allophene a phenotype not due to the mutant ge- netic constitution of the cells of the tissue in ques- tion. Such a tissue will develop a normal phenotype if transplanted to a wild-type host. See autophene.
allophenic mice chimeric mice produced by re- moving cleaving eggs from mice of different geno- types, fusing the blastomeres in vitro, and reim- planting the fused embryos into the uterus of another mouse to permit embryogenesis to continue.
Viable mice containing cells derived from two or more embryos have been obtained and used in cell lineage studies. See Appendix C, 1967, Mintz. alloplasmic referring to organisms or cells bearing chromosomes of one species and cytoplasm of a dif- ferent species; for example, bread wheat (Triticum aestivum) chromosomes and rye (Secale cereale) cy- toplasm. Compare with heteroplasmic, heteroplas- tidy.
allopolyploid (also alloploid) a polyploid organ- ism arising from the combination of genetically dis- tinct chromosome sets. See isosyndetic alloploid, seg- mental alloploid.
alloprocoptic selection a mode of selection in which association of opposites increases the fitness of the associates. An example involves the loci gov- erning alcohol dehydrogenase in Drosophila melano- gaster. The fertility is greater than expected when two mating individuals are homozygous for different alleles and smaller than expected when they are ho-
mozygous for the same allele. allostery the reversible interaction of a small mol- ecule with a protein molecule, which leads to changes in the shape of the protein and a consequent alteration of the interaction of that protein with a third molecule.
allosteric effectors small molecules that revers- ibly bind to allosteric proteins at a site different from the active site, causing an allosteric effect. allosteric enzyme a regulatory enzyme whose cat- alytic activity is modified by the noncovalent attach- ment of a specific metabolite to a site on the enzyme other than the catalytic site. allosteric protein a protein showing allosteric ef- fects.
allosteric site a region on a protein other than its active site (q.v.), to which a specific effector mole- cule may bind and influence (either positively or negatively) the functional activity of the protein. For example, in the lactose system of E.
coli, the lac re- pressor becomes inactive (cannot bind to the lac op-erator) when allolactose is bound to the allosteric site of the repressor molecule. See lac operon. allosyndesis the pairing of homoeologous chro- mosomes (q.v.) in an allopolyploid (q.v.).
Thus if the genetic composition of an alloploid is given by AABB, where AA represent the chromosomes de- rived from one parent species and BB the chromo- somes derived from the other parent species, then during meiotic prophase, A undergoes allosyndetic pairing with B.
Such pairing indicates that the A and B chromosomes have some segments that are ho- mologous, presumably because the two parent spe- cies have a common ancestry. In the case of autosyn- desis, A pairs only with A, and B with B.
Segmental alloploids form both bivalents and multivalents dur- ing meiosis because of allosyndesis. allotetraploid an organism that is diploid for two genomes, each from a different species; synonymous with amphidiploid (q.v.). allotypes proteins that are products of different alleles of the same gene. The term is often used to refer to serologically detectable variants of immu- noglobins and other serum proteins.
allotype suppression the systematic and long- term suppression of the expression of an immuno- globulin allotype in an animal induced by treatment with antibodies against the allotype. allotypic differentiation See in vivo culturing of imaginal discs.
allozygote an individual homozygous at a given locus, whose two homologous genes are of indepen-dent origin, as far as can be determined from pedi-gree information. See autozygote. allozymes allelic forms of an enzyme that can be distinguished by electrophoresis, as opposed to the more general term isozyme (q.v.).
See Appendix C, 1966, Lewontin and Hubby. alpha amanitin See amatoxins. alpha chain one of the two polypeptides found in adult and fetal hemoglobin (q.v.). alpha fetoprotein the major plasma protein of fe- tal mammals. AFP is a 70-kilodalton glycoprotein that is synthesized and secreted by the liver and the yolk sac.
The genes encoding AFP and serum albu- men arose in evolution as the result of a duplication of an ancestral gene (3-5) × 108 years ago. See al- bumen. alpha galactosidase an enzyme that catalyzes the hydrolysis of substrates that contain α-galactosidic residues, including glycosphingolipids and glycopro-