Encyclopedia of Biology

where one of the parents has chromosome 14 and 21
merge.
Approximately 4,000 children with a Down’s syn-
drome are born in the United States each year, or about
one in every 800 to 1,000 live births. The incidence is
higher for women over age 35, but the condition can
occur at any age for women.

drug Any substance presented for treating, curing, or
preventing disease in human beings or in animals. A
drug can also be used for making a medical diagnosis
or for restoring, correcting, or modifying physiological
functions (e.g., the contraceptive pill).

drug disposition Refers to all processes involved in
the absorption, distribution, METABOLISM, and excre-
tion of DRUGs in aliving organism.

drug latentiation The chemical modification of a
biologically active compound to form a new compound,
which in vivo will liberate the parent compound. Drug
latentiation is synonymous with PRODRUGdesign.

drug targeting A strategy aiming at the delivery of a
compound to a particular tissue of the body.

drumlin An oval or elongated hill of glacial drift that
looks like an overturned canoe from the air.

drupe A fleshy or pulpy fruit with a single seed
enclosed in a pit.

dual-action drug A compound that combines two
desired different pharmacological actions at a similarly
efficacious dose.

duodenum The first part of the small intestine;
short, wide, U-shaped, about 12 inches long, and clos-
est to the stomach. The bile duct (gallbladder) and pan-
creatic duct (pancreas) both open into the duodenum.

After food mixes with stomach acid, it moves into the

duodenum and mixes with digestive juices from the

pancreas, liver, and gallbladder, which starts the pro-

cess of breaking down food into its constituent parts.

duplication Aspecific kind of mutation: production

of one or more copies of any piece of DNA, including a

gene or even an entire chromosome. A chromosome

structural aberration from an error in meiosis. Duplica-

tion of a portion of a chromosome resulting from fusion

with a fragment from a homologous chromosome.

dwarfism Animals that evolve on islands are affected

by gigantism or dwarfism, the evolution of body form

102 drug

duplicated area

Before duplication After duplication

A duplication is a particular kind of mutation resulting in the pro-

duction of one or more copies of any piece of DNA, including a

gene or even an entire chromosome.(Courtesy of Darryl Leja,

NHGRI, National Institutes of Health)