12.1 Mendel’s experiments and the laws of probability

By the end of this section, you will be able to:

• Describe the scientific reasons for the success of Mendel’s experimental work
• Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles
• Apply the sum and product rules to calculate probabilities

Figure 12.1

Johann Gregor Mendel (1822–1884) ( [link] ) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system     (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization,

Johann Gregor Mendel, Versuche über Pflanzenhybriden Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr , 1865 Abhandlungen, 3–47. [for English translation see http://www.mendelweb.org/Mendel.plain.html]

in the proceedings of the Natural History Society of Brünn. However, Mendel’s work went virtually unnoticed by the scientific community at the time, and he was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

12.1a mendel’s model system

Mendel’s work was accomplished using the garden pea, Pisum sativum , to study inheritance. By experimenting with true-breeding     pea plants (plants that always produce offspring that look like the parent), Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

12.1b mendelian crosses

Mendel performed hybridizations     , which involve mating two true-breeding individuals that have different traits.

Plants used in first-generation crosses were called P     , or parental generation, plants ( [link] ). Mendel collected the seeds belonging to the P plants that resulted from each cross and grew them the following season. These offspring were called the F ₁     , or the first filial ( filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F ₁ generation of plants, he allowed them to self-fertilize naturally. This means that F ₁ individuals mated with other F ₁ s, to produce the F ₂     , or second filial, generation.