In fact, today we define genetics as the study of genes. Genes are located on the chromosomes. Genes are made up of DNA. Genes control the inherited characteristics. One gene for each characteristic is inherited from each parent.
Mendel conducted his experiments on Pisum sativum, the garden pea plant for the following reasons: i. Pea plants have several varieties. All the varieties have sharp contrasting characteristics, such as colour and shape of seeds. ii. Pea plant bears bisexual flowers. Each flower has both the male and the female parts.
iii. The structure of the flower is such that it completely encloses the reproductive organs until fertilization, which ensures self- pollination. iv. In pea plants, due to self-fertilization, it is easy to get pure lines (breeds) for several generations. v. As it is an annual plant, it is possible to study many generations within a short span of time. Mendel studied seven pairs of contrasting characteristics in his experiments on garden pea.
Mendel conducted his experiments in three stages.
Stage I: Mendel selected seven pairs of contrasting characteristics. He self-pollinated the pea plant for several generations. He observed that seeds from tall plants produce only tall plants and those from plants with pink flowers produce plants which always have pink flowers.
He could thus, obtain pure line (plants which bred true for the selected characteristic). Stage II: He crossed two plants showing contrasting expression of a single trait. He conducted reciprocal crosses (crosses with contrasting features), for example, he crossed a true-breeding tall stem variety plant (which he called tall plant) with a true-breeding short stem variety plant (dwarf plant). Such a cross between two parents representing contrasting forms of a single characteristic is called monohybrid cross and the offspring is called a hybrid. He performed the experiment by transferring pollen grains from the anther of the tall plant to the stigma of the short plant.
Self-pollination was prevented by removing all the stamens from the dwarf plant. The plants of parental generation were designated as P1. The seeds from the dwarf plant were then collected and sown. He allowed these plants to self fertilize. He found that all plants which grew from these seeds were tall plants. The plants in this generation were called F1 generation or first filial generation. Stage III: The plants of F1 generation were allowed to self pollinate and the seeds were collected.
When these seeds were sown, few plants were tall and few were short. These plants were called second filial generation (or F2 generation).