Question

A cross between a pea plant with purple flower (PP) and a pea plant with white flower (pp), what will be the phenotypic ratio?
A. All white
B. All purple
C.Half purple, half white
D. 1 PP, 2 Pp, 1 pp
E. All pp

Answer 1

Hint:A phenotype is a physical character of an organism such as flower color, seed shape, leaf color, etc. A phenotypic ratio is the relative number of certain physical characteristics of the offspring.
A monohybrid cross is a cross between two organisms that are having variations and differ in one trait only. For example a cross between a pea plant having green seeds and another pea plant having yellow seed. Here, the trait differs in yellow and green seeds.

Complete answer:
Each of the given options are discussed below:
- All white- In a cross between a pea plant with purple flower (PP) and a pea plant with white flower (pp), the offspring will be 4 Pp (heterozygotes) pea plants where the allele P will be dominant over allele p. The allele P is responsible for purple color and allele p is responsible for white color. Since all are of Pp genotype and purple phenotype therefore the flowers cannot be all white.
- All purple- The offsprings are of Pp (heterozygote) genotype. According to Mendel’s law of dominance, in a heterozygote one trait will hide or mask the characteristics of another trait of the same characteristic. Only the dominant allele can express itself. The recessive allele can only express itself in monozygotic conditions. Here, the purple color trait is dominant therefore hides the white color trait for the same characteristic that is flower color. Hence, all the flowers are purple.
- Half purple, half white- The offspring all have a Pp genotype. According to Mendel’s law of dominance, only purple color flowers will be expressed in the offspring, not half purple and half white.
- 1 PP, 2 Pp, 1 pp- This genotypic ratio for the offspring is not possible as the cross produces only Pp genotypes.
- All pp- Since the p allele is recessive therefore pp genotype gives white flowers. This recessive genotype is not possible in the given cross.

Therefore the correct answer is (B)- All purple.

Note:
The genotype is the genetic characteristic of an organism. For example here, PP, Pp, and pp are the genotypes that can produce purple, purple, and white phenotypes respectively. The dominant trait can express itself both in heterozygotes and homozygotes whereas the recessive can only express itself in homozygotes. The genotypic trait is non-observable whereas the phenotypic traits can be observed on the body of a particular organism.