Uploaded by ihatesuchin on Jul 05, 2004
Gregor Mendel was an Augustinian nineteenth century monk who, due to a series of momentous experiments, is now widely regarded as the forefather of genetics. Mendel studied the inheritance of seven contrasting characteristics of the species Pisum sativum, more commonly known as the garden pea. Each of the variables that Mendel experimented with were discontinuous; there were no intermediate forms. For example, one of the variables was length of stem, which was always either tall or short. From his experiments Mendel was able to draw solid conclusions about the inheritance of characteristics in organisms. With the advancements in genetics since his time we are now able to explain Mendel's principles in terms of chromosomes and genes. Understanding of these exact terms did not exist in Mendel's lifetime. However, Mendel's principles still form the basis of modern day genetics.
In his first series of experiments Mendel allowed Pisum to self-fertilise for several generations, so that he knew that these pea plants were purebred. He then cross-fertilised plants which were purebred for contrasting characteristics. For example, he crossbred pure-bred dwarf Pissum with pure-bred tall Pissum. He carried out reciprocal crosses. Even though these plants obviously showed many characteristics he only looked at one characteristic at a time. In collecting the results of his experiments, Mendel recorded the numbers of individuals in each class in the progeny, this established the ratios of the contrasting characters of many subsequent generations. In the F1 generation all the plants were tall. Mendel then left the F1 generation plants to self-fertilise. In the F2 generation there were both tall and dwarf plants in an approximate ratio of 3:1. The same ratio was found in the F3, F4, and F5 etc. generations. Mendel realised that because the 'dwarf' characteristic had disappeared in the F1 and had then reappeared in the F2, the controlling factor for 'dwarf' had remained intact and undiluted from one generation to another. It is never expressed, however, in the presence of a factor for 'tall'. He understood that there must be two independent factors for 'dwarf' and 'tall'. Mendel comprehended that the 3:1 ratio was the product of the binomial expression derived from randomly combining two pairs of unlike elements.
We now know Mendel's 'factors' to be genes found on homologous pairs of chromosomes in the nucleus of the cell. There are two or more forms of each gene known as alleles. In Mendel's experiments...