It’s a phenomenon in which the F1 hybrid exhibits intermediate characters of the parental gene.
It is seen in flower colours of Mirabilis jalapa (4 O'clock plant and Antirrhinum majus (Snapdragon).
In a cross between true breeding red flowered (RR) and true breeding white flowered plants (rr), the F1 (Rr) was pink. When the F1 was self-pollinated the F2 resulted in the following ratio 1 Red (RR) : 2 Pink (Rr) : 1 White (rr). Here the phenotypic ratio deviates from the Mendel's monohybrid cross.
Both phenotypic and genotypic ratio will be the same 1:2:1.