- Somatic cell hybridization is used to assign a gene to aparticular chromosome. When two cell lines from twodifferent species are fused, they form a heterokaryonwhich tends to Jose chromosomes as they divide,preferentially from one species. A panel of cell lines wascreated from mouse-monkey somatic cell fusions. Eachline was examined for the presence of monkeychromosomes and for the production of a given enzyme.The following results were obtained:
| Cell line | Presence of Enzyme | Presence of Monkey Chromosomes | |||||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | ||
| A | + | + | + | + | + | + | – | + | – | + | + |
| B | + | – | + | + | – | + | + | + | – | + | – |
| C | – | – | + | – | – | – | – | – | + | + | + |
| D | + | + | + | + | – | – | + | + | + | + | – |
| E | – | – | – | – | + | + | – | – | – | + | + |
| F | + | + | + | – | + | + | + | + | – | + | – |
On the basis of these results, which chromosome has the gene that codes for the given enzyme?
(1) Chromosome 10 (2) Chromosome 7
(3) Chromosome 1 (4) Chromosome 5
Answer: Chromosome 7. The enzyme appears only when monkey chromosome 7 is present across hybrid lines, and it is absent whenever chromosome 7 is lost, indicating the gene is on chromosome 7.
Introduction
Somatic cell hybridization maps genes by correlating the presence of a donor species’ chromosomes with expression of a gene product in interspecific hybrids. As hybrids divide, chromosomes from one species are preferentially lost, so co-segregation of a specific chromosome with the enzyme pinpoints the gene’s chromosomal location.
How the method works
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Fuse cells from two species to form heterokaryons and select stable hybrid lines. Over passages, chromosomes from one species are lost in different combinations.
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Score each hybrid line for the donor chromosomes retained and for the phenotype (here, enzyme activity). The chromosome whose presence always coincides with enzyme presence is assigned as the locus carrier.
Step-by-step solution
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Read the table: each cell line lists “Presence of Enzyme” (+/−) and monkey chromosomes 1–10 retained (+/−). The locus lies on the unique chromosome that is present in all enzyme-positive lines and absent in all enzyme-negative lines. This is the concordant segregation principle used in somatic cell hybrid mapping.
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Inspect enzyme-positive lines (A, B, D, F): the common chromosome across these lines is 7; other chromosomes vary or are missing in at least one positive line.
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Inspect enzyme-negative lines (C, E): chromosome 7 is absent in both, reinforcing that loss of 7 abolishes enzyme activity. The only chromosome perfectly concordant with enzyme presence is 7, so the gene is on chromosome 7.
Option-wise explanation
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Option (1) Chromosome 10: Rejected because enzyme-positive line(s) lack 10 or enzyme-negative line(s) retain 10, breaking concordance. A valid assignment must show presence only with enzyme and absence without it.
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Option (2) Chromosome 7: Correct; it is present in every enzyme-positive line and absent in every enzyme-negative line, matching the mapping rule for co-segregation.
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Option (3) Chromosome 1: Rejected because at least one enzyme-positive line lacks chromosome 1, so it cannot be required for enzyme expression.
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Option (4) Chromosome 5: Rejected because chromosome 5 is not consistently present with the enzyme and/or appears in an enzyme-negative line, violating the concordance criterion.
Key takeaways
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Concordant segregation between a donor chromosome and a phenotype across independent hybrid lines assigns the gene to that chromosome.
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Somatic cell hybridization is a classical approach widely used to map genes to specific chromosomes in interspecific hybrids because donor chromosomes are lost in different combinations across clones.
