92. Three somatic hybrid ceil lines, designated as X, Y and Z, have been scored for the presence or absence of chromosomes 1 through 8, as well as for their ability to produce the hypothetical gene product A, B, C and D) as shown In the following table:

Which of the following option has most appropriately assigned chromosomes for each of the given genes?
(1) Gene A on chromosome 5. Gene B on chromosome 3, Gene C on chromosome 8 and Gene D on chromosome 1
(2) Gene A on chromosome S and Gene B on chromosome 3 only
(3) Gene D on chromosome 8, Gene C on chromosome 1, Gene B on chromosome S and Gene A on chromosome 4
(4) Gene A on chromosome 5. Gene B on chromosome 3 and Gene D on chromosome 1

Introduction
Somatic cell hybridization questions in CSIR NET Life Sciences often test the ability to correlate presence or absence of human chromosomes with expression of specific gene products. By analyzing different hybrid cell lines, one can assign each gene to a particular chromosome with high accuracy, which is a classic application of somatic cell hybrid mapping.​

Question data interpretation

The question provides three somatic hybrid cell lines X, Y and Z, each containing a different subset of human chromosomes 1–8 and showing presence (+) or absence (−) of gene products A, B, C and D. The task is to assign each gene to its most likely chromosome based on co-segregation of chromosomes and gene products across the three cell lines.​

From the standard CSIR NET December 2015 version of this question, the correct mapping derived is:

• Gene A → chromosome 5

• Gene B → chromosome 3

• Gene C → chromosome 8

• Gene D → chromosome 1​

This pattern best explains which gene products are produced (+) or absent (−) in each of the X, Y and Z hybrid cell lines.​

Stepwise logic for gene assignment

The principle used is that a gene product will be expressed only in those hybrid cell lines that retain the chromosome carrying that gene. Conversely, if a chromosome is absent in a line, the corresponding gene product should also be absent, assuming no regulation or epigenetic silencing is involved.​

For each gene product (A, B, C, D):

• Compare the +/− pattern of that product across X, Y and Z with the +/− pattern of each chromosome 1–8.

• The chromosome whose presence/absence matches the gene product pattern is the most likely location of that gene.​

When this matching is done for all four gene products in the original CSIR NET table, the best overall fit is A on 5, B on 3, C on 8 and D on 1, which is provided as option (1) in the official key.​

Evaluation of option (1)

Option (1): Gene A on chromosome 5, Gene B on chromosome 3, Gene C on chromosome 8 and Gene D on chromosome 1.​

• For gene A, the presence pattern (+/− across X, Y, Z) most closely follows the presence of chromosome 5 in the three cell lines, so chromosome 5 is the best assignment for gene A.​

• For gene B, its expression pattern parallels that of chromosome 3; whenever chromosome 3 is present, B is expressed, and when 3 is missing, B is absent.​

• For gene C, expression correlates with chromosome 8, matching presence and absence in each cell line.​

• For gene D, the on/off pattern coincides with the presence of chromosome 1, supporting its assignment to chromosome 1.​

Since each gene product’s pattern is fully and uniquely explained by the corresponding chromosome pattern, this option is logically consistent and is the correct answer.​

Evaluation of option (2)

Option (2): Gene A on chromosome 5 and Gene B on chromosome 3 only.​

• This option correctly places gene A and gene B, because their patterns still match chromosomes 5 and 3, respectively.​

• However, it ignores gene C and gene D, even though the data clearly allow mapping them to chromosomes 8 and 1, respectively, using the same co-segregation logic.​

Because the question explicitly asks for appropriately assigned chromosomes for each of the given genes A, B, C and D, leaving C and D unassigned makes this option incomplete and therefore incorrect.​

Evaluation of option (3)

Option (3): Gene D on chromosome 8, Gene C on chromosome 1, Gene B on chromosome 5 and Gene A on chromosome 4.​

• Here the assignments are essentially shuffled among the chromosomes, so the gene expression patterns no longer match the respective chromosome presence patterns.​

• For example, the pattern of gene D expression does not coincide with the distribution of chromosome 8 across the three lines; there will be at least one line where 8 is present but D is absent, or vice versa.​

Since multiple mismatches arise between gene product patterns and chromosome patterns, this option contradicts the core principle of somatic hybrid mapping and is rejected.​

Evaluation of option (4)

Option (4): Gene A on chromosome 5, Gene B on chromosome 3 and Gene D on chromosome 1.​

• This option correctly gives the locations for A, B and D, and therefore matches part of the expression data set.​

• However, gene C is not assigned to any chromosome, even though the data permit a clear mapping to chromosome 8.​

As the question requires assignments for each of A, B, C and D, omitting gene C makes this option incomplete and thus not the best answer.​

Key exam takeaway

Somatic cell hybridization is widely used to map genes to specific chromosomes based on co-segregation of gene expression and chromosome retention in hybrid panels. For CSIR NET Life Sciences, always align each gene product’s +/− pattern with chromosome +/− patterns across all cell lines and choose the option that explains all genes simultaneously and completely. Here, only option (1) satisfies that condition, so it is the correct choice.