These are changes involving chromosome morphology, which results in changes in number and sequence of genes without altering ploidy or gene structure. Chromosome aberrations involve breaking of chromosome segments, their loss or union with same (intrachromosomal aberration) or different chromosomes (interchromosomal aberration). The important types are as follows:
It is the loss of chromosome segment. The lost segment may be terminal (deficiency or terminal deficiency) or intercalary (intercalary deficiency or deletion). The terms deficiency and deletion are also used interchangeably. Terminal deficiency involves a single break near the chromosome end. In intercalary/interstitial deficiency or deletion there are two breaks (double break), loss of separated segment and the union in the region of breaks. In heterozygous deficiency (deficiency in one out of two chromosomes of a homologous pair) the synapsed chromosome would show difference in length at one end (terminal deficiency) or loop (intercalary deficiency/deletion). Homozygous deficiency may be lethal if some vital gene is involved (e.g., sex-linked notch wing deletion in Drosophila). Notch wing heterozygous deficiency produces notched wing margins in female Drosophila. Even a recessive gene may express if corresponding gene on the other homologous chromosome has become deficient. The phenomenon is called pseudodominance.
It is the phenomenon of having one or more genes in excess of the normal complement due to presence of an extra chromosome segment.
i) Tandem/Repeat Duplication
The duplicating segment gets incorporated next to normal corresponding segment e.g., ABCDEFGHGH.
ii) Reverse Tandem Duplication
The duplicating segment is incorporated next to normal corresponding segment but the order is reversed e.g., ABCDEFGHHG.
iii) Displaced Duplication
The duplicating segment is incorporated away from the corresponding segment on the same chromosome.
iv) Transposed Duplication
The duplicating segment is incorporated in a non homologous segment. Duplication increases the number of genes, which increases genetic redundancy, allows development of variations and hence evolution. However, some duplications are harmful e.g., development of bar eye in Drosophila. Presence of single B gene on X-chromosome (both chromosome in female) produces normal eye with 780 facets. An extra B-gene due to duplication produces smaller and narrow bar eye with 350 facets in heterozygous state and 200 facets in homozygous state. If per chance there is shifting of one B-gene from one chromosome to its homologue, it results in three B-genes on one and one B-gene on second. This produces double bar eye which is very small. The change in phenotypic expression when genes are reallocated without altering their number is called position effect.
A chromosome segment having one or more genes gets separated and reinserted in its position but after inversion or rotation by 180˚ so that the order of genes is reversed. Inversion may include centromere (pericentric) or occur beyond centromere (paraentric). Chromosomes remain unsynapsed in the region of inversion with crossing over resulting in development of deficiencies, duplications, and bridge formation so that cross-over products are sterile. Therefore, inversion suppresses genetic recombinations. This is helpful in maintaining heterozygosity and balanced lethal systems.
It is an aberration in which a chromosome segment separates and gets attached to a nonhomologous chromosome.
i) Reciprocal translocation
Mutual exchange of segments between two nonhomologous chromosomes.
ii) Simple Translocation
One sided translocation. If a chromosome segment gets inserted in interstitial position of a non-homologous chromosome, it is known as shift translocation.
In humans, Philadelphia chromosome is produced by reciprocal translocation of a fragment of chromosome 9 (having c-abl-oncogene) into chromosome 22. It is invariably associated with myelogenous leukemia.
iii) Reciprocal translocation develops multivalents instead of bivalents at synapsis. During anaphase it breaks chromosomes, develops new linkages, produces a large number of sterile gametes, causes position effect but is useful in maintaining heterozygosity and balanced lethal system.
iv) Whole-arm Translocation or centric fusion reduces the number of chromosomes. Human chromosome number 2 has been produced by centric fusion of two chromosomes of ape-like ancestors.
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