20. Which of the following illustrations explain the correct pairing preceding recombination between a chromosome (ABC•DEFG/ABC•DEFG) and its inverted homologue (ABC•DGFE/ABC•DGFE). The dot in genotype represent the centromere.

Concept

• A linkage group = one chromosome; genes on the same chromosome are linked.

• The question says:

  • Order of genes in the genetic map changes.

  • Linkage group does NOT change (gene stays on the same chromosome).

This exactly describes an inversion.

Option‑wise explanation

1. Translocation

• Exchange or movement of a segment between non‑homologous chromosomes.

• This changes the linkage group because genes move to a different chromosome.

• So it does not fit the condition “does not alter its linkage group”.

2. Recombination (crossing over)

• Exchange between homologous chromosomes.

• It makes new combinations of alleles but does not change the physical order of loci along the chromosome; the map order remains the same.

3. Transposition

• Movement of transposable elements (and sometimes nearby genes) to new chromosomal locations, potentially to a different chromosome.

• This can change linkage group as well as local context, so it does not match the question.

4. Inversion – correct

• A chromosome segment breaks in two places and re‑inserts in the reverse orientation.

• The genes in that segment now appear in reverse order on the map.

• However, the entire segment stays on the same chromosome, so the linkage group is unchanged.

Thus, the aberration that changes gene order without changing linkage group is an inversion (option 4).