Principles of Genetics and Breeding

6.1.2.4. Chromosomal aberration

Four main types of chromosome aberrations are known.

1. Deletion – Fragment of chromosome is missing

2. Duplication – Fragment of chromosome becomes duplicated.

3. Inversion – Fragment detaches and reinserts in the reverse order

4. Translocation – A segment from one chromosome becomes connected to a non-homologous chromosome.

Deletions occur among fishes more frequently than duplications, but lost of the deletions lead to a drastic loss of viability,and the organisms carrying these deletions are rapidly eliminated from the populations.

Duplication of chromosomal regions is known to occur in fishes, although these events are probably infrequent.

• The presence of duplicated genes has been established by purely genetic techniques, since duplications cannot be detected under the microscope ;
• the most probable duplication mechanism involves unequal crossing over, i.e., the exchange by portions of imperfectly conjugated chromosomes.
• According to many authors, duplications play a particularly important role in the evolution of fishes.

Inversions can be classified into two main types.

• Paracentric inversions which do not involve the centromeric regions are difficult to detect. They do apparently occur in fishes quite frequently, but their presence can only be established by analyzing the inheritance of the linked genes.
• Pericentric inversions involving the centromere are quite frequent. If the two breakage events take place at equal distances from the centromere one cannot detect the inversion without the analysis of marker genes. When the sites of breakage are located asymmetrically, the relative length or even the absolute number of chromosomal arms will be changed.

Robertsonian translocations or centric fusions are very important and are apparently fairly frequent.

• The breakage of one acrocentric chromosome occurs near the centromere and another acrocentric chromosome is joined to the site of the breakage.
• One or two small regions adjacent to the centromere (with one of the centromeres) are lost and two acrocentric chromosomes fuse into a single metacentric one.
• The number of chromosomal arms remains unchanged.
• The reverse process of centric fissions is rarer because an additional centromere is required.
• According to recent data, however, the direct division of one centromere into two may be possible.