Mendelian Genetics

Overview

This module presents the basic principles of inheritance, as first outlined by Gregor Mendel. Mendel's four postulates will be presented, and will be used to demonstrate how the genetic make-up of parents can be used to predict the characteristics of their offspring. Examples of crosses will be presented involving one, two, and three separate genes.

Objectives

1. Learn the relationship between dominant and recessive alleles.
2. Learn Mendel's four postulates, and understand how the postulates apply to the behavior of genes and chromosomes.
3. Learn how to determine the phenotypes of offspring of monohybrid, dihybrid, and trihybrid crosses.
4. Learn how to use testcrosses to determine the genotypes of specific offspring from the above crosses.
5. Learn to use the product law to determine the probabilities of combinations of characteristics occurring together.

Gregor Mendel

Humans tried for more than 2000 years to understand the principles of heredity, so that domesticated animals and plants could be bred more efficiently to exhibit certain characteristics. The efforts to understand failed, but in spite of this, people learned to breed animals and plants empirically, with some success.

Finally, in 1856, Gregor Mendel, a monk and teacher of natural sciences, performed a successful study of inheritance in the pea plants that he tended in his garden. How did Mendel succeed where so many others had failed?

• First, he made a lucky choice of an organism to study. Pea plants are easy to grow and interbreed, they grow to maturity in one year, and they can reproduce by either self-fertilization or by crossing between plants.
• Second, as a monk (with lots of time on his hands?), Mendel was a meticulous record keeper. He was able to analyze large amounts of carefully recorded data.

Mendel published the results of his studies, but they were ignored for nearly 50 years. Finally, Mendel's work was discovered in the early 20th century, which led to the start of modern genetics.

So what exactly did Mendel do?

He picked seven pairs of traits of pea plants, such as tall vs. dwarf plants, yellow vs. green seeds, round vs. wrinkled seeds. He had strains of pea plants that 'bred true' for these characteristics, meaning that (for example) if a tall plant was bred with a tall plant, all of the offspring would be tall. Using these true breeding stocks, Mendel looked at each pair of traits separately.