Mendelian Genetics

Key concepts:
• Mendel chose varieties of pea plants whose purity was certain, and which had bred consistently for many generations under very stringent conditions.
• He chose two parent plants for each experiment that could be easily distinguished from each other by at least one characteristic.
• Mendel moved sex gametes from the male parent (pollen) to the female sex organ by hand.
• Special precautions were taken to ensure that there was no accidental fertilization by insects or other means.
• The resulting zygotes (seeds) were carefully collected, labeled and stored over the winter.
• The next spring Mendel planted these seeds, grew the plants and then counted the number of the offspring that showed a particular characteristic.
• These results were then recorded.

A Typical Cross - Round Two

Key concepts:
• New parents were chosen from the offspring of the first cross.
• In many crosses, these parents were indistinguishable from one another (same phenotypic appearance).
• Once again the fertilization (movement of pollen from the male parent to the female parent) was done by hand.
• The seeds were again collected, stored and planted next spring.
• The phenotypes of the offspring (grandchildren of the original parents) were recorded.

The Importance of Numbers and Ratios

Key concepts:
• It is a matter of chance which genes become part of a zygote (seed).
• Possible combinations are produced randomly.
• It is important therefore to examine large numbers of seeds/plants to ensure statistically representative results. (Try tossing two coins and recording the results of 1, 5, 15 and 25 tosses. See at what point the results begin to become equal; HH:HT:TT).
• But the absolute numbers don't tell the whole story. 352 Red flowered plants and 116 White flowered plants doesn't have as much meaning as "3 Red plants for every 1 White plant".
• Reducing the data from all experiments to a ratio allowed Mendel to compare the results from a large number of experiments.

The Results

Key concepts:
• Mendel made sure that the parents were "pure breeding". For generations the Red flowered plant had only produced Red flowered offspring and the White flowered plant had only produced White flowered offspring.
• He collected and planted hundreds of seeds from the cross.
• All of the offspring (called the "F1" generation) from these two parents showed a single phenotypic trait (in this case, Red flowers).
• The trait that "dominated" the F1 generation was called the Dominant trait while the one that "disappeared" was called the Recessive trait.
• When the Red flowered F1 plants were crossed, the grandchildren (F2 generation) showed a ratio of 3 plants with the dominant trait (Red flowers) to every 1 plant that showed the recessive trait (White flowers).

Interpreting the Results

Key concepts:
• Mendel made the assumption that a small particle he called an "elemente" controlled the production of a trait such as flower color.
• Historians of science have argued ever since where he got this idea from, but today we have isolated Mendel's elementes and called them genes.
• He knew about sexual reproduction, and knew that both male and female gametes came together to produce an offspring.
• He guessed that each sex gamete had to carry one of his controlling "elementes".
• The adult plants, therefore, must contain two "elementes", a condition we would call Diploid today.
• Mendel's third guess was that "elementes" came in different forms; one form produced a red flower, whereas a different form produced a white flower. Today we would call the altered forms of genes, mutants.

The Consequences

Key concepts:
• The combination of genes RR would give a Red flowered plant, and rr would give a white flowered plant, but, what would the Rr combination give? Do the experiment.
• RR plants could only produce sex gametes with R genes, and rr plants could only produce sex gametes with r genes.
• A cross between them would produce offspring with the Rr combination. What would they look like?
• Mendel did the experiment and the F1 offspring all had the Red flowered phenotype. The Rr combination, therefore produced only red flowers.
• However, there could be alternative explanations (one gene was destroyed, for example).
• When two Rr containing plants were crossed (second round), among their offspring were white flowered plants; the gene could not have been destroyed). But the interesting part were the ratios; only a quarter of the offspring were white flowered!
• If his guesses had been right so far, one quarter of the F2 offspring would be RR and "breed true" (never giving any white flowered offspring ever again). They did!
• Half of the offspring would have red flowers, but would keep producing white flowered offspring when crossed together. They did!! These were the Rr containing F2 plants.
• All of the results could be explained by Mendel's three guesses, so perhaps they were correct.