How many true-breeding pea plant varieties did Mendel select as pairs,which were similar except in one character with contrasting traits?

Genes are

Mendel conducted his hybridization experiments for how many years?

The segregation of Mendelian factors (no linkage,no crossing over) occurs during ............ .

What is a cross between two genotypes or phenotypes where the source of gametes is reversed in one cross called?

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.