Chapter 11, Problem 15A

Summary Introduction

Introduction:

According to Mendel’s law of inheritance, if an organism is heterozygous for a trait, its phenotype will be that of the dominant trait. Incomplete dominance is an inheritance pattern in which the heterozygous genotype results in an intermediate phenotype between the dominant and recessive phenotype. In cases such as radish, a heterozygous offspring of red and white radishes shows an intermediate phenotype (purple radish) between the two homozygous phenotypes.

Answer to Problem 15A

Correct answer :

The correct answer is option C. 1:2:1 red: purple: white

Explanation of Solution

Explanation/justification for the correct answer:

Option C. 1:2:1 red: purple: white−A purple radish ( Rr ) is produced by a cross between red ( RR ) and white ( rr ) radish plants due to incomplete dominance.

A Rr radish plant will produce two types of gametes- male and female- each with either the R allele or r allele. If a purple radish ( Rr ) is crossed with another purple radish ( Rr ), there are three possible genotypes; RR, Rr and rr . So the resulting phenotypic ratio will be 1: 2:1 red: purple: white as 1 red, 2 purple and 1 white radishes will be produced.

This can be checked by drawing a Punnett square as shown below.

	R	r
R	RR	Rr
r	Rr	rr

Hence this is the correct option.

Explanation for incorrect answer:

Option A. 2:2 red: white- When two heterozygous plants of radish are crossed, the following phenotypes are produced. So the resulting phenotypic ratio is 1: 2:1 red: purple: white as 1 red, 2 purple and 1 white radishes are produced.

	R	r
R	RR	Rr
r	Rr	rr

Hence this is not the correct option.

Option B. 1:1:1 red: purple: white- When two heterozygous plants of radish are crossed, the following phenotypes are produced. So the resulting phenotypic ratio is 1: 2:1 red: purple: white as 1 red, 2 purple and 1 white radishes are produced.

	R	r
R	RR	Rr
r	Rr	rr

Hence this is not the correct option.

Option D. 3:1 red: white- When two heterozygous plants of radish are crossed, the following phenotypes are produced. So the resulting phenotypic ratio is 1: 2:1 red: purple: white as 1 red, 2 purple and 1 white radishes are produced.

	R	r
R	RR	Rr
r	Rr	rr

Hence this is not the correct option.