Science at a Distance
a check up
Use this department to check up on the accuracy of your lecture notes. Make sure that you have written down the following definitions, explanations and important concepts in your notes.
Mendel - Part Two
A Typical Cross
A genetic cross is a breeding experiment carried out under carefully controlled conditions.
- 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
Offspring from the first round were then used as parents for a second round of crosses.
- 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
In his notebooks wrote down the actual numbers of seeds which grew into plants showing the experimental traits. Then he did some calculations.
- 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.
A Single Factor Cross. In a typical experiment Mendel chose as starting parents ones with distinctive differences; Red flowers on one parent and White flowers on the other parent.
- 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
Mendel was a genius. He was able to take the results seen above and deduce the nature, combination and inheritance patterns of the "elementes" that were controlling them.
- 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.
If Mendel was right with his three guesses, there were certain consequences that could be tested. If the tests/experiments gave the predicted results then the guesses could be correct.
- 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.
Science at a Distance
© 1997, Professor John Blamire