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Terminal Deletions

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Deletions occur when a specific chromosome loses its segment and is then termed to irreparable even with the enzymes. These deletions are of two types i. e. terminal deletions or interstitial deletions. Terminal deletions are the ones in which one end of a chromosome loses its segment whereas the interstitial deletions are the ones in which the chromosome loses its segment from the middlemost portion. This usually causes partial monosomy to occur in the chromosomes and can lead to many disease conditions. Some of the conditions which result because of deletions are DiGeorge Syndrome, Angelman Syndrome, and Prader-Willi Syndrome.     If the deletion of a gene that maps to chromosome 22q11 occurs DiGeorge Syndrome will possibly occur.

(Epstein 2001). The gene which is mutated in this syndrome is responsible for transcription factors of the T-family which help in the development of the branchial arch and the great vessels. Patients suffering from the syndrome have undeveloped third and fourth pharyngeal pouches. The fourth pharyngeal pouch forms the thymus and because of this thymus is not formed in these patients which leads to a T-cell deficiency.

Thus these patients have a poor defense against various bacterias and fungal infections. Angelman Syndrome is believed to be caused by an interstitial deletion of a band in the long arm of chromosome 15. The patients would inherit this chromosomal defect from their mothers because of which they would suffer from mental retardation and cat-like laughter. (Smith & Laan 2003). It is necessary that this defect is inherited from the maternal gene as if it is from the paternal side Prader-Willi Syndrome may occur. The children with Angelman Syndrome show poor motor development alongside with the mental retardation.

Prader-Willi Syndrome, on the other hand, is caused by a deletion of a gene on the paternal side. This deletion is again on the same band of chromosome 15 in the paternal genes. Patients suffering from Prader-Willi Syndrome show signs of mental retardation, lack of growth, hypotonia, hypogonadism, and obesity (Nicholls & Knepper 2001).

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