Chromosomal Aberrations

The chromosome of each species has a characteristic morphology and number. But sometimes due to certain irregularities at the time of cell division, crossing over, or fertilization, some alterations in the morphology and number take place. The slightest variation in the organization of the chromosome is manifold, phenotypically, and is of great genetic interest.

Duplication
The presence of a part of the chromosome in excess of the normal complement is known as duplication. During pairing, the chromosome bearing the duplicated segment forms a loop.

Examples

1. Bar eye in Drosophila. This is characterized by a narrow, oblong bar-shaped eye with few facets.
   It is associated with a duplication of a segment of the X-chromosome called 16A. Each added section intensified the bar phenotype.
2. A reverse repeat in the chromosome IV causes eyeless (Ey) morphology.
3. A tandem duplication in chromosome III causes confluens (Co), resulting in thickened veins.
4. Another duplication causes hairy wing (Hw).

Deletion
Loss of a broken part of a chromosome is called deletion. Deletion may be terminal or intercalary. During the pairing between a normal chromosome and a deleted chromosome, a loop is formed in the normal chromosome. This is known as the deletion loop.

Example
Notched wing mutation in Drosophila.

In the presence of deletion, a recessive allele of the normal homologous chromosome will behave like a dominant allele (pseudodominance).

Inversion
Inversion involves a rotation of a part of a chromosome or a set of genes by 180° on its own axis. It essentially involves the occurrence of breakage and reunion. The net result of inversion is neither gain nor loss in the genetic material, but simply a rearrangement of the sequence.
During pairing, an inversion loop is formed, in which one chromosome is in the inverted order and its homologue is in the normal order.

Significance
Chromosomes with inversions have practical applications for maintaining Drosophila stock. Crossing over is suppressed in such chromosomes and it is possible to maintain a gene in the heterozygous state that would cause death when present in the homozygous condition.

Translocation
The shifting of a part of a chromosome or a set of genes to a nonhomologous one is called translocation. During synapsis, a cross-shaped configuration is formed. H.J. Muller found one strain of Drosophila in which a group of genes, including scarlet, which normally is on the third chromosome, translocated to the second chromosome. Cytological examination showed that the third chromosome was much shorter than usual, while the second chromosome was longer than usual.