ment. The only cells in the body that do not have
chromosomes are the erythrocytes, which do not
have nuclei.
Autosomes carry the bulk of genetic code.
Thousands of genes line each autosome, each in
its ordained position. The sex chromosomes carry
several hundred genes. The GENEpositions, called
loci (in the singular, each position is a locus), are
constant. For example, the gene loci for the ABO
BLOOD TYPEare always on chromosome 9, those for
the rhesus (Rh) blood type are on chromosome 1,
and those for EYEcolor on chromosomes 15 and
19.
CHROMOSOME SIZE
The HUMANGENOMEPROJECT, completed in 2003,
revealed the structure of chromosomes to be
much larger and more complex than scientists
previously had theorized. Chromosome 1, the
largest CHROMOSOME, contains 2,968 genes. The
smallest chromosome, the Y chromosome, con-
tains 231 genes.Nomenclature
Geneticists designate the normal female chromo-
some complement as 46,XX and the normal male
chromosome complement as 46,XY. Deviations
from the norm are CHROMOSOMAL DISORDERSgeneti-
cists designate according to the deviation, for
example 47,XY,+21 denotes AUTOSOMAL TRISOMY 21
(DOWN SYNDROME) in a male. The designation 45,X
denotes TURNER SYNDROME, a monosomy disorder
(missing chromosome) affecting the SEX CHROMO-
SOMEin a female. A comprehensive standard of
nomenclature (naming) exists so all geneticists
can use a common “language” when describing
chromosomal and genetic configurations.
A chromosome’s structure consists of two
telomeres (end segments), a CENTROMERE(waistlike
indentation), and two arms (the segments above
and below the centromere). The centromere is
somewhat off-center, such that each chromosome
has a short arm (designated “p” for petite) and a
long arm (designated “q” because scientific
nomenclature is alphabetical). The regions of each
arm are numbered. Geneticists identify a gene’s
locus relative to its placement on the chromo-
some. The gene responsible for CYSTIC FIBROSIS, for
example, is identified as CFTR 7q31.2—cystic
fibrosis transmembrane conductance regulator
located in band 31, region 2, on the long arm of
chromosome 7.
For further discussion of chromosomes within
the context of the structures and functions of
genetics, please see the overview section “Genetics
and Molecular Medicine.”
See also AUTOSOME; ERYTHROCYTE; GAMETE; GENETIC
DISORDERS; GENOME; GENOTYPE; PHENOTYPE; SPERM;
TELOMERE.cloning The creation of exact copies of a GENE,
cell, or entire organism. Such exact copies occur
naturally when a ZYGOTEdivides to become identi-
cal multiples such as twins or, less commonly,
triplets. Manipulated cloning is primarily a
research method at present, though scientists use
cloning for therapeutic applications in creating
RECOMBINANT DNAproducts such as INSULIN. Insulin
was the first human gene cloned (1978) as well as
the first genetically engineered product approved
for use in the United States (1982). The cloning of
entire organisms, such as Dolly the sheep in 1997,
though sensational, is extraordinarily challenging.
Currently, cloned organisms appear prone to
numerous health problems and tend to die prema-
turely, which somewhat mystifies researchers
because natural clones such as identical twins do
not experience these challenges. Numerous ethical
issues surround the use of entire organism
cloning, particularly EMBRYOcloning.
Scientists create gene clones by removing the
DNAfrom a vector such as a bacterium cell and
replacing it with the DNA of choice. The bac-
terium rapidly replicates, creating multiple identi-
cal copies of the DNA. Similarly, this process can
create identical replicas of cells. Researchers are
hopeful that this technology will someday lead to
the ability to generate replacement tissues and
organs to treat various health conditions that
currently rely on therapies such as ORGAN TRANS-
PLANTATION. This technology further holds promise
for treating degenerative conditions such as
PARKINSON’S DISEASE and HUNTINGTON’S DISEASE.
Cloning is also one method of potential GENE
THERAPY.
See also CELL STRUCTURE AND FUNCTION; ETHICAL
ISSUES IN GENETICS AND MOLECULAR MEDICINE.cloning 119