Lesson 4
All of the breakthroughs, like the discovery of the structure of genes and how they function, could not have just happened. Just like any great discovery it must have had an origin, a beginning.
Genetics as we know it today began in a monestary in Austria in the 1860s. A monk named Gregor Mendel (1822-1884, he was educated in the fields of natural science and mathematics) was curious as to how animal and plant traits were inherited, so he began to study garden peas. He studied the peas for eight years and in this time he built the founding principles of heredity and ultimitaley what would be called genetics.
Garden peas usually self-pollinate, meaning that they have both male and female reproductive parts. But Mendel used a technique called cross-pollination to have pea offspring that inherited its traits from two different parents (essentially, this technique is done by collecting pollen from flowers of one of the pea plants and transfer them to another plant). He only paid attention to one trait at a time as he studied the patterns of inheritance. He also used many plants in order to lessen the influence of chance in his experiments (this will be discussed in detail in the lesson on statistics and probability).
Mendel collected several different strains of peas and made sure that each one was pure-bred (that it would produce offspring that was identical to the parent over a period of many generations). Mendel would use peas that expressed one out of two different forms of the same trait (for example, if a trait was the seed color of the pea, Mendel used yellow seeds and green seeds). Mendel throughout his study, worked with seven distinct traits:
Seed shape -> round vs. wrinkled
Seed color -> yellow vs. green
Seed coat-coloring -> colored vs. white
Pod shape -> inflated vs. constricted (tight)
Pod color -> green vs. yellow
Flower position -> axial vs. terminal (axial means that the flowers shoot off the sides of the stem, while terminal means that the flower is situated at the top of the stem)
Stem length -> long vs. short
In the first experiments, Mendel crossed pure-breeding plants that produced smooth seeds with those that produced wrinkled seeds. The outcome was seeds that were round. There were no more wrinkled seeds. Today the parent generation is symbolized by P1 and the next generation is symbolized by F1. Mendel let the offspring grow into plants and self-pollinate. This gave rise to the F2 generation (I think you get the way this notation goes. Just in case you're interested, the 'P' stands for parental, and the 'F' stands for filial).
The F2 generation produced a cool effect. Three quarters of the seeds produced were round, and one-fourth were wrinkled. Mendel coined the term 'dominant' for the round side of the trait, and 'recessive' for the wrinkled.
Mendel would go on to repeat the two-generation cross for the six other traits. The F2 generation would always produce traits that had the dominant to recessive ratio at 3:1. These results led Mendel to hypothesize that each pure-breeding plant had two identical copies of a unit or factor of a specific trait. He didn't know how they were structured or where they were located (that was for Watson and Crick to discover) but he went on to hypothesize that only one copy of the units went into each gamete created. Thus, if a parent were pure-bred for inflated pods the gametes that it generated would all have one inflated pod unit inside them. This led him to conclude that the offspring from a cross between wrinkled and round would contain one unit that was wrinkled and another that was round (the round one would be expressed, making dominant, the wrinkled unit can only be expressed when the plant is pure-bred for being wrinkled). Mendel would term this the "principle of segregation."