Mendel chose those characters that had different forms of expression. He selected seven pairs of contrasting characters. These characteristics were easily, distinguishable and bred true, generation after generation. A true breeding variety is one which produces offspring identical to the parents.
INHERITANCE OF SINGLE TRAIT
Pure Tall Plants
One of Mendel's varieties of peas grew about a meter tall, as contrasted with another that grew as a short. He crossed tall plants with tall plants for many generations and in this way obtained pure tall plants.
|Mendel’s seven pairs of contrasting traits in garden peas|
Pure Short (dwarf) Plants
Simultaneously he crossed the short (dwarf) plants with short plants for many generations and obtained pure short plants.
Crossing of Tall and Short Plants
Then he crossed the pure tall and pure short plant. The offspring were all tall plants. When the two tall offspring were crossed the result was different. He had grown 1064 plants and found that 787 of them were tall and 277 were short. The numbers struck him. They showed ratio of almost 3:1. It was certainly close to 798 to 266 have been exactly 3:1. Secondly Mendel was surprised at the occurrence of short plant as the offspring of two tall plants. This lead to a series of experiments. On the basis of his breeding experiments Mendel was able to propose that “each character occurs in pair” One part is given by the male and other by the female. When the characters (or factors as called by Mendel) are the same it is called homozygous and when the characters are different it is called heterozygous. When the tall plant get the character of tall from male and female, it will be homozygous tall. When it receives tall character from one parent and short character from another parent it will be heterozygous. In heterozygous condition only one character appears. The character which is seen is called a dominant character and the character which though remains but does not appear is called a recessive character.
In genetics the dominant character is shown by capital alphabet or by + and the recessive character is shown by small alphabet or by letters of the character e.g. vestigial (Vg, vg), arginine (Arg, arg) etc.
TT: It means the plant has one tall character from father (male) and another tall character from mother (female). It is homozygous tall.
Tt: It means that the plant is tail but it has tall character T and another short character t. Though the short character t is present but it is not exhibited. So the tall T character is dominant and short character t is a recessive one. The characters are therefore in the form of unit. The unit is now called pair of genes. (The term, gene, was introduced in 1909 by Wilhelm Johansson, then in 1912 Bateson introduced the word Genetics, Mendel used the word factor for gene). So the gene for tall is T and gene for short is t. Alternate forms of a genes are called alleles. Thus, in this example there is an allele for tallness (T) and an allele for shortness (t).
Genotype and Phenotype
The character seen externally is called phenotype e.g. tall and short. The genetic make-up of individual character is called genotype. The genotype may be homozygous or heterozygous e.g. tall homozygous TT, tall heterozygous Tt. Mendel did not use words phenotype and genotype, but he obviously understood the basic concept of visible and hidden characteristics.
The parents are shown by notation P, the offspring of the parents are known as first filial generation. (GK: Filial; offspring). It is represented as F. The offspring of this generation are called second filial generation. It is represented as F2.