Mendel proposed that two copies of factor (now called genes) controlling traits are present in sexually reproducing organism. These two may be identical, or may be different, depending on the parentage. The plants have genes which have the ability to produce enzyme for growth. If enzymes are efficient more growth hormones are produced thus plant grow in length. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone produced will be less and a plant will be short. Mendel used tall/dwarf plants since they were visible characters of garden peas. All plants were tall in F1 progeny - no 'medium-height' plants.
On self-pollination of F1 tall plants, the second-generation, or F2, progeny of the F1 tall plants are not all tall. Instead, one quarter of them are dwarf. This indicates that both the tallness and dwarfness traits were inherited in the F1 plants, but only the tallness trait was expressed. Thus, genes control the characteristics or traits in an organism.OR
Mendel's law of dominance can be deduced from the cross given below. The hybrid is heterozygous containing both the alternative alleles ( R and r ). However, only one trait, i.e. red colour appeared and the white colour trait was suppressed in the Fl generation. So, we can conclude that the red colour trait is dominant over the white colour trait. This explains Mendel's law of dominance.Thus 75% flowers will be Red ( RR and Rr) in colour and 25% flowers will be white (rr) in colour.