Page 206 - Concise Pathology for Exam Preparation ( PDFDrive )

8 Genetic and Paediatric Disorders 191

Q. Define genetic heterogeneity.

Ans. The phenomenon in which mutations at different genetic loci produce the same
result is called genetic heterogeneity, eg, retinitis pigmentosa, a disorder of abnormal
retinal pigmentation and visual impairment can be caused by several different types of
mutations.

Q. Define aneuploidy.
Ans. Humans have 46 chromosomes consisting of 22 pairs of autosomes or somatic
chromosomes and 2 sex chromosomes (XX 5 female and XY 5 male). The gametes
contain a haploid number of chromosomes (n 5 23). The union of two sex cells (egg
and sperm), each with only haploid number of chromosomes, results in a diploid
zygote (2n).
‘Hyperdiploidy’ is a chromosomal number more than diploid and ‘hypodiploidy’
is a chromosomal number less than diploid. Aneuploidy refers to the presence of an
uneven multiple of 23 chromosomes. It is most frequently due to nondisjunction, in
which one set of homologous chromosomes fails to separate during the first meiotic divi-
sion (one gamete has 22 chromosomes and the other 24 chromosomes).

Q. What is chromosomal translocation?
Ans. Chromosomal translocation (Fig. 8.2) is the transfer of a broken segment from one
chromosome to another nonhomologous chromosome. The process is usually reciprocal
(fragments are exchanged between two chromosomes). Translocations are indicated by ‘t’
followed by involved chromosome in numeric order.
• A special pattern of translocation involving two acrocentric (centromere at the
end; q very long, p very short) chromosomes is called centric fusion type or
Robertsonian translocation. The break occurs close to the centromere and affects
the short arm of both chromosomes. Transfer of the segment leads to a very large
and a very small chromosome. The short fragment is lost and the carrier has
45 chromosomes.
• Isochromosomes result when the centromeres divide horizontally and not vertically. One
of the two arms of the chromosome is lost and the remaining is duplicated, resulting in a
chromosome with two short arms or two long arms only.

Before After translocation
translocation
Derivative
Chromosome B chromosome B

Derivative
chromosome A

Chromosome A
FIGURE 8.2. Chromosomal translocation.

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