Bma Illustrated Medical Dictionary

cells instead of 2, causing physical
abnormalities, defective sexual develop-
ment, and infertility. A boy with 1 or more
extra X chromosomes has Klinefelter’s
syndrome,which causes defective sexual
development and infertility. The pres-
ence of an extra X chromosome in women
or an extra Y chromosome in men nor-
mally has no physical effect but increases
the risk of mild mental handicap.
Chromosomal abnormalities are diag-
nosed by chromosome analysisin early
pregnancy, using amniocentesisor chori-
onic villus sampling.
chromosome analysisStudy of the
chromosomesin body cells to discover
whether a chromosomal abnormalityis
present or to establish its nature. Fetal
cells for analysis can be obtained in the
uterus by amniocentesisor chorionic vil-
lus sampling. If a serious abnormality
such as Down’s syndromeis identified,
termination of the pregnancy and gen-
etic counsellingis offered. Chromosome
analysis is also carried out when a baby
is stillborn without an obvious cause,
or is born with abnormal physical char-
acteristics that suggest a chromosomal
defect, such asTurner’s sydrome.
Chromosome analysis in children and
adults uses white blood cells taken from
a blood sample. Analysis of the sex
chromosomes may be carried out to
establish the chromosomal sex of a child
in cases where the genitals have an
ambiguous appearance (see genitalia,
ambiguous); to confirm or exclude the
diagnosis of chromosomal abnormalities;
or to investigate infertility.
chromosomesThread-like structures in
the nuclei of cells. Chromosomes carry
inherited information in the form of
genes, which govern all cell activity and
function. Each chromosome contains
up to several thousand genes arranged
in single file along a long double fila-
ment of DNA. The sequence of chemical
units, or bases, in the DNA provides the
coded instructions for cellular activities.
All an individual’s body cells (with the
exception of egg or sperm cells) carry
precisely the same chromosomal mate-
rial copied by a process of cell division
from the original material in the fertil-
ized egg. Each human cell normally

contains 46 chromosomes made up of

23 pairs. Half of each pair is of maternal

and half of paternal origin. 22 pairs are

autosomal chromosomes, which are the

same in both sexes; the remaining pair

is made up of 2 sex chromosomes. In

females, the sex chromosomes are a

pair of X chromosomes. In males, 1 is

an X chromosome and the other is a Y

chromosome. One sex chromosome (an

X) originates from the mother’s egg and

the other (an X in girls; a Y in boys)

from the father’s sperm.

chronicA term describing a disorder or

set of symptoms that has persisted for a

long time. A chronic illness implies a

continuing disease process with little

change in symptoms from day to day.

(See also acute.)

chronic fatigue syndromeA condition

causing extreme fatigue over a prolonged

period, often over years. It is most com-

mon in women aged between 25 and 45.

The cause of the condition is unclear.

In some cases, it develops after recovery

from a viral infection or after an emo-

tional life event such as bereavement.

In other cases, there is no such preced-

ing illness or event. The main symptom

is persistent tiredness. Other symptoms

of the syndrome vary, but commonly

include impairment of short-term mem-

ory or concentration, sore throat, tender

lymph nodes, muscle and joint pain,

CHROMOSOME ANALYSIS CHRONIC FATIGUE SYNDROME

C

CHROMOSOMES

SET OF HUMAN CHROMOSOMES

Chromosome pair