 At its most fundamental level , inheritance in organisms occurs by passing discrete heritable units , called genes , from parents to progeny . This property was first observed by Gregor Mendel , who studied the segregation of heritable traits in pea plants . In his experiments studying the trait for flower color , Mendel observed that the flowers of each pea plant were either purple or white — but never an intermediate between the two colors . These different , discrete versions of the same gene are called alleles . 
