Certain characters/traits are governed and inherited by genes located in cytoplasmic organelles (chloroplast or mitochondrion) other than nucleus. This is called extra nuclear inheritance.

Plasmogenes are independent, self-replicating, extra-chromosomal units located in cytoplasmic organelles, chloroplast, and mitochondrion.

(a) Pleiotropism
(b) Poly genic inheritance

(a) Incomplete dominance
(b) Codominance
(c) Multiple alleles
(d) Pleiotropic genes are common examples for intragenic interaction.

The gene interactions may be
(a) Intragenic gene interaction. E.g.: Codominance
(b) Intergenic gene interaction. E.g.: Epistasis

(a) Dihybrid cross ratio = 9 : 3 : 3 : 1
(b) Dihybrid test cross ratio = 1 : 1 : 1 : 1

When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent to the other pair of characters. Genes that are located in different chromosomes assort independently during meiosis.

Seed shape, flower color, flower position & pod color.

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.

(a) Classical genetics
(b) Molecular genetics
(c) Population genetic
(d) Quantitative genetics

"Genetics" is the branch of biological science which deals with the mechanism of transmission of characters from parents to off springs.

(i) Back cross is a cross of F, hybrid with any one of the parental genotypes. 
(ii) They are of two types - Dominant back cross recessive back cross.

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

A true breeding lines means it has undergone continuous self pollination having stable trait inheritance from parent to off spring. 

Seven characters of pisum satiyum studied by Mendel.