Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Brassica juncea is an allotetraploid having AABB genome.
Reason R: Brassica juncea is produced when B. nigra is crossed to B. campestris.
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:

Remember the classic amphidiploid relationships in the genus Brassica.
\[ B.\ campestris = AA,\qquad B.\ nigra = BB \] \[ B.\ juncea = AABB \] Thus, \(AA \times BB\), followed by chromosome doubling, gives \(AABB\).
For Assertion--Reason questions, remember that when the Reason directly describes the origin or mechanism responsible for the condition stated in the Assertion, the correct choice is usually the option stating that both are true and the Reason correctly explains the Assertion.

Answer 1

The Correct Option is A

•

Step 1: Understanding the Question:
This question deals with the origin and genomic constitution of Brassica juncea through interspecific hybridisation and chromosome doubling.
Brassica juncea, commonly known as Indian mustard, is an allotetraploid species with the genomic constitution \(AABB\).
The question asks whether this statement is correct and whether its origin from B. nigra and B. campestris correctly explains its genome.

•

Step 2: Key Formula or Approach:
The important genomic constitutions of the relevant Brassica species are:
\[ B.\ campestris = AA \] \[ B.\ nigra = BB \] When these two species are crossed, the hybrid initially receives one \(A\) genome and one \(B\) genome.
\[ AA \times BB \longrightarrow AB \] Chromosome doubling of the hybrid produces:
\[ AB \longrightarrow AABB \]
• Detailed Explanation:

• Brassica campestris is a diploid species having the \(AA\) genome.

• Brassica nigra is another diploid species having the \(BB\) genome.

• When \(B.\ campestris\) and \(B.\ nigra\) are crossed, the resulting hybrid receives one \(A\) genome and one \(B\) genome.
\[ AA \times BB \longrightarrow AB \]
• The \(AB\) hybrid has chromosomes originating from two different species and therefore has difficulty undergoing normal meiosis because the chromosomes do not have complete homologous partners.

• Chromosome doubling restores homologous pairing by producing two copies of each genome.
\[ AB \longrightarrow AABB \]
• The resulting fertile allotetraploid is Brassica juncea.

• Therefore, B. juncea possesses the \(AABB\) genomic constitution and is correctly classified as an allotetraploid.

• Thus, Assertion A is true.

• The statement in Reason R is also true because B. juncea originated through hybridisation between B. campestris \(AA\) and B. nigra \(BB\), followed by chromosome doubling.

• Since this interspecific hybridisation and subsequent chromosome doubling directly explain the \(AABB\) genome of B. juncea, Reason R correctly explains Assertion A.

• Final Answer:
Both Assertion A and Reason R are true.
Furthermore, the cross between \(AA\) and \(BB\), followed by chromosome doubling, directly explains the allotetraploid \(AABB\) constitution of Brassica juncea.
Therefore, the correct answer is:
\[ \boxed{\text{(A) Both A and R are true and R is the correct explanation of A}} \]