Mendel’s experiments

Gregor Johann Mendel was a monk and teacher with interests in astronomy and plant breeding. He was born in 1822, and at 21, he joined a monastery in Brünn (now in the Czech Republic). The monastery had a botanical garden and library and was a centre for science, religion and culture. In 1856, Mendel began a series of experiments at the monastery to find out how traits are passed from generation to generation. At the time, it was thought that parents’ traits were blended together in their progeny.

Studying traits in peas

Mendel studied inheritance in peas (Pisum sativum). He chose peas because they had been used for similar studies, are easy to grow and can be sown each year. Pea flowers contain both male and female parts, called stamen and stigma, and usually self-pollinate. Self-pollination happens before the flowers open, so progeny are produced from a single plant.

Peas can also be cross-pollinated by hand, simply by opening the flower buds to remove their pollen-producing stamen (and prevent self-pollination) and dusting pollen from one plant onto the stigma of another.

Traits in pea plants

Mendel followed the inheritance of 7 traits in pea plants, and each trait had 2 forms. He identified pure-breeding pea plants that consistently showed 1 form of a trait after generations of self-pollination.

Mendel then crossed these pure-breeding lines of plants and recorded the traits of the hybrid progeny. He found that all of the first-generation (F1) hybrids looked like 1 of the parent plants. For example, all the progeny of a purple and white flower cross were purple (not pink, as blending would have predicted). However, when he allowed the hybrid plants to self-pollinate, the hidden traits would reappear in the second-generation (F2) hybrid plants.

Dominant and recessive traits

Mendel described each of the trait variants as dominant or recessiveDominant traits, like purple flower colour, appeared in the F1 hybrids, whereas recessive traits, like white flower colour, did not.

Mendel did thousands of cross-breeding experiments. His key finding was that there were 3 times as many dominant as recessive traits in F2 pea plants (3:1 ratio).

Traits are inherited independently

Mendel also experimented to see what would happen if plants with 2 or more pure-bred traits were cross-bred. He found that each trait was inherited independently of the other and produced its own 3:1 ratio. This is the principle of independent assortment.

Find out more about Mendel’s principles of inheritance.

The next generations

Mendel didn’t stop there – he continued to allow the peas to self-pollinate over several years whilst meticulously recording the characteristics of the progeny. He may have grown as many as 30,000 pea plants over 7 years.

Mendel’s findings were ignored

In 1866, Mendel published the paper Experiments in plant hybridisation (Versuche über plflanzenhybriden). In it, he proposed that heredity is the result of each parent passing along 1 factor for every trait. If the factor is dominant, it will be expressed in the progeny. If the factor is recessive, it will not show up but will continue to be passed along to the next generation. Each factor works independently from the others, and they do not blend.

The science community ignored the paper, possibly because it was ahead of the ideas of heredity and variation accepted at the time. In the early 1900s, 3 plant biologists finally acknowledged Mendel’s work. Unfortunately, Mendel was not around to receive the recognition as he had died in 1884.