Normally, each human has 46 chromosomes that are distributed
in 23 pairs, 22 pairs of autosomes and 1 pair of sex chromosomes
(XX in females and XY in males). Thus, each individual has two copies
of each chromosome (ie, diploid). A normal chromosome
constitution is termed the euploid state, whereas
an abnormal chromosome complement is called aneuploidy. The
autosomes are numbered 1 to 22, with the numbers assigned in descending
order of length, size, and centromere position of each chromosome
pair, although chromosome 21 is actually smaller than chromosome
22. The position of the centromere varies among chromosomes and
is classified into three categories: metacentric chromosomes have
centromeres in the middle, submetacentric chromosomes display centromeres
closer to one end, and acrocentric chromosomes have centromeres
at the end of the chromosome. For example, the chromosomes 13, 14,
15, 21, and 22 are referred to as acrocentric.
The centromere (cen) divides a chromosome into
a short (p) arm and a long (q)
arm. Each arm ends in a terminus (ter). Thus, the
end of the short arm is called pter. The arm of
each chromosome is divided into regions, these regions are divided into
bands, and bands into subbands. When a karyotype is reported, the
total number of chromosomes is presented first, followed by the
sex chromosomes’ constitution, and then any numerical and
structural anomalies are indicated to the level of the subbands.
The International System for Human Cytogenetic Nomenclature (ISCN) 2005
provides additional information.3 Some examples
of normal and abnormal karyotype reports are given in Table
173-1.