J

Jasmonic acid: An important molecule in plant defense against herbivores.
Joule (J): A unit of energy: 1J = 0.239 cal; 1 cal = 4.184J.
Junk DNA: Sequences that accumulate by mutation and that are neutral or deleterious.
Just-so stories: Untestable explanations for adaptations.
Juvenile hormone: A hormone in arthropods, secreted by the corpora allata glands, that promotes the retention of larval characteristics.
Juxtaglomerular apparatus (JGA): A specialized tissue that releases the hormone renin when blood pressure or  blood volume drops in the afferent arteriole that supplies blood to the glomerulus.
Juxtamedullary nephrons: Nephrons with well develop loops of Henle that extends deeply into the renal medulla.