Question

Write an essay on chromosomal aberrations giving their cytological and genetic effects.

Answer 1

Hint: All organisms have a specific type and number of chromosomes. Mitotic and meiotic cell division in eukaryotes, as well as binary fission in bacteria, work to ensure that the offspring also have the same chromosomal patterns or karyotypes.

Complete Answer:
- Any change in the number of chromosomes or in the structure is an aberration. Changes in number include aneuploidy where a single chromosome changes, and polyploidy where an entire set of chromosomes changes in number.
- Down’s syndrome is a well-studied example of aneuploidy where chromosome 21 is present in an extra number, and is called trisomy 21.
- Changes in chromosome structure are deletions, duplications, inversions, and translocations. In duplication a part of a chromosome is repeated which means that the genes in that region are now present in more copies.
- Very simply, this would lead to production of more protein products of those genes. Depending on the locations, duplication could be tandem or misplaced. Sometimes these actually help with evolution as seen with the human globin gene family.
- A part of a chromosome is lost. The effect of this will depend on how much is lost and if it is centred on the centromere. Some genes require both alleles to present for effective functioning, so a deletion causes haploinsufficiency.
- In inversions, a part of the chromosome is flipped around. The disorder haemophilia involves a chromosomal inversion. In human karyotypes inversions can easily be seen with changes in the G banding patterns. Some inversions have absolutely no phenotypic effect, but others can inhibit crossing over. Inversions have also been seen to be important in primate evolution.
- Chromosomal translocations involve a part of a chromosome breaking off and attaching to another. These are easily visible in karyotypes. The Philadelphia chromosome is seen in acute myeloid leukaemia, where a part of chromosome 8 breaks and joins chromosome 11.

Note: Other than chromosomal aberrations, point mutations within the chromosomes can also take place. In this a single substitution, addition, or deletion of one nucleotide affects the functioning of a gene.