homeosis The replacement of one body part by
another caused by mutations or environmental factors
initiating developmental anomalies.
homeostasis The ability of an organism to automati-
cally maintain a constant internal condition regardless
of the external environment.
homeothermic The process of maintaining a con-
stant body temperature.
homeotic gene Controls the overall body plan of ani-
mals by controlling the developmental fate of groups of
cells. A homeotic mutation results in the replacement of
one type of body part in place of another.
See alsoHOMEOBOX.
hominoids Acollective term used for humans and
apes.
homogamy The tendency for similar types to mate
with similar types.
homologous chromosomes A pair of chromosomes
(homologues); contains the same length, gene position,
centromere location, and same characters at corre-
sponding loci, with one homologue coming from the
mother and another from the father. Homologous
chromosomes line up with each other and then sepa-
rate during meiosis.
See alsoCHROMOSOME.
homologous structures Characteristics or parts in
different animals that may have served the same gener-
al function and are shared with related species and
inherited from a common ancestor. In related species,
they may have the same evolutionary origin, but their
functions may differ. Examples include the front fins of
a whale, forelimb of a bat or horse, and human and
chimp arm bones.
homologue Used to describe a compound belonging
to a series of compounds differing from each other by a
repeating unit, such as a methylene group, a peptide
residue, etc. Also refers to one member of a chromo-
some pair.homology The similarity of characteristics that
result from a shared ancestry; the relationship between
structures in different organisms that are united by
modification of the same structure, gene, or set of genes
of a common ancestor.homolysis(homolytic cleavage or homolytic fission)
The cleavage of a bond so that each of the molecular
fragments between which the bond is broken retains
one of the bonding electrons.homonomy(serial homology) Organs that are iden-
tical or of similar construction within the same organ-
ism (e.g., segments in annelid worms).homoplasy The possession by two or more species
of a similar characteristic that has not evolved in those
species from a common ancestor. Instead, these charac-
teristics derive from convergent evolution, parallel evo-
lution, or character reversal. An example is the wings
of insects and the wings of the flying dinosaurs, the
pterosaurs.Homoptera An order of insects with beaklike pierc-
ing-sucking mouthparts that include cicadas, aphids,
tree and leaf hoppers, and scale insects. The forewings
are either wholly membranous or wholly leathery. The
wings rest on the back in the shape of a tent, e.g.,
cicadas, frog-hoppers, and aphids. They are found
worldwide and are plant feeders.homosexual Having an affection for members of the
same sex, i.e., male attracted to male or female attract-
ed to female (lesbianism).
See alsoHETEROSEXUAL.homosexual 165