A plant breeder is trying to develop a deep purple violet. She knows the violets come in deep purple, light purple, and white. She crosses two violets hoping to produce only the deep color. Which parent plants should she use? At least one parent must be pp. Both parents must be Pp. At least one parent must be Pp. Both parent must be PP.

The correct answer is: Both parent must be PP.

Explanation:

The given question states that for flower color of the violet plants there are three phenotypes possible, purple, light purple and white. The flower color coding gene is governed by two variants or alleles in which there is an absence of complete dominance of one allele over the other as, if complete Mendelian dominance was there, only two phenotypes for flower color would have existed, purple and white, without the existence of any intermediate color, light purple. This indicates the existence of a Non-Mendelian pattern of inheritance with respect to the gene encoding for the violet flower color. As the intermediate phenotype of flower color is light violet, it indicates that in these plants neither of the two alleles are capable of showing dominance and this is an example of Incomplete dominance. Let the two alleles of the flower color coding gene be: P and p. Then the possible genotypes are: PP, Pp, pp. The corresponding phenotypes are: Purple, Light purple and White. Crossing all possible parents,

1. P P

P PP PP

Phenotypes (Purple) (Purple)

P PP PP

Phenotypes (Purple) (Purple)

2. P P

P PP PP

Phenotypes (Purple) (Purple)

p Pp Pp

Phenotypes (Light Purple) (Light Purple)

3. P p

P PP Pp

Phenotypes (Purple) (Light Purple)

p Pp pp

Phenotypes (Light Purple) (White)

4. P P

p Pp Pp

Phenotypes (Light Purple) (Light Purple)

p Pp Pp

Phenotypes (Light Purple) (Light Purple)

5. P p

p Pp pp

Phenotypes (Light Purple) (White)

p Pp pp

Phenotypes (Light Purple) (White)

6. p p

p pp pp

Phenotypes (White) (White)

p pp pp

Phenotypes (White) (White)

According to the results of the above crosses, we find that the plant breeder should use such parents which are homozygous for the P allele, that is, both the parents must have the PP genotype. Then only all the offspring produced by crossing these parents will develop purple colored violet only.