Biology of Disease

Akaryotype is the characteristic number, size and shape of chromosomes of a
species. A karyogram is a photographic representation of these chromosomes
stained and arranged in order, as for example in Figures 15.2and 15.20. An
idiogram is a diagrammatic representation or interpretive drawing of the
chromosomes based on the physical features seen in the karyogram. The
karyotype of normal humans is 46. Human autosomal chromosomes are
divided into seven groups (Table 15.6) on the basis of their sizes and the
positions of their centromeres.

CHROMOSOMES AND THE HUMAN KARYOTYPE

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )'.

Figure 15.19 A single human A2 (Table 15.6)

chromosome showing G banding. Courtesy of J.S.

Haslam and K.P. O’Craft, Tameside General Hospital,

Ashton under Lyne, UK.

Chromosome group Chromosome numbers Structures

A 1 to 3

B 4 to 5

C 6 to 12, X

D 13to15

E 16to18

F 19to20

G 21 to 22, Y

Table 15.6The seven groups of human chromosomes

Cytogenetics is the microscopic study of chromosomes. Small lymphocytes
isolated from a blood sample or cells obtained by amniocentesis or chorionic
villus sampling (Section 15.10) are stimulated to divide by treatment with
the plant lectin, phytohemagglutinin (PHA) and mitosis is then arrested at
metaphase by an inhibitor such as colchicine. Metaphase chromosomes are
visible by microscopy when stained in one of several different ways to allow
their accurate identification when examined by microscopy. For routine
karyotyping, Giemsa (G) staining is usually the preferred procedure since
this produces a pattern of alternating dark and light bands characteristic for
each pair of chromosomes. These patterns reflect differences in the detailed

Figure 15.20 Karyogram of a normal human female showing G banding. Courtesy of J.S.
Haslam and K.P. O’Craft, Tameside General Hospital, Ashton under Lyne, UK.