A genetic disease is a disorder caused by a variation in a person’s DNA — a mutation in a single gene, a chromosomal abnormality, or a complex interplay of many genes with the environment. In nutrition and metabolism, the classic examples are inborn errors such as phenylketonuria, where the body cannot break down phenylalanine, and galactosaemia, where galactose cannot be metabolised — conditions managed principally through diet. Others with dietary relevance include coeliac disease, where genetic susceptibility plus gluten triggers intestinal damage; lactose intolerance from lactase gene variants; and haemochromatosis, causing iron overload. Genetic testing increasingly identifies such susceptibilities, moving nutrition toward personalised dietary advice based on genotype.