4. Suppose a chromosomal aberration in a chromosome AB.CDEFG leads to AB.CDEFCDFEG. The probable reason is
(1) Duplication and followed by EF inversion
(2) Duplication followed by pericentric inversion
(3) Only duplication
(4) Only Inversion

Step-by-step analysis

Original chromosome:
AB.CDEFG

Mutant chromosome:
AB.CDEFCDFEG

Compare:

• The original segment between C and G is DEFG.

• In the mutant, between C and G we see DEFCDFE:

  • first: DEF (forward order)

  • then: CDFE (where the order is C–D–F–E, showing that E and F are reversed in this duplicated block).

So:

1. There is an extra block containing C, D, E, F → this indicates a duplication of CDEF.

2. In that extra block, the sub‑order is CDFE instead of CDEF → meaning within the duplicated region, E and F have undergone an inversion.

Thus, the most reasonable sequence of events is:

• First, duplication of the CDEF segment.

• Then, inversion of the EF part in the duplicated segment, producing CDFE.

This matches option (1) Duplication followed by EF inversion.

Why other options are incorrect

• (2) Duplication followed by pericentric inversion

  • Pericentric inversion includes the centromere. The notation AB.CDEFG does not show the centromere inside EF, and the observed rearrangement only reverses a small internal part (E–F), not a large centromere‑spanning block.

• (3) Only duplication

  • Simple duplication of CDEF would give AB.CDEFCDEFG, but the mutant is AB.CDEFCDFEG, with the order of E and F reversed in the duplicated copy. So duplication alone cannot explain it.

• (4) Only inversion

  • An inversion does not change copy number; it only reverses order. Here, gene segments are present in extra copy (C and D appear three times total), so an inversion by itself is insufficient.

Therefore, the aberration is best explained as duplication followed by inversion of EF, corresponding to option (1).