Source: The History of Biology de Erik Nordenskiöld, Ed. Knopf, 1928 (domaine public), Wikimedia Commons

Gregor Mendel’s work provided the foundation for the study of genetics.

Mendel made the following three important conclusions, which still hold true to date:

• “Units” or “factors” that are passed unchanged on to offspring, determine the inheritance of each trait. (These traits are now called genes.)
• An individual inherits one “unit” (gene) from each parent for each trait.
• A trait may not show up in an individual but can still be passed on to the next generation.

There are exceptions to the principles that Mendel proposed. Patterns of inheritance that do not follow Mendel’s principles are called non-Mendelian inheritance. Examples of non-Mendelian inheritance include incomplete dominance, codominance, polygenic traits, multiple alleles, and sex-linked traits.