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,he got both red and white flowered plants in a $3:1$ ratio. Explain the basis of using $RR$ and $rr$ symbols to represent the genotype of plants of the parental generation.

(A) When crossing red and white flowered plants,only red-flowered plants appeared in the $F_1$ generation. However,white-flowered plants reappeared in the $F_2$ generation,which was derived from self-pollinating the $F_1$ individuals.
Mendel reasoned that there is a 'factor' (now called a gene) for every character. Accordingly,there must be one factor $(R)$ for the red flower trait and another factor $(r)$ for the white flower trait.
If an organism possessed only one copy of the gene,the reappearance of the white flower trait in the $F_2$ generation would be impossible. Furthermore,the observed phenotypic ratio of $3:1$ (red to white) indicates that each organism must possess two copies of a particular gene,which segregate during gamete formation. This observation forms the basis of Mendel's Law of Segregation and the Law of Dominance.