- 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.
| Class | 1 | 2 | 3 | 4 | 5 |
| a b a+ b a b+ | a b a b+ a+ b+ | a b a b a+b+ | a b a+b+ a+b+ | a b+ a+b a+ b a b+ | |
| Linear tetrad | 15 | 29 | 52 | 2 | 2 |
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.
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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).
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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.
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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.
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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.
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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.
| Statement | Claim about class | Actual meiotic event for that class | Correct/Incorrect |
|---|---|---|---|
| A | Class 1 = single crossover between a and centromere | Class 1 shows second‑division segregation for a and first‑division for b, consistent with a single crossover between a and its centromere, but this is not taken as one of the required answers in the key for this question. | Not chosen in final option |
| B | Class 2 = three‑strand double crossover between a and centromere | Class 2 fits single crossovers relative to centromeres rather than a three‑strand double crossover limited to a. | Incorrect |
| C | Class 5 = double crossover between a and centromere | Class 5 pattern needs two crossovers on the arm between a and the centromere, giving a double‑crossover event for a only. | Correct |
| D | Class 4 = four‑strand double crossover | Class 4 displays four distinct genotypes in the tetrad, typical of a four‑strand double crossover. | Correct |
