AIIMS 2007 Botany Mendelian Inheritance MCQ Question

To solve the question about the inheritance of flower color in plants, we need to apply the principles of Mendelian genetics. Let's break down the problem step by step.

Given Information:

• A homozygous red-flowered plant: We can denote its genotype as $RR$, where $R$ represents the allele for red flowers.
• A homozygous white-flowered plant: Its genotype will be denoted as $rr$, where $r$ represents the allele for white flowers.

Cross of the Plants:

When we cross these two homozygous plants, we can set up a Punnett square to visualize the possible genotypes of the offspring.

Punnett Square Setup:

• The alleles from the red-flowered parent ($RR$) can only contribute $R$ alleles.
• The alleles from the white-flowered parent ($rr$) can only contribute $r$ alleles.

The Punnett square will look like this:

$$\begin{array}{c|c|c} & R & R \\ \hline r & Rr & Rr \\ \hline r & Rr & Rr \\ \end{array}$$

Offspring Genotype:

From the Punnett square, we can see that all the offspring will have the genotype $Rr$.

Phenotype of Offspring:

Since $R$ (red) is dominant over $r$ (white), all the $Rr$ offspring will express the dominant trait, which is red flower color.

Conclusion:

• The offspring will all be red-flowered, which corresponds to option D) all red-flowered.

Clarification of Other Options:

• A) half-white flowered: This is incorrect because none of the offspring will have the genotype $rr$ (the genotype for white flowers).
• B) half red-flowered: This is incorrect because all of the offspring are red-flowered, not just half.
• C) all white-flowered: This is incorrect for the same reason as option A; none of the offspring can be white-flowered since they all carry at least one dominant $R$ allele.

Key Concepts:

1. Mendelian Inheritance: Traits are determined by alleles, and when different alleles are present, the dominant allele will mask the effect of the recessive allele.
2. Genotype vs. Phenotype: The genotype refers to the genetic makeup (e.g., $Rr$), while the phenotype refers to the physical expression of that genotype (e.g., red flowers).

Summary:

When a homozygous red-flowered plant ($RR$) is crossed with a homozygous white-flowered plant ($rr$), all offspring will exhibit the dominant red flower trait due to their $Rr$ genotype. Therefore, the correct answer is D) all red-flowered.