If you go a hundred or two years back then there were basically two 'genres' of music: classical and folk. Classical music was learned through formal apprenticeship or academic training from 'established' theories of melody, harmony and composition. Most often played from a written score with little opportunity for improvisation. Conversely, folk music was typically based around rythmic and melodic patterns learned informally 'by ear' from family members or local community. Songs and tunes, even if they did have a single 'author' quickly morphed and changed through improvisation and the imperfect nature of human memory.
If you accept this skeletal taxonomy of 'cultural transmission' then most popular genres of music throughout the 20th century could be seen as subgenres of 'Folk Music', including Blues, Jazz, Country, Rock, Soul, Metal etc. Once recorded music was a thing commonly available to everyone (say post WWII) then formal 'academic/classical' music began to seriously inform 'popular' genres and vice versa.
Nowadays, a 'metal' or 'jazz' musician is as likely to have as much formal training, sight reading ability and thorough grasp of 'theory' as any 'classical' musician.