In the early 20th century scientists unlocked the secrets of the atom that changed the world forever, but life remained a great mystery. Among the life’s most profound secrets were inheritance, how the characteristics like hair colour, eye colour, height and other traits were passed on to the next generation.

From Mendel’s published work on inheritance, it took 135 years of research to the completion of the human genome.

We have all heard of the Roman Catholic monk, Gregor Mendel's extensive studies of thousands of pea plants (Pisum sativum). The work that he did close to over a nine-year period to study the inheritance of traits.
Gregor Mendel

The Natural Science Society in Brno published his results in 1866, with the title Experiments on Plant Hybrids. Today, he is considered as the “Father of Genetics” by explaining the principles of inheritance: (1) The law of segregation, (2) the law of independent assortment and (3) the law of dominance. He reasoned that genes came in pairs and inherited in separate parts, he called them dominant and recessive factors. Today, we call these factors genes. Mendel used the term factor instead. The name gene was coined much later, in 1903 by Wilhelm Johannsen Danish botanist, plant physiologist, and geneticist.

Mendel demonstrated that traits are inherited corresponding to specific numerical ratios. Today, we use terms, such as Mendelian traits, according to Mendelian laws or in short Mendelian, when we mean that the inheritance pattern follows the numerical ratios described by Mendel. That is when a single gene locus controls a trait. These inherited characteristics are for example blood type, face freckles, and diseases such as sickle-cell anemia, cystic fibrosis and multiple single-gene disorders associated with stroke. In cases of multiple gene loci controlling a trait, such as hair color or height, the pattern of inherited traits will not comply with the numerical ratios described by Mendel. In other words, the inheritance is not Mendelian.

Today we have incredible resources available, such as Online Mendelian Inheritance in Man (OMIM). This resource has all known Mendelian disorders, full-text descriptions over 15,000 genes. It is also updated daily.

Although most people ignored Mendel's work during his time, everyone knew that children inherit characteristics from parents, but what transferred these characteristics was mystery. Some of the children's traits could be similar to father and some similar to mother or a mix of both. In some cases, children seemingly didn't have any of the mother's or father's traits. Scientists would not solve this mystery until 87 years later.