Monohybrid Cross

In this experiment, Mendel brought about the cross between two pea plants with only one pair of contrasting characters. This type of cross is called a monohybrid cross. To study the monohybrid ratio, let us consider the characteristic ‘plant height’ with a pair of contrasting characteristics: tall plant and dwarf plant. 

• The Tt plant is heterozygous for genes controlling one character (height); it is a monohybrid and the cross between TT and tt is a monohybrid cross.
• When a pure tall pea plant (TT) produces gametes by meiosis, each gamete has the allele 'T'. Similarly, when a pure dwarf pea plant (tt) produces gametes by meiosis, each gamete has the allele 't.'
• During fertilisation, the two alleles, 'T' from one parent, say through the pollen, and t from the other parent, then through the egg, are united to produce zygotes that have one T allele and one t allele. In other words, the hybrids have Tt.
• Since these hybrids contain alleles that express contrasting traits, the plants are heterozygous. 

In the F₂ generation, when F₁ plants are self-pollinated, the recessive trait reappears.
This is because the recessive parental trait is expressed without any blending in the F₂ generation. While during gamete formation, these alleles segregate and each gamete receives only one allele from each pair of factors. This segregation of alleles is a random process and so there is a 50% chance of a gamete containing either allele.
For example:

When fertilisation takes place, pollen grains with genotype 'T' have a 50% chance to pollinate eggs with genotype 'T' as well as genotype 't'. Similarly, pollen grains with genotype 't' have a 50% chance to pollinate eggs with genotype 'T,' as well as of genotype 't'. As a result of random fertilisation, resultant zygotes formed by selfing of the F₁ generation have genotypes TT, Tt, or tt. (TT: pure tall, Tt: hybrid tall, tt: dwarf).

Monohybrid cross

Phenotypic ratio: 3 (tall): 1 (dwarf)

Genotypic ratio: 1 (TT): 2 (Tt): 1 (tt).

Generally, a gene is represented by alphabetical symbols. The capital letter is used for the trait expressed at the F₁ stage, and the small alphabet for the other trait. For example, in the case of the character of height, "T" is used for the tall trait and 't' for the ‘dwarf." Both T and t are alleles of each other. Hence, in plants, the pair of alleles for height would be TT, Tt, or tt. 

• Mendel also proposed that in a true-breeding, tall or dwarf pea variety, the allelic pair of genes for height are identical or homozygous, TT and tt, respectively.
• When an individual has dissimilar alleles (Tt) for a particular trait, it is known as 'heterozygous' or 'hybrid' for a specific trait.
• Thus, the genetic constitution for a particular trait (i.e., 'TT,' Tt, or 'tt') is known as the 'genotype,' and the type of character it represents (i.e., tallness and dwarfness) is known as the 'phenotype.'
• As Mendel found the phenotype of the F₁ heterozygote Tt to be exactly like the TT parent in appearance, he proposed that in a pair of dissimilar factors, one dominates the other (as in the F₁) and hence is called the dominant factor while the other factor is recessive. For example, T (for tallness) is dominant over t (for dwarfness), which is recessive.
• It is convenient to use the capital and lowercase of an alphabetical symbol to remember this concept of dominance and recessiveness.