A

allele Any of the variations of a GENEthat 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 deter-
mines 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 chromo-
some 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 indi-
vidual is heterozygous. In a heterozygous individ-
ual generally one allele is dominant and the other
recessive. Occasionally each allele in a pairing has
equal dominance, a circumstance called codomi-
nance. The 9q34i allele (type O) is recessive; the
9q34IAand 9q34IBalleles (type A and type B) are

dominant. When the 9q34IAand 9q34IBalleles

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 con-

text 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 DNAfragments,

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-

113

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