Encyclopedia of Health and Medicine

A
allele Any of the variations of a GENE that may
occupy the same position (locus) on a CHROMO-
SOME. The gene controlling a particular trait or
function always occupies the same locus on the
same chromosome. Genes occur as pairs, with one
gene coming from each parent. The pairing determines
how the gene’s traits are expressed in the
individual. For example, the gene for BLOOD TYPE
occurs at region 34 on the long arm of chromosome
9, indicated as 9q34. This gene has three
alleles, identified as 9q34IA, 9q34IB, and 9q34i
(which geneticists sometimes abbreviate as IA, IB,
and i, respectively). These alleles can occur in one
of six pairings to produce the blood type A, B, O,
or AB.
When the two alleles at the same locus are the
same the individual is said to be homozygous for
that gene; when the alleles are different the individual
is heterozygous. In a heterozygous individual
generally one allele is dominant and the other
recessive. Occasionally each allele in a pairing has
equal dominance, a circumstance called codominance.
The 9q34i allele (type O) is recessive; the
9q34IA and 9q34IB alleles (type A and type B) are
dominant. When the 9q34IA and 9q34IB alleles
pair, their expression is codominant. The possible
allele pairings for blood type can produce any of
these expressions.
For further discussion of alleles within the context
of the structures and functions of genetics,
please see the overview section “Genetics and
Molecular Medicine.”
See also GENOTYPE; INHERITANCE PATTERNS; PHENO-
TYPE.
apoptosis The natural mechanism through
which a cell engages in actions that lead to its
death, often called programmed cell death or cell
suicide. Apoptosis appears linked to SENESCENCE, an
inherent limitation on the number of times a cell
can divide. Both apoptosis and senescence play
significant roles in the aging process. Once the cell
initiates apoptosis there is no reversal; the process
proceeds until the cell dies.
Apoptosis begins when the cell’s DNA fragments,
signaling or switching the rest of the process in
motion. Once activated apoptosis sets in motion
the subsequent events result in the cell’s disman-
EXAMPLE ALLELE PAIRINGS AND EXPRESSION: BLOOD TYPE
Allele Pairing Expression Blood Type
IaIa (A+A) Homozygous dominant Type A
Iai (A+O)
Heterozygous dominant
IbIb (B+B) Homozygous dominant Type B
Ibi (B+O)
Heterozygous dominant
ii (O+O) Homozygous recessive Type O
IaIb (A+B) Heterozygous codominant Type AB
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