Gregor Mendel selected seven pairs of characters in Pisum sativum (garden pea)

Heredity - Transmission of characters from parents to off springs.
Variations - organisms belonging to the same natural population or species that shows a difference in the characteristics.

"Genetics" is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings. The term Genetics was introduced by W. Bateson in 1906.

(i) Hugo de Vries of Holland
(ii) Carl Correns of Germany
(iii) Erich von Tschermak of Austria

Seven characters of pisum satiyum studied by Mendel.

To identify the plant genotype whether homozygous or heterozygous Mala can perform test cross, where the individual is crossed with homozygous recessive for the trait. If the plant is heterozygous then the resultant progenies would be in the ratio 50:50 (i.e., 1: 1)

He chooses pea plant because,
(i) It is an annual plant & bears contrasting characters controlled by a single gene separately.
(ii) Self-fertilization occurred in garden pea plants.
(iii) Mendel used both self-fertilization and cross-fertilization.   
(iv) The flowers are large hence emasculation & pollination are very easy for hybridization.

i) They help individuals to adapt themselves to the changing environment.
ii) Variations allow breeders to improve better yield, quicker growth, increased resistance, and lesser input.
iii) They constitute the raw materials for evolution.

It is based on independent, self-replicating extra chromosomal unit called plasmogene located in the cytoplasmic organelles, chloroplast & Mitochondrion.

Incomplete Dominance	Co-dominance
When one allele is not completely dominant to another allele.	lt occurs due to simultaneous expression of both alleles in the heterozygote.
E.g: Flowers colour of Mirabilis Jalapa	E.g:  ABO blood group system in human beings.

1. It is found in 40'ciock plant (Mirabilis jalapa).
2. Two types of variegated leaves.
(i) Dark green leaved plants
(ii) Pale green leaved plants
3. When the pollen of dark green leaved plant (male) is transferred to the stigma of pale green leaved plant (female) and pollen of pale green leaved plant is transferred to the stigma of dark green leaved plant, the F₁ generation of both the crosses must be identical as, per mendelian inheritance.
4. But in the reciprocal cross the F₁ plant differs from each other.
5. In each cross, the F₁ plant resembles female plant.
6. This inheritance is not due to nuclear gene.
7. This is due to the chloroplast gene found in the ovum of the female plant which contributes the cytoplasm.
8. During fertilization male gamete contribute only the nucleus but not cytoplasm.

(i) The Pleiotropic gene-influences a number of characters simultaneously & such genes are called pleiotropic gene.
(ii) Mendel noticed pleiotropy while performing breeding experiment with peas (Pisum sativum).
(iii) Peas with purple flowers were cross with white flowers.
(iv) Brown seeds were crossed with light coloured seeds.
(v) Dark spot on the axils of the leaves were crossed with no spot on the axils of the leaves.
(vi) The above characters all were inherited together as a single unit.
(vii) This is due to the pattern on inheritance where the 3 traits were controlled by a single gene with dominant & recessive alleles.
(viii) E.g. Sickle cell anemia.