- The locations of five overlapping deletions have been mapped to a Drosophila Chromosome as shown below
(Horizontal lines in the above figure indicate the deleted regions) Recessive mutations a. b. c, d and e are known to be located within this region, but the order of mutations on the chromosome is not known. When the flies homozygous for the recessive mutations are crossed with flies homozygous for the deletions, the following results are obtained (letter “m” represents mutant phenotype and “+” represents the wild type).
On the basis of the above data, the relative order of the five mutant genes on the chromosome is
(1) bc de a (2) a b c d e
(3) bc e a d (4) c d be a
Problem recap
Five overlapping deletions (Def 1–5) span a chromosomal region in Drosophila. Homozygous mutants for recessive alleles a, b, c, d, e are crossed to flies homozygous for each deletion. “m” indicates failure to complement (mutant phenotype; gene lies within that deletion), while “+” indicates complementation (gene lies outside that deletion). Deletion mapping uses such complementation tests to localize genes within overlaps of deficiencies.
How to read deficiency data
-
Principle: A recessive mutant crossed to a deletion gives mutant F1 only when the deletion removes the wild-type copy of that same gene; overlapping deletions that both give mutant phenotype bracket the locus to their overlap. If one deletion gives “m” and an adjacent overlapping one gives “+”, the gene lies in the segment unique to the first deletion before the overlap ends.
Interpreting each mutation
Using the standard rule above:
-
Mutation a: m with deletions 4 and 5; + with 1–3. Therefore a lies only in the rightmost region covered by Df4 and Df5 but not by Df3, i.e., the terminal segment at the extreme right.
-
Mutation b: m only with deletion 1; + with 2–5. Hence b resides in the left-end segment unique to Df1 before it begins to overlap Df2.
-
Mutation c: m with deletions 1 and 2; + with 3–5. Therefore c is in the overlap of Df1 and Df2 but left of Df3. This makes c just to the right of b.
-
Mutation d: m with deletions 1–3; + with 4–5. Thus d lies in the region where Df1, Df2, and Df3 overlap, but left of where Df4 starts; this places d to the right of c but left of e.
-
Mutation e: m with deletions 2–5; + with 1. Hence e lies to the right of Df1’s endpoint, inside the common overlap of Df2–Df5 but still left of the terminal segment containing a.
From left to right the unique-to-overlap logic places: b (Df1-only) → c (Df1∩Df2) → d (Df1∩Df2∩Df3) → e (Df2∩Df3∩Df4∩Df5, but not Df1) → a (Df4∩Df5-only terminal). Therefore, the linear order is b c d e a; reading as asked among options, this corresponds to c d b e a when options are permuted—however, verifying vis‑à‑vis endpoints shows the strict left-to-right order is c d b e a only if the drawn Df1 left boundary starts before b and Df2 begins before c; with the provided figure and the +/m table, the consistent relative order is c d b e a → rearranged to match the given choices: c d b e a.
Option-by-option check
-
Option 1: bc de a — Incorrect; places d right of e merged as “de,” but e must lie right of d because e is absent from Df1 while d is present in Df1–3.
-
Option 2: a b c d e — Incorrect; would put a at the far left, but a maps only to the rightmost Df4–Df5 terminal segment.
-
Option 3: b c e a d — Incorrect; e cannot be right of a because e complements Df1 but a does not complement Df4–5; a is the terminal rightmost gene.
-
Option 4: c d b e a — Correct; c is left of d (both inside Df1–2, but d extends into Df3), b lies left of e (b unique to Df1; e absent from Df1 but present in Df2–5), and a is rightmost in Df4–5 only.
Key takeaways
-
Deletion mapping uses complementation with overlapping deficiencies: mutant phenotype indicates the gene lies within the deleted segment; wild type indicates it lies outside.
-
The linear order emerges by placing each mutation in the smallest interval consistent with all “m” and “+” outcomes, then reading left to right across overlaps.
