Definition : Mutations which results in changes in the structure of individual chromosomes or results in changes in the number of chromosomes, named as chromosomal mutations.
Introduction : Changes in the structure of chromosomes can take place, due to loss of one or more genes viz. election or duplication arrangement of genes viz. inversion or may occur due to changes in the number of chromosomes named Heteroploidy.
Changes in the Structure of Chromosomes :
(a) Changes in the number of genes :
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Deletion of deficiency : Loss of one or more genes
- Heterogyaes condition – i.e. one chromosome affected, other normal, organism is viable with some abnormality.
- Homozygous condition – i.e. both chromosomes of a pair get affected – likely to be lethal.
- Duplication : Addition of one or more genes additional genes are added to the chromosome complement which might convert a lethal (homozygous) conditions into a tolerate one.
(b) Changes in the arrangement of genes :
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Inversion : Rotation of a block/set of genes in a chromosome by 1800 – sequence gets inverted. Effect of inversion is not so drastic as in deletion r duplication as the number of genes remains constant.
- Effects may be produced due to Position effect i.e. a gene at one location on a chromosome may not have the same action/effect that it would have at another position.
- Inversions suppress crossing over and retain the original combination of genes (The two arms of chromosomes may not pair during synapsis) – Evolutionary significance is that if inversion results in a superior combination of genes, it might be advantageous to retain the arrangement .
Example : In Decosophila Melanogoster and D. Simulans these are completely distinct species formed due to inversion. They are morphologically identical but produce completely sterile hybrids.
- Translocation : Exchange of parts between (non) homologous chromosomes. Or, Any kind of aberration in which a fragment of one chromosome becomes detached and then attached to a non-homologous chromosome is called as Translocation.
Non- homologous may interchange part in reciprocal translocation. It occurs when parts of chromosome belonging to members of two different pairs become exchanged hence, Reciprocal translocation, results in changes number of chromosomes and thus formation of new species.
Example : Comparative cytogenetic studies of Drosophila sp.
- Changes in the number of chromosomes (Heteroploidy)
(i) Changes involving entire set of chromosomes (Euploidy)
Polyploidy : Organism has more than two sets of chromosomes viz. triploid, tetraploid, hexaploid etc.
Haploid : Loss of one set of chromosomes.
(ii) Changes involving the number of chromosomes in a set (Aneuploidy) – due to non – disjunction of chromosomes during cell division.
Monosomic : One chromosome is present singly – loss of 1 chromosome (2n-1)
Polysomic : Addition of 1 or more chromosome : (2n + 1 ) or (2n + 2)
Nullisomie : Loss of both the chromosome of a pair (2n-2)
- Examples of polyploids : More significant examples in evolution of plants.
- By Karpachenko, cross between raddish, Raphanus and cabbage, Brassica : only few hybrids were fertile, these had a tetraploid situation – doubling of chromosome in the zygote – Raphanobrassica
- Species of wheat : Different chromosome complements
Eichron wheat : 2n : 7 pairs
Hard wheat : 28 chromosome : 14 paris (tetraploid)
Vulgare : hexaploid.