The given situation is an example of incomplete dominance where phenotype found in F1 generation do not resemble either of the two parents. The genotype of the two plants used for cross will be
The incomplete dominance of dominant allele (here 'R') over recessive allele (here 'r') could be due to mutations (insertion, deletion, substitution or inversion of nucleotides). The mutant allele generally produces a faulty or no product. This modification in the product may lead to incomplete dominance of the (unmodified) wild type dominant allele.