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

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)

i) He applied mathematics and statistical methods to biology and laws of probability to his breeding experiments.
ii) He followed scientific methods and kept accurate and detailed records that include quantitative data of the outcome of his crosses.
iii) His experiments were carefully planned and he used large samples.
iv) The pairs of contrasting characters which were controlled by factor (genes) were present on separate chromosomes.
v) The parents selected by Mendel were pure breed lines and the purity was tested by self crossing the progeny for many generations.

Pleiotropy - A single gene affecting many traits. Here the single gene AA controls the traits for flower colour, seed colour and leaf axil spot.

The phenomenon in which two alleles are both expressed in the heterozygous individual is known as codominance. The codominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flavonoid substance. Example: Gossypium hirsutum and Gossypium sturtianum, their F ₁ hybrid (amphiploid) was tested for seed proteins by electrophoresis. Both the parents have different banding patterns for their seed proteins. In hybrids, additive banding pattern was noticed. Their hybrid shows the presence of both the types of proteins similar to their parents.

A cross between homozygous parents that differ in three gene pairs (i.e. producing trihybrids) is called trihybrid cross.
F₂ Phenotypic ratio - 27: 9 : 9 : 9 : 3 : 3 : 3 : 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.