 Discussions of heredity invite confusions between mechanisms responsible for individual development and mechanisms responsible for the transmission of traits from one generation to the next. Genes are the standard units of inheritance discussed in biology. Genes are also taken to be the most important causal component in the development of an organism’s traits. Methods derived from population genetics to assess heritability provide no information about the causal mechanisms contributing to the development of an individual’s traits. Population geneticists study the patterns of transmission of traits in populations from one generation to the next. Molecular biologists identify coding sequences of DNA and hence the proteins that these sequences produce in the developing organism. Working together, molecular biologists and population geneticists can produce a convergent account of a particular gene, providing both its pattern of transmission and an account of its role in development. For example, medical geneticists may discover a pattern of inheritance for a disease in a family that leads them to hypothesize that there is a gene (or a number of genes) responsible for the development of the trait in individual humans. Molecular analysis may then lead to the discovery of a sequence of DNA that codes for an unusual protein that is in part responsible for the development of the symptoms of the disease. Finally, population genetics techniques, such as heritability analysis, may then be applied to mechanisms discovered by molecular biologists.