Explanation

The genotypic and phenotypic ratio of 1 : 2 : 1 with red, pink and white flowers are produced in $Mirabilis\ jalapa$, when red flowered plants (RR) are crossed with white flowered (rr). It occurs due to allelic gene interactions, called, incomplete dominance. In which, both of the allelomorphic genes will have partial or incomplete dominance and $F_{1}$-hybrid will show mixture of characters of two parents.