 A heritable trait is most simply an offspring’s trait that resembles the parents’ corresponding trait more than it resembles the same trait in a random individual in the population. Inheritance or heredity was a focus of systematic research before its inclusion as a key concept within evolutionary theory. An influential 18th and early 19th century theory of heredity was preformationism. This view took several forms, each maintaining that organisms were passed on from one generation to the next, miniature and yet fully formed, and development was simply the growth of the miniature organism. Subsequent accounts of heredity included the theory that organisms inherited traits that their parents had developed through response to various environmental pressures. This view was widely held during the 19th century and usually attributed to Lamarck. Although Darwin also at times defended aspects of Lamarck’s view, he also clearly articulates and defends the view that evolutionary change results from natural selection acting upon inherited traits under variation. Weismann’s experimental refutation of the inheritance of acquired traits paved the way for the combination of Darwin and Mendel’s views of the nature of heredity. The systematic study of heredity in the 20th century focused on the gene as the unit of heredity. (There is a vast amount of helpful work on the history of heredity including Keller (2002), Griesemer (1994), Morange (1998), Moss (2003), Sapp (2003), Sarkar (1998), Wade (1992), Winther (2000; 2001) and contributors to Buerton et al. (eds.) (2000).) Two traditions now dominate the study of heredity: population genetics and molecular biology. The notion of a quantitative measure of the heritability of any given trait comes from population genetics and heritability measures are commonly used in behavioral genetics (Plomin et al. 1990 and Plomin et al. 1997 and see Panofsky 2014, Tabery 2014 and Taylor 2014 for critical reflection on behavioral genetics and heritability analysis). The idea that what is inherited is a stock of DNA, or the information contained in the DNA sequence, comes from molecular biology.