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.
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.
- 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.
- A reverse repeat in the chromosome IV causes eyeless (Ey) morphology.
- A tandem duplication in chromosome III causes confluens (Co), resulting
in thickened veins.
- Another duplication causes hairy wing (Hw).
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.
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 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.
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.
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
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.