1931 in genetics, from German allel, abbreviation of allelomorph "alternative form of a gene" (1902), coined from Greek allel- "one another" (from allos "other;" from PIE root *al- (1) "beyond") + morphē "form," a word of uncertain etymology.
1670s, "tending to recede, going backward," from Latin recess-, past-participle stem of recedere "to go back, fall back" (see recede) + -ive. Linguistics sense in ancient Greek grammar is from 1879; in genetics, of a hereditary trait present but not perceptibly expressed in the individual organism, 1900, from German recessiv (Mendel, 1865). Related: Recessively; recessiveness.
early 15c., "act of passing back or returning," from Latin regressionem (nominative regressio) "a going back, a return," noun of action from past-participle stem of regredi "to go back" (see regress (n.)).
From 1590s as "act of returning toward a point of departure" (of fluids, spirits, actions, etc.); from 1640s as "return (to or into) a certain state or condition, relapse." Genetics sense is by 1885.
late 14c., mutacioun, "action or process of changing," from Old French mutacion (13c.), and directly from Latin mutationem (nominative mutatio) "a changing, alteration, a turn for the worse," noun of action from past-participle stem of mutare "to change" (from PIE root *mei- (1) "to change, go, move"). The genetics sense "process whereby heritable changes in DNA arise" is from 1894. The linguist's i-mutation is attested from 1874; earlier was i-umlaut (1869), from German, for which mutation was Sweet's English substitute.
"brother or sister," 1903, modern revival (in anthropology) of Old English sibling "relative, kinsman," from sibb "kinship, relationship; love, friendship, peace, happiness," from Proto-Germanic *sibja- "blood relation, relative," properly "one's own" (source also of Old Saxon sibba, Old Frisian, Middle Dutch sibbe, Old High German sippa, German Sippe, Gothic sibja "kin, kindred"), from PIE *s(w)e-bh(o)- (source also of Old Church Slavonic sobistvo, Russian sob "character, individuality"), an enlargement of the root *swe- "self" (see idiom). Related to the second element in gossip.
The word 'sib' or 'sibling' is coming into use in genetics in the English-speaking world, as an equivalent of the convenient German term 'Geschwister' [E.&C. Paul, "Human Heredity," 1930]
In Old English, sibb and its compounds covered grounds of "brotherly love, familial affection" which tended later to lump into love (n.), as in sibsumnes "peace, concord, brotherly love," sibbian (v.) "bring together, reconcile," sibbecoss "kiss of peace." Sibship, however, is a modern formation (1908). Sib persisted through Middle English as a noun, adjective, and verb expressing kinship and relationship.