The technology to rapidly scan genetic variation in whole human genomes—and even to sequence genomes completely—has revolutionized geneticists’ ability to study the inheritance of complex traits. The description of these traits as ‘complex’ is actually something of a misnomer as so-called simple inheritance, involving traits such as cystic fibrosis, phenylketonuria,
or Huntington disease, has complexities of its own. The distinction really lies in the nature and number of factors influencing the traits. Complex traits, which are often quite common, tend to be quantitative in nature, and like height and blood glucose show a continuous range of values (e.g., low to high) when examined in populations. Such traits are generally influenced by
multiple genes, and the environment may play a significant role in the expression of the trait. Nature and nurture work together. Precise predictions about complex phenotypes such as height are not feasible because of the myriad possible interactions that can occur. For example, consider that many genes may contribute directly to the phenotype, other genes may contribute
indirectly by the activation and inactivation of other genes, plus the environment can affect these interactions. By contrast, traits that are largely influenced by the actions of a single gene (and are more resistant to environmental variation) lend themselves to more precise predictions because the trait segregates with the causative allele(s). However, even these Mendelian
traits may be physiologically and medically complex; age of onset and severity of symptoms for Mendelian diseases can vary greatly. A sound understanding of genetics requires knowledge of both Mendel’s laws (and how they are explained by meiosis) and an understanding of principles arising from modern genomics.