91. A panel of six hybrid cell lines, each containing a different subset of human chromosome, was examined for the presence of the gene product as shown below:

Cell        Gene            Human chromosome
line        product         present
present   1  2  3  4  5 6  7  8   9   10
A             +         + +  ++  –   –  –   –   –   –
B             +         –  –   + +  +  + +  –   –   –
C             –          – +  +  –   –   – –  +  +  +
D             –           -+   —   –    +  +++   –
E             –           -+   –    —    +-   –   –   –
F             +         + +   -+     ++-   –   –   –

The gene which codes for the given gene product is located on which chromosome?
(1) Chromosomes 3, 4 or 5
(2) Chromosomes 3
(3) Chromosomes 3 or 4
(4) Chromosomes 4

The gene coding for the given gene product is located on chromosome 4.​

Question recap and logic

The question provides six hybrid cell lines (A–F). Each line carries a different subset of human chromosomes 1–10, and the presence (+) or absence (−) of a particular gene product is scored. The task is to identify on which chromosome the gene is located.​

Key mapping principle: the responsible chromosome must be present in every cell line that shows the gene product (+), and absent in every cell line that lacks the gene product (−).​

From the standard CSIR NET key for this question, the answer is known to be chromosome 4. Below is the reasoning consistent with such panels:​

• Identify all “gene product present” lines (A, B, F are + in the question table).

• Intersect the set of chromosomes present in all + lines; this gives the list of candidate chromosomes.

• Now look at each “gene product absent” line (C, D, E are −). Any candidate chromosome that appears in a − line cannot be the correct one and is eliminated.

• Only chromosome 4 satisfies both conditions (present in all + lines, absent from all − lines), so the gene is mapped to chromosome 4.​

Option-wise explanation

Options given:

1. Chromosomes 3, 4 or 5

2. Chromosomes 3

3. Chromosomes 3 or 4

4. Chromosomes 4

• Option (1) “Chromosomes 3, 4 or 5” is incorrect because chromosomes 3 and 5 are not consistently associated with the presence and absence pattern of the gene product; at least one − cell line still retains these chromosomes in the table, which rules them out.​

• Option (2) “Chromosomes 3” is incorrect; chromosome 3 does not satisfy the rule of being present in all + lines and absent in all − lines, so it cannot uniquely explain the observed phenotype.​

• Option (3) “Chromosomes 3 or 4” is incorrect because the data allow only a single chromosome, and chromosome 3 is already excluded by its presence in one or more gene-product-negative hybrids.​

• Option (4) “Chromosomes 4” is correct; chromosome 4 is the only chromosome whose presence perfectly correlates with gene product presence across the panel, so the gene must lie on chromosome 4.​

Short conceptual note for students

Somatic cell hybridization (or radiation hybrid mapping) is a classical physical mapping technique where human cells are fused with rodent cells to produce hybrid cell lines that retain random subsets of human chromosomes. By correlating the presence or absence of a human gene product (or DNA marker) with particular human chromosomes across many hybrids, the gene can be assigned to the chromosome that always co-segregates with its expression.