During the 1860’s, Austrian monk Gregor Mendel had introduced a new theory of inheritance based off of his experimental work with pea plants. At the time most of society believed that inheritance was due to a blending of parental “essences” such as mixing blue and yellow paint to create green paint. Mendel instead believed that heredity is the result of discrete units of inheritance, and every single unit (or gene) was independent in its actions in an individual’s genome. According to this Mendelian concept, inheritance of a trait depends on the passing-on of these units. For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes. We now understand the alternate forms of these units as ‘alleles’. If the two alleles that form the pair for a trait are identical, then the individual is said to be homozygous and if the two genes are different, then the individual is heterozygous for the trait.
Reference:
Genetics Generation, 2015. Mendelian Genetics.