C h a p t e r 5
Genetic Control of Cell Function and Inheritance
97
If the centromere is in the center and the arms are of
approximately the same length, the chromosome is said
to be
metacentric;
if it is not centered and the arms are of
clearly different lengths, it is
submetacentric;
and if it is
near one end, it is
acrocentric.
The short arm of the chro-
mosome is designated as “p” for “petite,” and the long
arm is designated as “q” for no other reason than it is the
next letter of the alphabet. The arms of the chromosome
are indicated by the chromosome number followed by
the p or q designation (e.g., 15p). Chromosomes 13, 14,
15, 21, and 22 have small masses of chromatin called
satellites
attached to their short arms by narrow stalks.
The banding patterns of a chromosome are used in
describing the geographic position of a gene on a chro-
mosome, which can be useful, for example, in com-
municating the location of genes involved in genetic
diseases. Each arm of a chromosome is divided into
regions, which are numbered from the centromere
outward (e.g., 1, 2). The regions are further divided
into bands, which are also numbered (Fig. 5-10). These
numbers are combined to designate the position of a
gene; for example, Xp22 refers to band 2, region 2 of
the short arm (p) of the X chromosome.
FIGURE 5-8.
Crossing over of DNA at the time of meiosis.
Centromere
Chromosome
arm
Chromatid
Satellite
Metacentric
Submetacentric
Acrocentric
FIGURE 5-9.
Three basic shapes and the component parts
of human metaphase chromosomes.The relative size of
the satellite on the acrocentric is exaggerated for visibility.
(Adapted from Cormack DH. Essential Histology. Philadelphia,
PA: J.B. Lippincott; 1993.)
Ocular albinism
Hypophosphatemia, hereditary
Duchenne/Becker muscular dystrophy
Chronic granulomatous disease
Retinitis pigmentosa
Wiskott-Aldrich syndrome
SCID, X-linked
Agammaglobulinemia
Fabry disease
Hemophilia B
Fragile X syndrome
Color blindness
Diabetes insipidus, nephrogenic
G6PD deficiency
Hemophilia A
22
2
p
q
1
1
2
21
11
13
21
25
28
FIGURE 5-10.
Localization of inherited diseases as represented
on the banded karyotype of the X chromosome. Notice the
nomenclature of arms (P, Q), regions (1, 2), and bands (e.g., 22
[region 2, band 2]). G6PD, glucose-6-phosphate dehydrogenase
deficiency; SCID, severe combined immunodeficiency disease.
(Modified from Peiper S, Strayer DS. In: Rubin R, Strayer DS,
eds. Rubin’s Pathology: Clinicopathologic Foundations of
Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health |
Lippincott Williams &Wilkins; 2012:251.)
SUMMARY CONCEPTS
■■
The genetic information in a cell is organized,
stored, and retrieved as small structures called
chromosomes.There are 46 chromosomes
arranged in 23 pairs, 22 of which are alike for males
and females, and 1 pair of sex chromosomes, with
XX pairing in females and XY pairing in males.
■■
Cell division involves the duplication of the
chromosomes. Duplication of chromosomes in
somatic cell lines involves mitosis, in which each
daughter cell receives 23 pairs of chromosomes.
Meiosis is limited to replicating germ cells (ovum
and sperm) and results in formation of a single
set of 23 chromosomes.
■■
A karyotype is an image of a person’s
chromosomes. It is prepared by special laboratory
techniques in which body cells are cultured, fixed,
and then stained to display identifiable banding
patterns. A photomicrograph is then made. Often
the individual chromosomes are cut out and
regrouped according to chromosome number.
