* GRAPHIC: Home DNA testing takes off https://tmsnrt.rs/2M6KGyl. : LONDON (Reuters) - How much is your DNA worth? As millions of people pay for home tests to check on ancestry or health risks, genetic data is becoming an increasingly valuable resource for drugmakers, triggering a race to create a DNA marketplace. GlaxoSmithKline’s (GSK.L) decision to invest $300 million in 23andMe and forge an exclusive drug development deal with the Silicon Valley consumer genetics company crystallizes the value locked up in genetic code.