In a cross between red-flowered (RR) and white-flowered (rr) plants of snapdragon, the $F_1$ plants were all pink-flowered.
What is the genotype of the $F_1$ and the reason for this phenotype?

The correct option is D Rr; Incomplete dominance
With respect to the inheritance of flower colour in snapdragon, allele ‘R’ imparts red colour to the petals of the flower and the allele ‘r’ imparts white colour to the petals of the flower.

However, the allele ‘R’ is not completely dominant over ‘r’ and due to this, a cross between true-breeding red-flowered (RR) and white -flowered plants (rr) results in pink-flowered plants (Rr).

Hence, the progeny of this cross does not resemble either of the two parents with respect to the flower colour, rather an intermediate phenotype is seen. This phenomenon is known as incomplete dominance.

In the case of co-dominance, the $F_1$ generation resembles both the parents.

Generally, a gene has two alleles, but certain genes have more than two alleles and these are called multiple alleles. However, in an individual, only two alleles are present.

Hence, inheritance of flower colour in snapdragon is an example of incomplete dominance.