To determine the genotype of a tall plant Mendel crossed the plants from F₂ with the homozygous recessive dwarf cross This is called a test cross.

The progenies of the test cross can be easily analysed to predict the genotype of the plant or the test organism.
Thus in a typical test cross an organism (Pea plants) showing dominant phenotype is crossed with the recessive parent.
Test cross is used to identify whether an individual is homozygous or heterozygous for dominant character.

(i) In one experiment, the tall pea plants were pollinated with the pollens from a true-breeding dwarf plant, the result was all tall plants.
(ii) The parental types were reversed, which also yielded only tall plants

Matings were done in both ways called reciprocal crosses.
(i) The results of this crosses are the same.  It was concluded that the trait is not sex dependent.
(ii) The results of Mendel's monohybrid crosses were not sex dependent.

a) Dominance
b) Incomplete dominance

a) supplementary genes - 9 : 3 : 4
b) Complementary genes - 9 : 7
c) Dominant epistasis - 12 : 3 :1

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

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)

(a) F ₁ plants produce yellow colour flower plants.
(b) Law of dominance and Law of segregation

Atavism is a modification of a biological structure whereby an ancestral trait reappears after having been lost through evolutionary changes in the previous generations. Evolutionary traits that have disappeared phenotypically do not necessarily disappear from an organism's DNA. The gene sequence often remains, but is inactive. Such an unused gene may remain in the genome for many generations. As long as the gene remains intact, a fault in the genetic control suppressing the gene can lead to the reappearance of that character again. Reemergence of sexual reproduction in the flowering plant Hieracium pilosella is the best example for Atavism in plants.

Male sterility found in pearl maize (Sorgum vulgare) is the best example for mitochondrial cytoplasmic inheritance. So it is called cytoplasmic male sterility. In this, male sterility is inherited maternally. The gene for cytoplasmic male sterility is found in the mitochondrial DNA.

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

Four gametes - RY, Ry, rY, ry

The Law of Segregation (Law of Purity of gametes): Alleles do not show any blending. During the formation of gametes, the factors or alleles of a pair separate and segregate from each other such that each gamete receives only one of the two factors. A homozygous parent produces similar gametes and a heterozygous parent produces two kinds of gametes each having one allele with equal proportion. Gametes are never hybrid.

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.