Page 207 - Concise Pathology for Exam Preparation ( PDFDrive )

192 SECTION I General Pathology

Q. Define deletion.

Ans. Deletion is loss of a portion of a chromosome. A single break may delete a terminal
segment. Two breaks with loss of an intervening segment is called an interstitial deletion.
Two interstitial breaks with reunion of the proximal and distal segments may result in
formation of a ring chromosome. After loss of segments from each end of the chromosome,
the arms unite to form a ring.

Q. Define inversion.
Ans. Inversions occur when there are two interstitial breaks in a chromosome and the
segment reunites after rotation.

Q. Define mosaicism.

Ans. When nondisjunction occurs during mitosis of autosomal cells, the result is
mosaicism (presence of two or more genetically different cell populations in the same
patient; a common occurrence in Turner syndrome).

Q. What are alleles?
Ans. Alternative forms of the same gene are called alleles. Genes with the same alleles are
called homozygous, while those with different alleles are called heterozygous.

Q. Enumerate the cytogenetic disorders involving autosomes.
Ans. Cytogenetic disorders involving autosomes include the following:
1. Autosomal trisomies affecting chromosomes 21 (Down syndrome), 18 (Edwards
syndrome) and 13 (Patau syndrome).
2. One deletion (cri du chat) syndrome due to partial deletion of short arm of chro-
mosome 5 (characterized by mental retardation, a cat-like cry, and ventricular
septal defects) .
3. Trisomies and deletions affecting 22q (‘DiGeorge syndrome’—thymic hypoplasia,
decreased T-cell immunity, parathyroid hypoplasia, hypocalcaemia; ‘Velocardiofacial
syndrome’—congenital heart disease, facial dysmorphism, delayed development).

Q. Write briefly on Down syndrome.
Ans. Down syndrome is a cytogenetic disorder affecting chromosome 21.

Incidence
1 in 700 live births.

Genetic Abnormalities
• Trisomy of chromosome 21 (47, XX, 121).
• Extra chromosome because of Robertsonian translocation of the long arm of chromo-
some 21 to another acrocentric chromosome like 22 or 14 [46, XX; der(14;21)
(q10;q10); 121].
• Least common mosaic pattern having some cells with 46 chromosomes and some with
47 chromosomes due to mitotic nondisjunction of chromosome 21 during early stage
of embryogenesis (46, XX/47, XX, 121).
• Trisomy of chromosome 21 is influenced by mother’s age. Increased incidence is noted
after 30 years of age.
• The extra chromosome is derived from nondisjunction of chromosome 21 during first
meiotic division in ovum.

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