Page 209 - Concise Pathology for Exam Preparation ( PDFDrive )

194 SECTION I General Pathology

• Polydactyly.
• Congenital heart defects.
• Renal malformations.
• Umbilical hernia.
• Rocker bottom feet.

Q. Enumerate the cytogenetic disorders involving sex chromosomes.
Ans. Cytogenetic disorders involving sex chromosomes:
1. One X chromosome (maternal or paternal) may get randomly inactivated in the course
of development (Lyon’s hypothesis)
2. Klinefelter syndrome
3. Turner syndrome
4. Hermaphroditism and pseudohermaphroditism

Q. Write briefly on Lyon’s hypothesis.
Ans. Lyon’s hypothesis:
1. Only one of the X chromosomes is genetically active; the other X chromosome of either
maternal or paternal origin undergoes hyperpyknosis and is rendered inactive.
2. Inactivation of either the maternal or paternal X chromosome occurs at random among
all the cells of the blastocyst, on or about the 16th day of embryonic life.
3. Inactivation of the same X chromosome persists in all the cells derived from one precur-
sor cell.
(a) The inactivated X chromosome is selectively reactivated in germ cells before
first meiotic division, as both X chromosomes are required for normal
oogenesis (‘Modified Lyon’s hypothesis’—modification is based on the obser-
vation that women continue to express many genes from their inactive
X chromosome).
(b) The inactive X chromosomes can be seen in the interphase nucleus as the darkly
staining small mass in contact with the nuclear membrane known as Barr body or
X chromatin.
(c) It is present in all cells of a normal female. A buccal smear is made to demonstrate it.
(d) Normal females have one Barr body and normal males none.
(e) A male with Klinefelter syndrome and an XXY genotype has one Barr body.

Q. Write briefly on Klinefelter syndrome.
Ans. Klinefelter syndrome is characterized by male hypogonadism due to presence of two
or more X chromosomes and one or more Y chromosome.

Incidence
1 in 850 live male births.

Genetic Abnormalities

• Classical type 47, XXY karyotype (82% cases).
• Maternal nondisjunction is slightly more common than paternal nondisjunction of sex
chromosomes; increases with increase in age.
• Other variants: 46, XY/47, XXY; 47, XXY/48, XXXY; may have more number of X chro-
mosomes.

Clinical Features
• Distinctive body pattern and other features emerge after puberty.
• Elongated body due to increased length between the soles and the pubic bone.
• ‘Eunuchoid body habitus’: long legs, small atrophied testes and small penis.

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