Allergic reaction
An inflammatory response triggered by a weak antigen (an allergen) to which most
individuals do not react; involves the release of large amounts of histamine from mast cells.
Allometric growth
The variation in the relative rates of growth of various parts of the body, which helps shape
the organism.
Allopatric speciation
A mode of speciation induced when the ancestral population becomes segregated by a
geographical barrier.
Allopolyploid
A common type of polyploid species resulting from two different species interbreeding and
combining their chromosomes.
Allosteric site
A specific receptor site on an enzyme molecule remote from the active site. Molecules bind
to the allosteric site and change the shape of the active site, making it either more or less
receptive to the substrate.
Allozymes
Slightly different versions of the same enzyme, distinguishable via gel electrophoresis.
Alpha helix
A spiral shape constituting one form of the secondary structure of proteins, arising from a
specific hydrogen-bonding structure.
Alternation of generations
A life cycle in which there is both a multicellular diploid form, the sporophyte, and a
multicellular haploid form, the gametophyte; characteristic of plants.
Alternative splicing
In alternative splicing, the same pre-mRNA molecule, which consists of introns and exons, is
spliced in different ways to produce mature mRNAs of different lengths and different
functionality.
Altruistic behavior
The aiding of another individual at one's own risk or expense.
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