In a monohybrid cross of plants with red and white flowered plants, Mendel got only red flowered plants. On self-pollinating these $F_1$ plants got both red and white flowered plants in $3:1$ ratio. Explain the basis of using $RR$ and $rr$  symbols to represent the geno type of plants of parental generation.

On crossing red and white flower only red colour flower appeared in the $F_{1}$ generation. But the white colour flower again appear in the $\mathrm{F}_{2}$ generation which is raised out of the $\mathrm{F}_{1}$ individuals only. Mendel reasoned that there is a factor of each and every character. Accordingly, there has to be one factor ($R$) for red flower and other one factor ($r$) for white flower. In case, an organism possess only one copy of the gene then the possibility of reappearance of white flower in the $\mathrm{F} 2$ generation of the given cross is not there. Also the ratio ($3:1$ of red and white) indicates that each organism must possess two copies of a particular gene. This is Mendel's first law of inheritance i.e law of dominance.