What is a genetic disorder?

• A genetic disorder is an illness caused by changes in a person’s DNA.
• These mutations can be due to an error in DNA replication or due to environmental factors, such as cigarette smoke and exposure to radiation, which cause changes in the DNA sequence.
• The human genome is a complex set of instructions, like a recipe book, directing our growth and development.
• However, unlike a printed book, the human genome can change.
• These changes can affect the individual bases (A, C, G or T) or much larger chunks of DNA or even chromosomes.
• Our DNA provides the code for making proteins, the molecules that perform most of the functions in our body.
• However, when a section of our DNA is changed in some way, the protein it codes for is also affected and may no longer be able to carry out its normal function.
• Depending on where these mutations occur, they can have little or no effect, or may profoundly alter the biology of cells in our body, resulting in a genetic disorder.

Genetic disorders can be grouped into three main categories:

1. Single gene disorders: disorders caused by defects in one particular gene, often with simple and predictable inheritance patterns.

• Dominant diseases: single gene disorders that occur when an individual has one altered copy of the relevant gene and one healthy copy. For example, Huntington’s disease.
• Recessive diseases: single gene disorders that only occur when an individual has two altered versions of the relevant gene. For example, cystic fibrosis.
• X-linked disorders: single gene disorders that reflect the presence of an altered gene on the X chromosome. X-linked disorders are more common in males because they only have one X chromosome. As a consequence males only need one copy of the altered gene for symptoms to occur. For example, muscular dystrophy.

2. Chromosome disorders: disorders resulting from changes in the number or structure of the chromosomes.

• For example, Down’s syndrome, which results from an extra chromosome 21 (trisomy 21: three copies of chromosome 21).

Down’s syndrome is a chromosome disorder resulting from an extra chromosome 21.
Image credit: Shutterstock

3. Multifactorial disorders (complex diseases): disorders caused by changes in multiple genes, often in a complex interaction with environmental and lifestyle factors such as diet or cigarette smoke.

• For example, cancer.