Gregor Mendel’s Laws – Teacher Notes (new March 5, 2019)

History about Gregor Mendel’s investigations

Gregor Mendel was an Austrian monk who proposed his Principles of Inheritance in 1865.

Mendel was curious about how traits passed from one generation to the next. He chose to experiment with regular garden pea plants because they reproduce relatively quickly and he could control their fertilization by transferring pollen with a small paintbrush. The peas were also a useful organism to experiment with because they can self-pollinate. (i.e. the sperm cells in pollen fertilize the egg cells in the same flower.)

He started by observing the plants that formed through self-fertilization first to ensure their visible characteristics were consistent and reliable. He ended up looking at seven characteristics/traits, but we’ll just look at seed colour.

Mendel confirmed the purity of his plants by verifying that the green pea plants only made green children and the yellow pea plants only made yellow children. Because these characteristics/traits were tracked to be consistent over multiple generations, Mendel considered them “pure-breeders”.

Mendel then began to cross-pollinate. (Aside - first he stopped self pollination by cutting out the male parts.) He pollinated green plants with yellow plants, and breeding back again.

TEACHER NOTE – Chart with generations:Generation Plants How Fertilized?# 1(P generation, parental = pure breeders)

1 Yellow X 1 Green Crossbreed

Generation 2(F1 generation, first filial or first son)

4 Yellow Self-pollinate

Generation 3(F2 generation, 2nd filial or second son)

3 yellow, 1 green3 yellow, 1 green3 yellow, 1 green3 yellow, 1 green

Another example of Mendel and his Peas

To discuss Mendel’s Findings – we need some Genetic Vocabulary first :)

Allele – extra note: arise by mutation and are found at the same place on a chromosome

Mendel’s Findings:

Law of Dominance: Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.

Law of Segregation: Every individual carried two alleles for each trait. During production of gametes (sex cells), alleles for each gene segregate from each other so that each gamete carries only one allele for each gene. (See pic below)

Law of Independent Assortment: The alleles for different traits will separate independent of each other. For example, the allele for pea colour will separate completely independent of the allele for plant height.

Mendel’s Garden Pea Plant Experiment

Mendel’s success as a scientist

Mendel’s results with the pea plants were useful, but could not complete our understanding of heredity and inheritance. Some consider his largest contribution to genetics was his scientific practice. Mendel did three key things to advance the study of genetics:

1. He ensured he had a consistent group of pure breeders to act as a strong control group.

2. He carefully controlled the reproduction of the peas to ensure he was only testing a single variable at a time. This allowed him to be sure that any changes in his results were due to his actions.

3. He formed a set of methodical testing careful application of mathematical models to ensure he got reproducible results.