Method used by Mendel

Although he was not the first researcher to study hybridization in plants, Mendel succeeded in his experiments by getting some questions right that previous researchers had not considered:

• Using the pea Pisum sativum, an autogamous species, which allowed him to use pure races (homozygous individuals, AA and aa). For this, before conducting his experiments, he had to control the reproduction of the peas until all descendants were of the same type.
• He studied seven easily distinguishable characters: smooth or rough, yellow or green seed, flowers in axial or terminal position, yellow or green pod, smooth or rough pod, and long or short stem. Although this character is highly variable, if the plants are grown under the same conditions, the variations when crossing plants of about 2-3 meters with others of less than 50 centimeters do not matter.
• Each experiment tried to analyze only one character, obtaining easily identifiable numerical results 1: 3, 1: 2: 1.
• He used statistical methods to analyze the results, studying the offspring of each plant separately.
• For the third law or principle of independent combination, you chose characters that are not bound or that at least behave as such. Although it may be that he was lucky with the election, he also had the wisdom to discard the results that did not interest him, because they were due to another cause.

By Mariana Ruiz LadyofHats (Spanish Translation El Ágora) [Public domain], via Wikimedia Commons

After checking how these characters were transmitted in successive generations, Mendel deduced that each alternative of a differential character (such as the smooth or rough seed, for example) is determined by a hereditary factor (now we would call it a gene). Each individual has two genes that provide information about each character. Thus, a constant parental form (as Mendel would say, or homozygous or purebred) would be AA or aa, while the hybrid form would be Aa. Therefore, each pair of Mendelian differential characters is determined by two genes, A and a, also called alleles. Each pair of alleles A already for a character constitutes an allelic pair.

Mendel crossed pure breeds that differed in one character and observed the hybrid offspring that he obtained (F₁). Afterwards, he made the hybrids self-fertilize to obtain the second filial generation (F₂).

You can watch the video because it explains the basic concepts, the method that Mendel used and Mendel's laws. But be careful, in peas, yellow is dominant over green, not as it is said in the video that it can be another plant. Yes, I know that in Spain we only have green peas.