Question

The genotype of a plant showing the dominant phenotype can be determined by

• A. test cross
• B. dihybrid cross
• C. pedigree analysis
• D. back cross.

Answer 1

The Correct Option is A

To determine the genotype of a plant showing the dominant phenotype, a test cross is used. Let's delve into what a test cross is and why it is used for this purpose.

Understanding the Problem

In genetics, the phenotype refers to the observable characteristics of an organism that result from the interaction of its genotype with the environment. A dominant phenotype is one that expresses itself in the presence of just one copy of the allele responsible for the trait. However, it is usually undetermined whether the organism is homozygous dominant (has two copies of the dominant allele) or heterozygous (has one dominant and one recessive allele).

What is a Test Cross?

A test cross involves crossing the individual with an unknown genotype with another individual that is homozygous recessive for the trait in question. The key is that if any offspring manifest the recessive phenotype, the unknown parent must be heterozygous.

Step-by-Step Explanation:

1. Identify the Purpose: The test cross aims to reveal the unknown genotype of an individual demonstrating the dominant phenotype. This is crucial in plants where we cannot visibly determine the genotype.
2. Select the Appropriate Cross: The individual with the dominant phenotype is crossed with a homozygous recessive individual (genotype: rr).
3. Interpret the Results:
   • If all offspring show the dominant phenotype, the test subject is likely homozygous dominant.
   • If there is a 1:1 ratio of dominant to recessive phenotypes in the offspring, the test subject is heterozygous.

Ruling Out Other Options:

• Dihybrid Cross: This involves studying two traits simultaneously and is not used solely for determining unknown genotypes of one trait.
• Pedigree Analysis: This is more suitable for studying genetic patterns in families rather than determining an individual's genotype directly.
• Back Cross: While similar, a back cross refers to the crossing of a hybrid with one of its parents and is not exclusively intended to determine dominant phenotype genotypes.

Conclusion:

Considering the possible approaches to reveal an unknown genotype for a dominant phenotype, the test cross accurately serves this purpose by identifying whether the dominant phenotype individual is homozygous or heterozygous.