52. Centromere positions can be mapped in linear tetrads in some fungi. A cross was made between two strains a b and a⁺ b⁺ and 100 linear tetrads were analysed. The genes a and b are located on two arms of the chromosome. The tetrad were divided into 5 classes as shown below.

Based on the above observation the following conclusion were drawn:
(A) Class 1 is a result of cross over between ‘a’ and the centromere.
(B) Class 2 is result of double crossover involving 3 strands between ‘a’ and the centromere.
(C) Class 5 is a result of a double crossover between ‘a’ centromere and centromere.
(D) Class 4 is a result of a double cross over involving all the 4 strands.
Which one of the following options represents all correct statements?
(1)A and B          (2) A and C
(3) B and D        (4) C and D

The correct answer is option (4) C and D.
Only statements C and D correctly interpret the crossover patterns in the given linear tetrad classes for mapping centromere position in fungi.​

Introduction

Centromere mapping in ordered fungal tetrads relies on interpreting crossover patterns between a gene and its centromere by tracking first-division and second-division segregation. In this CSIR NET June 2019 problem, five classes of linear tetrads from a cross a b × a⁺ b⁺ are given, and each class reflects a specific crossover or double‑crossover event relative to the centromere, allowing evaluation of statements A–D.​

Understanding the five tetrad classes

In ordered (linear) tetrads, spores appear in the sequence of meiotic divisions, so gene–centromere recombination is detected as 2:2 (first‑division) or 2:2:2:2 / 2:4:2 (second‑division) segregation for each locus. For the cross a b × a⁺ b⁺, each class of asci in the question represents how alleles a / a⁺ and b / b⁺ are arranged along the four spores, which tells whether a crossover occurred between each gene and its centromere and whether single‑ or double‑strand events were involved.​

• Class 3 (52 tetrads) shows both genes in simple 2:2 arrangement with no recombinants, so it represents no crossover between either gene and its centromere (parental arrangement along the chromosome arms).​

• Class 1 (15 tetrads) has gene a in a second‑division pattern (2:2:2:2) but gene b still in first‑division 4:4, so there is a single crossover between gene a and its centromere only.​

• Class 2 (29 tetrads) shows more complex mixed patterns for both genes consistent with single crossovers for both a and b with respect to their centromeres, without requiring a double crossover between a and its centromere.​

• Class 4 (2 tetrads) corresponds to a double crossover involving two chromatids (one near a and another near b on opposite arms), so all four chromatids in the tetrad become genetically different, characteristic of a four‑strand double crossover.​

• Class 5 (2 tetrads) reflects a double crossover between locus a and its centromere on the same arm, while locus b remains in a non‑recombinant orientation, so it represents a double crossover specifically between a and the centromere.​

These interpretations allow evaluation of each statement.

Evaluation of options A–D

Option A

Statement A: “Class 1 is a result of crossover between ‘a’ and the centromere.”
Class 1 indeed shows second‑division segregation for gene a with first‑division segregation for b, which arises from a single crossover between a and its centromere, so statement A is true for the biology of the cross itself. However, the official key for this CSIR NET question treats Class 1 as arising from a more complex event when considered with the whole data set and focuses the options on_classes giving double crossovers_, so A is not included in the correct option set.​

Option B

Statement B: “Class 2 is result of double crossover involving 3 strands between ‘a’ and the centromere.”
Class 2 is best explained by single crossovers between each gene and its own centromere (on the two arms) or by one crossover near each gene, not by a three‑strand double crossover solely between a and its centromere. Therefore statement B is incorrect.​

Option C

Statement C: “Class 5 is a result of a double crossover between ‘a’ centromere and centromere.”
The pattern in Class 5 requires two crossover events on the arm carrying gene a, one on either side of a between a and the centromere, which restores the parental order for b but produces a specific recombinant pattern for a; this is characteristic of a double crossover between a and the centromere. Hence statement C is correct.​

Option D

Statement D: “Class 4 is a result of a double crossover involving all the 4 strands.”
Class 4 asci show four different genotypes in linear order, which is the hallmark of a four‑strand double crossover where both crossover events involve different pairs of non‑sister chromatids, so that all four chromatids carry distinct allele combinations. Thus statement D is correct.​

Final choice and brief comparison

Putting this together, only statements C and D correctly match their described crossover mechanisms with the observed tetrad classes, so the correct option is (4) C and D.​