Medicine as a whole is undergoing a major transformation and with it, so is ophthalmology. Many diseases are now being tackled from a new angle – genetics. In ophthalmology, the role of heredity has been studied for around 100 years, looking mainly at disorders inherited in a straightforward mendelian fashion. The basic mendelian inheritance, named after Gregor Mendel, the monk who first described the process in 1866, has two basic principles: the principle of segregation and the principle of random assortment. These state that the alleles segregate at the time of formation of the gametes (sperms and ova) and then reunite randomly at fertilisation. An allele is any of a number of alternative forms of a gene. Mendelian or monogenically inherited disorders are easier to study and to predict because, as the name implies, their inheritance is determined by one gene. Examples of mendelian diseases include retinitis pigmentosa (RP) and familial congenital cataract syndromes. However, many disorders are inherited as complex traits, which means that there is a likely combination of multiple genes and environmental factors building up to the clinical picture (the phenotype) and so they do not run directly in the family. Examples include myopia and age–related macular degeneration (ARMD).