: The Human Genome

Structure: Structure of Genetic Material

Process: Protein Synthesis

Process: Cell Division

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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