A genetic disorder is a disease caused by changes, or mutations, in an individual’s DNA sequence. Genetic disorders can be divided into three different categories: single gene, chromosomal or complex disorders.
Single gene disorders are caused by defects in one particular gene. There are over 10,000 human disorders caused by a change, known as a mutation, in a single gene.
Individually, single gene disorders are each very rare, but as a whole, they affect about one per cent of the population.
Since only a single gene is involved, these disorders often can be easily tracked through families and geneticists can predict the risk of them occurring in later generations.
Chromosomal disorders result from changes in the number or structure of the chromosomes. Changes in the number of chromosomes happen when there are more or fewer copies of a particular chromosome than usual. Changes in chromosome structure happen when the material in an individual chromosome is disrupted or rearranged in some way. This may involve the addition or loss of parts of a chromosome.
Complex disorders (also known as multifactorial or polygenic) are those that are caused by the simultaneous effect of many different genes, often in a complex interaction with environmental and lifestyle factors such as diet.
Many of the common diseases of adult life, such as diabetes mellitus, hypertension, schizophrenia, and most common developmental abnormalities, such as cleft lip and congenital heart defects, have a strong genetic component and are caused by more than one genetic change.
Because polygenic diseases involve more than one gene, inheritance patterns are diverse and complex. If a parent has a disease, it does not necessarily mean a child will develop the same disease. On the other hand, an individual may not be born with a disease but may be at a higher risk of developing it. This is known as genetic predisposition or susceptibility.
Although a person’s genetic makeup cannot be altered, some lifestyle and environmental changes (such as having more frequent disease screenings and maintaining a healthy weight) may be able to reduce disease risk in people with a genetic predisposition.