Factor: The unit of inheritance and expression of a particular character is controlled by inheritable units called factor (gene) which are present in pairs in parental cells and singly in the gametes.

Gene: A segment of DNA molecule which determines the unit of inheritance and expression of a particular character.

Alleles or Allelomorphs: Two or more alternative forms of a gene are called alleles. For example in pea plant, the gene for producing seed shape may occur in two alternative forms: smooth (S) and wrinkled (s). Genes for smooth wrinkled seeds are alleles of each other, and occupy same locus on homologous chromosomes.

Trait: It is the morphologically or physiologically visible character, e.g. colour of flower, and shape of seed.

Dominant trait: Out of the two alleles or allelorrorphs of a trait, the one which expresses itself in a heterogygous organism in the F1 hybrid is called the dominant trait (dominant allele) and the one that remains masked in F1 individual but gets expressed in the next generation (F2), is called recessive. Thus, if the allelic combination in an organism is Tt, and T (tallness) expresses itself but t (dwarfness) cannot, so T is the dominant allele, and tallness is dominant on dwarfness represented by 't'.

Recessive trait: Out of the two alleles for a trait, the one which is suppressed (does not express) in the F1 hybrid is called the recessive trait (recessive allele). But the Recessive allele does express itself only in the homozygous state (tt).

Genotype: A class of individuals recognized based on its genetic constitution and breeding behaviour is called the genotype. For example, the genotype of pure smooth seeded parent pea plant is SS and it will always breed true for smooth-seeded character, but plants having Ss on selfing would give rise to a population represented by 3:1 ratio for smooth seeded plants and wrinkled seeded plants.

Phenotype: A class of individuals recognized based on outward appearance of a trait in an individual is the phenotype. For example, Smooth-seeded shape or wrinkled shape of seeds represent two different phenotypes.

Homozygous: An individual possessing identical alleles for a trait is termed homozygous. For example, SS is homozygous condition for smooth seeded character in garden-pea.

Heterozygous: An individual with dissimilar alleles for a trait is termed heterozygous. For example, Ss represents the heterozygous condition for smooth seeded character in garden pea.

Parent generations: The parents used for the first cross represent the parent (or P1) generation.

F1 generation: The progeny produced from a cross between two parents (P1) is called First filial or F1 generation.

F2 generation: The progeny resulting from self pollination or inbreeding of F1 individuals is called Second Filial or F2 generation.

Monohybrid cross: The cross between two parents differing in a single pair of contrasting characters is called monohybrid cross and the F1 offspring is the Monohybrid. The phenotypic ratio of 3 dominants : 1 recessive obtained in the F2 generation from the monohybrid crosses by Mendel was mentioned as 3:1 monohybrid ratio.

Dihybrid cross: The cross in which two parents differing in two pairs of contrasting characters are considered simultaneously for the inheritance pattern is called dihybrid cross. The phenotypic ratio obtained in the F2 generation from a dihybrid cross is called Mendelian dihybrid ratio (9:3:3:1), and the F1 individual is called dihybrid (Ss Tt).

Hybridisation: Crossing organisms belonging to different species for getting desirable qualities in the offspring.

Test cross: It is the Crossing of the F1 progeny with the homozygous recessive parent. If F1 progeny is heterozygous, then test cross always yields the ratio of 1:1 between its different genotypes and phenotypes.

Reciprocal cross: It is the cross in which the sex of the parents is reversed. That is if in the first cross father was dwarf and mother tall, then in the reciprocal cross, dwarf parent will be female and tall parent male.