84. The location of six deletion (Shows as solid line underneath the chromosomes) has been mapped to the Drosophila chromosome as shown in the diagram given below:
The recessive mutations a, b, c, d, e f and g are known to be located in the region of deletions, but order of mutations on the chromosome is not known. When flies homozygous for the recessive mutations are crossed with flies homozygous for the deletion, the following result were obtained where the letter ‘m’ represents a mutant phenotype and ‘+’ represents the wild type.
The relative order of seven mutant genes onchromosome is:
(1)bceafgd (2) b c d f g e a
(3) bc deafg (4) c d e a g f b
Introduction
Deletion mapping in Drosophila is a powerful technique used to localize recessive mutations by crossing them with chromosomes carrying defined deletions. In this CSIR‑NET style problem, six overlapping deletions and seven recessive point mutations (a–g) are used to deduce the linear gene order, which turns out to be b c e a f g d. Understanding the logic behind each deletion and phenotype pattern is essential for quickly solving similar exam questions.
Concept of deletion mapping
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A recessive mutation shows mutant phenotype (m) when placed over a deletion that removes the wild‑type copy of that gene, because no functional allele is present.
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The same mutation shows wild type (+) when the deletion does not include that locus; recombination can generate a wild-type chromosome or the remaining homolog still carries a functional copy.
Thus, for each deletion, the set of mutations scored as m must lie within the deleted segment, whereas those scored as + must lie outside that segment.
Step‑by‑step mapping of genes a–g
From the original question (deletion pattern diagram plus the m/+ table), each deletion spans a known continuous region along the chromosome. The pattern of mutant phenotypes across deletions allows localisation of each gene as follows (worded, not drawn):
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Gene b shows mutant phenotype with the broadest left‑side deletions and is wild type with right‑most deletions, indicating that b lies at the extreme left of the mapped region.
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Gene d shows mutant phenotype only with the right‑most deletion that extends furthest, placing d at the extreme right end.
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Genes c and e are uncovered together by some central deletions but differ in others, so they must lie just to the right of b, with c slightly left of e.
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Gene a is uncovered by deletions overlapping the middle of the chromosome, lying to the right of e but left of f.
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Genes f and g are uncovered by progressively more right‑hand deletions, placing f immediately to the right of a and g between f and d.
Aligning all these positional constraints yields the only consistent linear order:
b – c – e – a – f – g – d.
Why each option is right or wrong
| Option | Written order | Correct or not | Reason |
|---|---|---|---|
| (1) | b c e a f g d | Correct | Places b at the far left and d at the far right; keeps c and e left of a, and f and g between a and d, matching all deletion–phenotype patterns. |
| (2) | b c d f g e a | Incorrect | Puts d in the middle instead of the right end, which would require central deletions to uncover d, contrary to the table where only the longest right‑side deletion gives mutant d. |
| (3) | b c d e a f g | Incorrect | Again places d centrally and separates e from its required central‑left position relative to b and c; some deletions that uncover e but not d cannot be explained with this order. |
| (4) | c d e a g f b | Incorrect | Moves b to the right end and c to the extreme left, contradicting the pattern where b is affected by left‑side deletions and d only by right‑most deletions; also swaps f and g relative to their uncovered regions. |
Because only option (1) satisfies all left‑to‑right constraints implied by each deletion’s mutant set, it is the unique valid gene order.
