Gene Expression

The given question involves understanding the functioning of the lac operon and involves determining the accuracy of both the assertion and the reason provided. Let's break down the question step-by-step.

1. Understanding the Assertion:

   Assertion [a]: lac operon is an inducible operon.

   An inducible operon is one that is usually in an "off" state, but can be turned "on" by an inducer. The lac operon in E. coli is a classic example of an inducible operon. In the absence of lactose, a repressor protein binds to the operator region of the operon, preventing transcription. When lactose is present, it acts as an inducer by binding to the repressor protein, causing it to change shape and release from the operator, allowing transcription to proceed. Hence, the assertion that the lac operon is inducible is true.

2. Understanding the Reason:

   Reason [r]: lac operon is not induced when the repressor protein remains bound to the operator DNA sequence.

   This is a correct statement. The function of the repressor is to prevent the expression of the genes in the operon by binding to the operator region. When the repressor is bound, the RNA polymerase cannot transcribe the downstream genes required for lactose metabolism. Therefore, the operon remains "off," confirming that this reason is true.

3. Analyzing the Relationship Between Assertion and Reason:

   Both the assertion and the reason are statements about the lac operon, and both are true. However, the reason does not directly explain why the lac operon is classified as an inducible operon. Instead, it explains a mechanism of how the operon is kept "off" in the absence of the inducer (lactose). Therefore, the reason, while true, is not the correct explanation for the assertion being true.

4. Conclusion:

   The correct answer is: Both [a] and [r] are true but [r] is not the correct reason for [a].

The question asks, "A random fluctuation in gene frequency is called?" The correct answer is Genetic drift. Let's explore this in more detail:

Explanation

Genetic drift is a concept in biology that refers to random changes in the frequency of alleles (gene variants) within a population. These changes are influenced by chance events rather than natural selection, mutation, or migration.

• Genetic Drift: This is the correct answer. It occurs in all populations of non-infinite sizes but its effects are most pronounced in small populations. Random fluctuations can lead to the increase or decrease of certain alleles over generations, sometimes even leading to their fixation or complete loss. It is a mechanism of evolution and differs from natural selection because it is not driven by environmental pressures or survival advantages.
• Genetic Load: This refers to the presence of unfavorable genetic mutations in a population. Unlike genetic drift, it does not involve random fluctuations but is more about the genetic burden on a population.
• Panmixis: This describes a population where all individuals are potential mates without restrictions, leading to random mating. It is not related to fluctuations in gene frequency but rather to the mating patterns within populations.
• Genetic Shift: This term is not commonly used in the context of allele frequency fluctuations. Genetic drift might sometimes be confused with genetic shift; the latter more specifically relates to significant evolutionary changes, often in viral populations.

In conclusion, genetic drift is distinct because it is a random process that can have significant evolutionary consequences in certain population dynamics, primarily due to its stochastic nature.