48. Segregation of alleles can occur either at anaphase I or anaphase II of meiosis. Which one of the following is an ideal model system for identifying the stage at which allelic segregation occurred?
(1) Arabidopsis thaliana
(2) Drosophila melanogaster
(3) Neurosporacrosso
(4) Saccharomyces cerevisiae

Introduction:
Segregation of alleles during meiosis can occur at either anaphase I or anaphase II, depending on the position of the gene relative to the centromere and the occurrence of crossing over. To pinpoint the exact meiotic division where allelic segregation takes place, an organism is needed in which all meiotic products can be observed in an ordered manner. Among common model organisms, Neurospora crassa provides this unique advantage, making it the ideal system for such analysis.​

Correct answer:
(3) Neurospora crassa

Concept: Allelic segregation at anaphase I vs anaphase II

During meiosis I, homologous chromosomes separate at anaphase I, which can cause first-division (centromere) segregation of alleles if no crossover occurs between the gene and the centromere. If crossing over happens between a gene and its centromere, sister chromatids may carry different alleles, so the final separation of alleles happens at anaphase II, giving second-division segregation.​

To know at which division (anaphase I or II) a particular allele pair segregated, one must:

• Recover all the products of a single meiosis, and

• Know their order reflecting the sequence of nuclear divisions.​

This is exactly what an ordered tetrad/octad system like Neurospora crassa provides.​

Option (3): Neurospora crassa – Why it is ideal

Neurospora crassa is a filamentous ascomycete fungus that forms asci containing eight linearly arranged ascospores (an octad) that correspond directly to the four products of meiosis plus a postmeiotic mitosis. The linear order of ascospores in each ascus preserves the spatial history of chromosome segregation, so patterns like 4:4 or 2:2:2:2 directly report first-division vs second-division segregation.​

Key reasons Neurospora crassa is ideal for identifying stage of allelic segregation:

• Ordered asci: Ascospores are arranged in a strict linear order corresponding to the order of chromatids on the meiotic metaphase plate.​

• Tetrad/octad analysis: All four meiotic products (and their mitotic derivatives) can be recovered and genotyped, allowing distinction between first-division segregation (e.g., 4A:4a blocks) and second-division segregation (e.g., 2A:2a:2A:2a patterns).​

• Classic system for centromere mapping: The frequency of second-division segregation is directly used to estimate gene–centromere distance in Neurospora, explicitly relying on whether segregation occurred at anaphase I or II.​

Because of these features, standard genetics texts and exam resources consistently identify Neurospora crassa as the ideal model system for determining whether allelic segregation occurs at anaphase I or anaphase II.​

Therefore, option (3) Neurospora crassa is the correct answer.​

Option (1): Arabidopsis thaliana – Why it is not the best here

Arabidopsis thaliana is a premier model plant and is widely used for meiotic studies and recombination mapping, but it does not naturally present all four meiotic products in a simple, ordered linear structure like an ascus. In wild-type Arabidopsis, meiotic products become separate pollen grains or ovules, so the direct, ordered recovery of all four daughter nuclei from a single meiosis is not straightforward.​

Some special mutants (such as quartet1, qrt) keep the four pollen products attached, enabling tetrad analysis in Arabidopsis, but:

• The question generally refers to the classical, textbook ideal model system for identifying whether alleles segregate at anaphase I or II, which is Neurospora crassa, not Arabidopsis.​

• Even with qrt, the system is more complex and not the standard teaching model for centromere vs gene segregation patterns, whereas Neurospora is the classical and most direct system.​

Thus, although Arabidopsis can be adapted for tetrad analysis, it is not considered the ideal or classical system for identifying the stage of allelic segregation in the context of the exam question.​

Option (2): Drosophila melanogaster – Why it is not ideal here

Drosophila melanogaster is a powerful genetic model organism, especially for linkage, recombination, and developmental genetics, but it does not allow routine recovery of all four products of a single meiosis in an ordered fashion. In Drosophila, meiotic products contribute to gametes that are separated; tetrad analysis is generally not feasible except in special cytogenetic setups (e.g., attached X chromosomes) and even then not as cleanly ordered as in Neurospora.​

Important limitations in this context:

• No natural ordered tetrads: There is no ascus-like structure with linearly arranged meiotic products, so one cannot directly “read off” first- vs second-division segregation patterns.​

• Allelic segregation is inferred indirectly from progeny classes rather than directly from the ordered products of a single meiosis.​

Because the question specifically asks for an ideal model to identify whether segregation occurred in anaphase I or II, Drosophila does not meet the core requirement of ordered meiotic products and is therefore not the correct choice.​

Option (4): Saccharomyces cerevisiae – Good but not the ideal teaching model here

Saccharomyces cerevisiae (budding yeast) is an excellent eukaryotic genetic model, and its asci contain four spores (a tetrad), all of which can be dissected and analyzed. Tetrad analysis in yeast is extensively used for mapping genes and studying recombination, and in principle, first- vs second-division segregation can also be inferred with appropriate centromere markers.​

However, there are reasons it is not selected as the ideal system in this standard question:

• Yeast asci are typically unordered with respect to the four spores (in many practical situations the precise linear order relative to the meiotic divisions is not used the way it is in Neurospora).​

• Classical textbook discussions of identifying first- vs second-division segregation and gene–centromere distances highlight Neurospora crassa, specifically because of its ordered, linear asci.​

Although Saccharomyces cerevisiae is powerful for tetrad analysis, Neurospora’s strict linear order of ascospores makes it the archetypal model for clearly distinguishing whether allelic segregation took place at anaphase I or anaphase II.​

Summary for exam perspective

• Requirement: To identify whether alleles segregated at anaphase I or anaphase II, the organism must allow recovery of all meiotic products in an ordered manner that preserves the history of chromatid segregation.​

• Neurospora crassa uniquely provides ordered asci with linearly arranged ascospores, making patterns of first- vs second-division segregation directly visible.​

• Other options (Arabidopsis thaliana, Drosophila melanogaster, Saccharomyces cerevisiae) are important genetic models but do not serve as the classical, ideal system for this specific purpose in standard genetics and CSIR-NET style questions.​

Hence, the correct answer is:
(3) Neurospora crassa.​