Question

In Snapdragon, a cross between true-breeding red flowered (RR) plants and true-breeding white flowered (rr) plants showed a progeny of plants with all pink flowers.
(a) The appearance of pink flowers is not known as blending. Why?
(b) What is this phenomenon known as?

Answer 1

Hint:Blending is the combination of two colours, but red and white colours appear independent at the cellular level in this example.

Complete answer:
First we should know about true breeding to answer this question. A true breeding is a form of breeding in which the parents will bear offspring carrying the same phenotype. This suggests that with each characteristic, the parents are homozygous. In plants, true breeding happens when plants self-pollinate and produce only offspring of the same variety. A plant that has blue flowers, for example, can only develop seeds that will develop into plants that have blue flowers. Through true breeding, the trait is passed on to all generations that follow. For this to happen, the parents are homozygous for a trait, implying that the parents must be both dominant and recessive.
(a) The appearance of the pink flower is not known as blending because during gamete formation, different alleles do not combine with each other in the pink flower and segregate with each other (Law of Segregation)
(b) This condition is known as incomplete dominance.

Note: In the punnett square, we will get 25% (1/4) of the overall number of plants that carry red flowers (RR), 25% (1/4) that carry white flowers (rr) and 50% (2/4 = 1/2) that carry pink flowers (Rr). Hence, the ratio of genotypes = 1: 2: 1 [RR: rr: rr]
And the ratio of phenotypes = 1: 2: 1 [red: pink: white]
This is attributed to incomplete tolerance because here the dominant gene (red colour) is not fully dominant over the recessive gene (white colour).