Gregor Mendel's Laws of Inheritance Changed our Understanding of Genetics

Gregor Johann Mendel (1822-1884) was a scientist who died having no idea of the importance of his discoveries. He spent his life running experiments on pea plants, analyzing their genes and traits. Through his work he was able to better understand the role of dominant and recessive traits. It was this work which formed the basis for modern genetics.

The Laws of Heredity

Mendel's work is summarized by his Laws of Heredity, which are as follows:

1. The Law of Segregation: Each trait that is inherited is defined by a gene pair. Parental genes are randomly separated to germ cells, which in turn only have one gene of the pair. Offspring of given parents inherit one allele from each parent during fertilization

2. The Law of Independent Assortment: Genes for different traits are sorted separately so that individual traits are transmitted independently of each other. 

3. The Law of Dominance: Organisms that have alternate forms of the same gene will express the form that is dominant.

What Brought Mendel to Study Genetics

Mendel grew up with an interest in science and mathematics, but no financial means with which to pursue it. By age 21, he had run out of money and one of his teachers suggested that he become a monk so that he could continue his studies of science without starving. 

Mendel joined the Abbey of St. Thomas which was a diverse and intellectual community. The director was interested in studying the science of heredity.

In 1846, when Mendel was 24, he started taking fruit-growing classes at the local Brünn Philosophical Institute, where his professor was a known authority on plant breeding.

As a monk, Mendel also trained as a teacher. However, he initially struggled to pass his teaching exams and was sent to the University of Vienna, where he studied physics and math, as well as botany, before returning to teaching.

In 1854, he began to plan a major experimental program in plant hybridization at the monastery. Although previous work demonstrated that the offspring of hybrids tended to revert to the originating species, both plant and animal breeders had long shown that crossbreeding could produce new forms.

This knowledge was of particular interest to the monastery, which bred Merino sheep and was concerned about competing with wool being imported from Australia. The aim of Mendel's program was to trace the transmission of hereditary characteristics in successive generations of hybrid progeny with the aim of eventually breeding better sheep.

The genetic experiments that Mendel performed involved the growing of 10,000-30,000 individual pea plants, tracing the transmission of seven different traits and crossing varieties that differed in one trait (eg short vs tall), through several different generations of hybrids.

In 1865, Mendel presented his results in two separate lectures to the Natural Science Society in Brunn, and his work was published in the society's journal the following year. At the time, most people who read it didn't recognize that it was new work and felt it merely demonstrated what was already widely assumed—that hybrid offspring revert to their originating forms.

Most scientists overlooked the implications of his work for the role of variability in evolution. This research would sit for nearly 50 years before it's importance would be recognized.

After this, Mendel devoted himself to running a monastery, as abbot.

The Discovery of Modern Genetics

In his work, Mendel was initially encouraged by two professors he met while studying at university, Friedrich Franz and Johann Karl Nestler. People at the time had long known about selective breeding of animals and plants, but they had little idea of what caused it or influenced it.

The main theories at the time were that every offspring from parents was a blend of each of the parent's traits. This would have made sense at the time, but we now know this isn't the case.

Gregor Mendel's study of pea plants was highly systematic. He bred specific types of peas that were closely related so that only a few traits were different between each.

Mendel chose to study 7 specific traits of pea plants:

The results got more interesting from here though. As he took these hybrid offspring and bred them with one another, some plants grew with white flowers. 

These discoveries made Mendel realize that the first generation of all purple flowers retained some kind of instructions that could produce white flowers in the next generation.

He was also able to statistically predict the ration of purple to white flowers in the 3rd generation: 75% purple to 25% white. This result was repeatable and consistent, meaning that the purple-colored flower trait must somehow be dominant over the white trait.

All of Mendel's research, more than just the one example we provided above, can be boiled down into his key conclusions.

• That the inheritance of each trait is determined by something passed from the parent to the child. This 'thing', now understood as genes, doesn't mix upon fertilization, but rather remains unchanged. 
• An organism inherits one gene from each parent for each individual trait.
• While some individuals may not exhibit certain traits, the trait can still be there and be present in future generations.

Today, these common patterns of inheritance are called Mendelian Inheritance.

Genetics Today

Mendel's work wasn't recognized as the groundbreaking study that it was until the year 1900.

That year, three scientists who were independently carrying out heredity research found what they thought were new results in the field. However, after concluding their findings, they realized their results actually provided confirmation of Mendel's work of 35 years earlier.

The scientists, Carl Correns, Hugo de Vries, and Erich von Tschermak, brought Mendel's original work to light and helped to found our modern understanding of genetics.