In 1856, the Austrian monk Gregor Mendel began a series of experiments that were to reveal the basic principles of genetics.
Mendel studied the inheritance of seven contrasting traits in pea plants. For each trait there were only two forms. For example, plants were either tall (6 to 7 feet [1.8 to 2.1 meters]) or short (9 to 18 inches [23 to 46 centimeters]), with no intermediate forms. The ripe seeds could be either smooth and round or wrinkled. The peas could be either yellow or green, again with no intermediate colors.
When Mendel began his experiments, one of the prevailing beliefs about heredity was the "paint-pot" theory. According to this theory, the traits of the two parents blended in their children much as two pigments are blended in a can of paint. Children were therefore a unique mixture of their parents, and when these children married and reproduced, their traits would blend with those of their spouses. Through his experiments with pea plants, Mendel discovered that discrete particles or units determine heredity. Although traits could disappear in one generation, they reemerged in their original form in later generations. Mendel crossbred pure strains of tall and short plants. Their offspring were all tall. This was the first descending, or first filial, generation, designated F1. Mendel then interbred the plants of the F1 generation to produce a generation of grandchildren, the F2 generation .In this generation, short plants reappeared. Among thousands of plants in the F2 generation, there was approximately one short plant for every three tall ones.
From similar results with the other six traits, Mendel concluded that although a dominant form could mask the other form in hybrid, or mixed, individuals, the dominated traitâ€”the recessiveâ€” was not destroyed; it wasn't even changed. Recessive traits would appear in unaltered form in later generations because genetic traits were inherited as discrete units.
These basic genetic units that Mendel described were factors located on chromosomes.
In Mendel's experiments, genes on seven different pairs of chromosomes determined the seven contrasting traits. The gene for height occurred in one of the seven pairs. When Mendel crossbred pure tall and pure short plants to produce his F1 generation, each of the offspring received an allele for tallness (T) from one parent and one for shortness (t) from the other. These off springs were mixed, or heterozygous, with respect to height; each had two dissimilar alleles of that gene. Their parents, in contrast, had been homozygous, possessing two identical alleles of that In the next generation (F2), after the mixed plants were interbred, short plants reappeared in the ratio of one short to three tall. Knowing that shorts only produced shorts, Mendel could assume that they were genetically pure. Another fourth of the F2 plants produced only tall. The remaining half, like the F1 generation, was heterozygous; when interbred, they produced three tall for each short. Dominance produces a distinction between genotype, or hereditary makeup, and phenotype, or expressed physical characteristics. Although some of our genetic traits follow Mendelian laws, with only two formsâ€”dominant and recessiveâ€”other traits are determined differently.
Through additional experiments, Mendel also formulated his law of independent assortment. He discovered that traits are inherited independently of one another. For example, he bred pure round yellow peas with pure wrinkled green ones. All the F1 generation peas were round and yellow, the dominant forms. But when Mendel interbred the F1 generation to produce the F2, four phenotypes turned up. Round greens and wrinkled yellows had been added to the original round yellows and wrinkled greens.
The independent assortment and recombination of genetic traits provide one of the main ways by which variety is produced in any population. Recombination is important in biological evolution because it creates new types on which natural se- lection can operate.
Source: Anthropology Appreciating Human Diversity: Conrad Philip Kottak