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.
Parents
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.
