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  Section: Genetics » Structural Changes in Chromosomes
 
 
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Chromosome Duplications

 
     
 
Content
Structural Changes in Chromosomes
Deficiencies
Duplications
Translocations 
Inversions 

Duplications are obtained due to addition of a part of a chromosome. If duplication is present only on one of the two homologous chromosomes, at meiosis (pachytene), cytological observations characteristic of deficiency will be obtained in duplication also (Fig. 19.6). Duplication of a chromosome segment, may be brought about by addition at any of the following positions : (i) in adjacent region (Fig. 19.7a), (ii) at a displaced position of the same arm (Fig. 19.7b), (iii) on the different arm of the same chromosome (Fig. 19.7c) or (iv) on a different chromosome (Fig. 19.7d). Sometimes the duplication may be found as a reverse repeat (Fig. 19.7e).

One of the classical examples of duplication in Drosophila is Bar eye (Fig. 19.8). Bar eye is a character, where eyes are narrower as compared to normal eye shape. This phenotypic character is due to duplication for a part of a chromosome. By the study of giant salivary gland chromosomes, it could be demonstrated that 'Bar' character was due to a duplication in region 16A of X-chromosome. Barred eyes will have slightly different phenotype in heterozygous and homozygous individuals (Fig. 19.9). Barred individuals (16A 16A)gave rise to ultrabar (16A 16A 16A)and normal -wild type (16A) due to unequal crossing over (Fig. 19.10).
 
Chromosome pairing in a duplication heterozygote.
Fig. 19.6. Chromosome pairing in a duplication heterozygote.

Some other duplications known in Drosophila lead to following phenotypic effects : (i) a reverse repeat in chromosome 4 causes eyeless dominant (Ey); (ii) a tandem duplication in chromosome 3 causes confluens (CO)resulting in thickened veins, and (iii) another duplication causes hairy wing (Hw).

Different kinds of duplications in chromosomes.
Fig. 19.7. Different kinds of duplications in chromosomes.
 
A normal eye, bar eye and a double bar eye with corresponding chromosome segments (salivary gland chromosomes) showing duplications.
Fig. 19.8. A normal eye, bar eye and a double bar eye with corresponding chromosome segments (salivary gland chromosomes) showing duplications.

Shapes of eyes in normal, heterozygous bar and homozygous bar Drosophila flies.
Fig. 19.9. Shapes of eyes in normal, heterozygous bar and homozygous bar Drosophila flies.
 
An ultrabar condition resulting from 'Bar' due to unequal crossing over.
Fig. 19.10. An ultrabar condition resulting from 'Bar' due to unequal crossing over.

 
     
 
 
     




     
 
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