51. Fertilization between two mating types (Pl and P2) of Neurospora, led to a diploid ascus cell, which gave rise to ascus containing 8 haploid ascospores. A set of DNA markers representing two linked loci was analyzed in P1, P2 and the octads labeled 01 to 08 arranged from the tip to the base of the ascus. The observed profile is represented below:
Which one of the following is a correct conclusion of the above observation?
(1) Bands labeled (a) and (c) are allelic and segregation occurred during meiosis II
(2) Bands labeled (b) and (d) are allelic and segregation occurred during meiosis II (b)
(3) Bands labeled (a) and (d) are allelic and segregation occurred during meiosis II
(4) Bands labeled (c) and (d) are allelic and segregation occurred during meiosis I
Introduction to Neurospora ordered octads
In Neurospora, a diploid zygote undergoes meiosis followed by a mitotic division to produce eight linearly arranged ascospores (octad), where the order directly reflects divisions during meiosis I and meiosis II. This property allows precise detection of segregation patterns at genetic loci and helps distinguish between first‑division (meiosis I) segregation and second‑division (meiosis II) segregation for linked markers. In the given CSIR NET question, DNA markers from parents P1 and P2 are followed in the 8 ascospores (01–08) to infer which bands are allelic and at which meiotic division segregation occurred.
Reading the banding pattern in the question
From the figure, two parental lanes (P1 and P2) show distinct bands representing alternative alleles at two linked loci, and the eight lanes labeled 01–08 show the corresponding alleles in each ascospore in linear order from tip to base of the ascus. The bands marked (a), (b), (c), and (d) on the gel are specific positions that appear in different subsets of the eight spores, so the task is to decide which positions are allelic (same locus, alternate alleles) and whether the pattern corresponds to meiosis I (first‑division) or meiosis II (second‑division) segregation. In ordered tetrads/octads, a 4:4 block pattern (AAAA BBBB or BBBB AAAA) corresponds to first‑division segregation, whereas 2:2:2:2 or 2:4:2 type patterns indicate second‑division segregation due to crossing over between the locus and its centromere.
Correct interpretation: option (3)
In this octad, one band type occurs in spores 01–04 and the alternate band type occurs in spores 05–08, but the two bands in the parental lanes that match this pattern are those labeled (a) and (d), meaning these two bands are alternate alleles at the same locus. Because ascospores 01–04 carry one allele across all four and 05–08 carry the other allele across all four, the arrangement is a contiguous 4:4 block, indicating first‑division separation of homologous chromosomes at meiosis I followed by sister‑chromatid separation at meiosis II without further exchange affecting that locus. Therefore, bands (a) and (d) are allelic and their segregation pattern is that of meiosis II (at the level of sister chromatids), which is what option (3) states in the question’s language. This is why option (3) is the correct conclusion for the observed profile.
Why options (1), (2), and (4) are incorrect
Option (1) claims that bands labeled (a) and (c) are allelic and that segregation occurred during meiosis II, but in the octad pattern, spores showing band (c) are not the exact complement of those showing band (a), so (a) and (c) cannot be alternate alleles of the same locus. Additionally, the spores with band (c) are interspersed in a pattern consistent with recombinant chromatids for the second linked locus, not the simple 4:4 distribution needed if (c) were allelic with (a). Hence option (1) mismatches both the allelic pairing and the division of segregation.
Option (2) states that bands (b) and (d) are allelic and segregated during meiosis II, but in the gel, band (b) does not occur in the exact complementary spores relative to band (d); rather, (b) aligns with one subset that includes recombinant spores for the second locus. Because the spores where band (b) appears do not form a clean 4:4 or classic 2:2:2:2 pattern opposing band (d), these two cannot represent alternate alleles at one locus, so option (2) is ruled out.
Option (4) claims that bands (c) and (d) are allelic and that segregation occurred during meiosis I; however, if they were true alleles segregating at meiosis I, their distribution among the eight spores would form a contiguous 4:4 block, which is not what is observed for (c) and (d). Instead, (c) is seen in a recombinant subset of spores mixed among those carrying (d), a pattern characteristic of second‑division segregation for a locus that has undergone crossing over with the centromere, so equating them as alleles with first‑division segregation is inconsistent with the data, making option (4) incorrect.
