Heredity

The Work of Gregor Mendel

Around 1843, a young man named Gregor Mendel entered an Augustinian monastery to study theology, and to become a teacher. But after failing his teaching examination, he traveled to the University of Vienna to study science. As part of his education, he learned how to use mathematics to explain natural phenomena.

After a time, Mendel returned to the monastery, where he joined a local science research society. Having read of Knights work at the University of Vienna, Mendel decided to repeat his work, with the objective of trying to predict when white flowers, and when violet flowers would result from a cross (breeding).

Like Knight, Mendel chose the garden pea, Pisum sativum, but from there Mendel did things a bit differently from Knight. Mainly, Mendel applied his knowledge of Mathematics to help him predict the outcome of crosses (breeding).

In choosingP. sativum, Mendel had the good fortune. For one, P. sativum were readily available in his area. Also, the peas could be cultivated quickly, and with little effort. Important from a mathematical view point, P. sativum produced a large number of offspring.

P. sativum had 32 varieties from which to select from. Mendel chose seven. More importantly, several traits had only two contrasting forms. For example, the seeds of the pea plant were either round or wrinkled. The plants themselves were either tall or short. The flowers were either violet or white. There were no mixtures of traits.

Just as important, P. sativum was a plant that was easy to work with. The flower contains stamens (the male part) that produce pollen, and it contains s a pistil (the female part) that contains the eggs. Because of the shape of the petals of the flower often trapped the pollen, causing it to fall on the pistil and fertilize the eggs. This process is called self-pollination. This simple process allowed Mendel to produce several groups of "true-breeding" plants, plants whose offspring bare the same traits as the parents.

Both the clear traits, and having true-breeding plants gave Mendel the ability to label traits, and clearly follow how these traits were passed on to future generations. After many experiments, Mendel was finally able to accurately predict what traits the offsprings would have, knowing the traits of the parents.

Lets follow how Mendel did this using one trait, the color of P. sativum' flower. Mendel crossed two true-breeding plants, one that was violet, and the other that was white. He called the parents the the parental, or P, generation. Their offspring formed the first filial (FIHL ee uhl), or F1, generation.

Mendel found that every single plant in the F1 generation was tall. Now mathematically, Mendel knew that three possible traits could have appeared in violet flowers, white flowers, or pale violet flowers. He eliminated the pale violet flowers earlier from common sense and observations. No P. sativum had pale violet flowers. But what happened to the short peas? The contrasting trait - shortness-seemed to have disappeared.

Mendel allowed the members of the F1 generation to self-pollinate, producing the second filial, or F2, generation. When he examined the offspring, he found that about three-fourths of the plants in the F2 generation were violet, while about one-fourth were white. The trait that had vanished in the F1 generation had reappeared in the F2 generation.

Mendel knew that the offspring resulted from combining the pollen from the stamens to the eggs in the pistols. So he assumed that the trait for violet flowers was passed to the offspring on an allele, while the trait for white flowers was passed on a different allele. If this is the case, reasoned Mendel, then a flower has two alleles. Mathematically, then, all of the F1 generation, and half of the F2 generation should have both alleles, one for white and one for violet flowers. Yet all of the F1 generation, and three-fourths of the F2 generation had violet flowers. Mendel reasoned that the allele for violet flowers was dominant over the allele for white flowers. Thus, Mendel developed his Principle of Dominance. When two different allele for a trait are together, and only the dominant trait will appear. Mendel called the other trait the recessive trait, because it hides behind the dominant trait.

This observation also made Mendel realizes that the alleles must separate at some point in the parents. This realization led to Mendel's Law of Segregation, which simply states that the alleles forming a genotype will separate when gametes are formed, during meiosis.

Remember that Mendel followed seven different traits. Each set of traits had its own dominant trait, and its own recessive trait. Also, each trait followed the law of segregation without fault, and independently of each other. This led to Mendel's Law of Independent Assortment, which states that pairs of alleles separate independently of one another during gamete formation.