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Traits in living organisms are divided into quantitative and qualitative. Quantitative traits can be measured—that is, they are the kind of trait where how much you have can vary: body weight, height, and the thickness of bones, for example. Quantitative traits are usually not the product of one gene, but instead are coded by several pairs of genes and have a so-called polygenetic inheritance.

Qualitative traits, on the other hand, are usually monogenetic or influenced by a single gene. The phenotype is either/or—that is, you have one variant of the gene or another that dictates how it manifests. Pigments and blood type are examples.

When it comes to genes working nicely together (or not), there are other terms to become familiar with and to aim to understand:

 Penetrance is the ability of a gene to show itself phenotypically. It can be either complete (manifested in each individual that carries the gene) or incomplete (not phenotypically expressed by all carriers).

 Pleiotropic action is when one gene is responsible for two or more phenotypic traits.