A group of genes that together determine a characteristic of an organism.

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

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.

(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.

Interlocus interactions take place between the alleles at different loci i.e. between alleles of different genes
Dominant Epistasis: It is a gene interaction in which 2 alleles of a gene at one locus interfere & suppress or mask the phenotypic expression or mask the phenotypic expression of a different pair of alleles of another gene at another locus.
The gene that suppresses or masks the phenotypic expression of a gene at another locus is known as epistatic. The gene whose expression is interfered by non-allelic genes & prevents from exhibiting its character is known as hypostatic.
When both the genes are present together, the phenotype is determined by the epistatic gene and not by the hypostatic gene.
In the summer squash, the fruit colour locus has a dominant allele 'W' for white colour & a recessive allele 'w' for coloured fruit.
'W' allele is donminant that masks the expression of any colour. In another locus hypostatic allele 'G' is for yellow fruit and its recessive allele 'g' for green fruit.
In the first locus the white is dominant to colour where as in the second locus yellow is dominant to green.
When the white fruit with genotype WWgg is crossed with yellow fruit with genotype wwGG, the F1 plants have white fruit and are heterozygous (WwGg).
When F1 heterozygous plants are crossed they give rise to F2 with the phenotypic ratio of 12 white:3 yellow: 1 green.

Since W is epistatic to the alleles 'G' and 'g'. the white which is dominant, masks the effect of yellow or green.
Homozygous recessive ww genotypes only can give the coloured fruits (4/16).
Double recessive 'wwgg' will give green fruit (1/16).
The Plants having only 'G' in its genotype (wwGg or wwGG) will give the yellow fruit(3/16).

Law of Dominance: The characters were controlled by discrete units called factors which occur in pairs. In a dissimilar pair of factors one member of the pair is dominant & the other is recessive.
(i) This law gives an explanation to the monohybrid cross.
(a) the expression of only one of the parental characters in F₁ generation.
(b) the expression of both in the F₂ generation
(ii) It also explains the proportion of 3:1 obtained at the F₂

Monohybrid Cross 
(i) Monohybrid inheritance is the inheritance of a single character i. e, plant height.
(ii) It involves the inheritance of two alleles of a single gene.
(iii) When the F₁ generation was selfed Mendel noticed that 7847 of 1064 F₂ plants were tall, while 277of 1064 were dwarf.
(iv) When tall pea plants were pollinated with the pollens from a true breeding dwarf plants, the result was all tall plants.
(v) Dwarf trait appears in the F₂ generation.
(vi) When the parental types were reversed, the result was the same - All tall plants.
(vii) Tall (♀) \(\times\) Dwarf (♂) and Tall (♂) \(\times\) Dwarf (♀) mating's are done in both ways.
(viii) It was concluded that the trait is not sex dependent.
(xi) The gene for plant height has two alleles: Tall (T) \(\times\) Dwarf (t). (TT, Tt, Tt: tt)

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