Let us take the example of one such hybridisation experiment carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene. He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial 1 progeny or the F1. Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf. He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.

Mendel then self-pollinated the tall F1 plants and to his surprise found that in the F2 generation some of the offspring were ‘dwarf’; the character that was not seen in the F1 generation was now expressed. The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offspring were either tall or dwarf, none were of in-between height.

Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. Now we call them as genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.

If we use alphabetical symbols for each gene, then the capital letter is used for the trait expressed at the F1 stage and the small alphabet for the other trait. For example, in case of the character of height, T is used for the Tall trait and t for the ‘dwarf’, and 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 plant the allelic pair of genes for height are identical or homozygous. TT and tt are called the genotype of the plant while the descriptive terms tall and dwarf are the phenotype. What then would be the phenotype of a plant that had a genotype Tt? 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 and hence is called the dominant factor while the other factor is recessive.

The production of gametes by the parents, the formation of the zygotes, F1 and F2 plants can be understood from a diagram called Punnett Square. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.