Some diseases are written in the genome. Pathological genetics is the study of how changes in DNA — inherited or acquired — cause disease, and it has moved from a niche specialty to something central to modern medicine as genetic testing and targeted therapies have matured.
Types of genetic change
Disease-causing changes range from the tiny to the wholesale: point mutations (a single DNA letter altered), insertions and deletions, and large-scale chromosomal abnormalities (extra, missing, or rearranged chromosomes). Whether a change causes disease depends on what it does to the protein the gene encodes — or to gene regulation.
Patterns of inheritance
- Single-gene (Mendelian) disorders — caused by one faulty gene, inherited in recognisable patterns (dominant, recessive, X-linked). Examples include cystic fibrosis and sickle-cell disease.
- Chromosomal disorders — from whole-chromosome changes, such as the extra chromosome 21 in Down syndrome.
- Multifactorial / polygenic — many genes plus environment, behind common conditions like heart disease and diabetes. Most disease risk is of this kind, not single-gene.
Most disease isn't 'a gene for X' — it's many genetic influences interacting with environment. The single-gene cases are the exception that taught us the rules.
Cancer is a genetic disease
Cancer is fundamentally a disease of the genome — an accumulation of mutations that unleash cell growth. Two gene classes are central: oncogenes (accelerators stuck on) and tumour-suppressor genes (brakes that have failed). Understanding which are altered in a given cancer increasingly guides targeted treatment.
Why it matters clinically
Genetic understanding now drives diagnosis (genetic testing), risk prediction (knowing a patient's inherited risk), reproductive counselling, and a growing wave of targeted and personalised therapies aimed at the specific molecular fault. Pathological genetics is where the future of precision medicine is being built.
