Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
• [Assertion (A):] Incomplete penetrance and variable expressivity confuse the relationship between the genotype and phenotype. 
• [Reason (R):] Progeny test for more than one generation may be used to study the nature of penetrance and expressivity, and to establish the genotype of a plant beyond reasonable doubt. 
In light of the above statements, choose the correct answer from the options given below. 
 

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To evaluate Assertion-Reason questions of this type:
- Ask yourself: Does the Reason statement answer "Why" the Assertion is true?
- Here, the progeny test is a *solution* to find the genotype, not the *biological cause* of incomplete penetrance or variable expressivity.
- This confirms that both are true, but the Reason is not the correct explanation.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In classical Mendelian genetics, a specific genotype is expected to produce a predictable phenotype.
However, phenomena such as incomplete penetrance and variable expressivity alter this direct, one-to-one relationship.
Penetrance refers to the proportion of individuals with a specific genotype who express the expected phenotype.
Expressivity refers to the range of severity or intensity with which a particular genotype is expressed in different individuals.

Step 3: Detailed Explanation:
Let us analyze the Assertion and Reason statements:
- Assertion (A) is true: Incomplete penetrance and variable expressivity alter the phenotypic expression of genes.
When penetrance is incomplete, some individuals carrying a dominant mutant allele appear phenotypically normal, resembling homozygous recessive individuals.
When expressivity is variable, individuals with identical genotypes display different phenotypes.
These factors obscure or "confuse" the relationship between genotype and phenotype, making it difficult to infer an individual's genotype simply by observing its phenotype.
- Reason (R) is true: To overcome this phenotypic confusion, geneticists use progeny testing over multiple generations.
By crossing the suspect plant and observing the segregation ratios among its offspring across generations, researchers can track the inheritance of the gene.
This multi-generational analysis allows them to calculate the penetrance percentage, observe the range of expressivity, and identify the true genotype of the parent plant.
- Evaluating the connection: While both statements are scientifically true, the Reason describes a method used to resolve the phenotypic confusion caused by these phenomena.
The Reason does not explain *why* incomplete penetrance and variable expressivity confuse the genotype-phenotype relationship.
The confusion itself is caused by modifier genes, environmental influences, and gene-interaction networks, not by progeny testing.
Therefore, both statements are true, but (R) is not the correct explanation of (A).

Step 4: Final Answer:
Both (A) and (R) are true, but (R) is not the correct explanation of (A), matching Option (B).