tling, assimilation, and recycling. In some respects
cells become endlessly renewable resources for the
body. Specialized cells called phagocytes break
down dying and dead cells into basic components
such as amino acids that other the body can use to
construct new cells.
Apoptosis is necessary for growth, develop-
ment, and change in the body. The process of the
death of cells that experience injury or damage is
called necrosis and by definition occurs outside
the natural order of cell life expectancy. Extrinsic,
rather than intrinsic, factors initiate necrosis.
See also CELL STRUCTURE AND FUNCTION; METABO-
LISM; PHAGOCYTE; PHAGOCYTOSIS; STEM CELL.
autosomal trisomy A chromosomal disorder in
which there are three instead of the normal two
copies of an AUTOSOME(nonsex chromosome). An
autosomal trisomy may be complete (affect all
cells) or mosaic (affect only some cells). The most
commonly occurring complete autosomal trisomies
that are survivable are those involving chromo-
somes 21, 18, and 13, which result in the chromo-
somal disorders DOWN SYNDROME (trisomy 21),
EDWARDS SYNDROME(trisomy 18), and PATAU’S SYN-
DROME(trisomy 13). These trisomy disorders may
also occur as a mosaic. Mosaic autosomal trisomies
typically produce less severe, though still signifi-
cant, physical and mental impairments. Complete
autosomal trisomies affecting other chromosomes
are often lethal, nearly always causing death early
in development and well before birth.
Though the risk for autosomal trisomy disor-
ders increases with a woman’s age at the time she
becomes pregnant, most autosomal trisomy disor-
ders occur in pregnancies in younger women
because the rate of CONCEPTION is significantly
higher among younger women. The risk is highest
for women who have previously given birth to a
child with a trisomy disorder. Obstetricians can
detect fetal trisomy disorders generally within the
first and early part of the second trimesters of
PREGNANCYwith prenatal tests. The diagnostic path
usually incorporates a combination procedures
including- BLOODtests that look at the levels of proteins
the FETUSand placenta are making - ULTRASOUND, which shows physical anomalies
that suggest a chromosomal disorder - CHORIONIC VILLI SAMPLING (CVS) and
AMNIOCENTESIS, which permit examination of
fetal cells
See also BIRTH DEFECTS; CHROMOSOMAL DISORDERS;
CONGENITAL ANOMALY; GENETIC COUNSELING; GENETIC
TESTING; MOSAICISM; PREGNANCY.autosome A CHROMOSOMEthat appears as a pair
in which both chromosomes are the same in
either sex, also called a nonsex chromosome. In
contrast, the sex chromosomes appear as a pair
that is different in males and females. The human
GENOMEcontains 22 autosomes and one pair of sex
chromosomes for a total complement of 46 chro-
mosomes as 23 pairs.
For further discussion of autosomes within the
context of the structures and functions of genetics,
please see the overview section “Genetics and
Molecular Medicine.”
See also GENE; GENOTYPE; KARYOTYPE; PHENOTYPE;
SEX CHROMOSOME.114 Genetics and Molecular Medicine