16 allelic frequency tion within the segmented Ig genes of immature lymphocytes. In any one B lymphocyte (q.v.), a light chain or heavy chain can be synthesized from a maternal or paternal homolog, not both. See immunoglobulin genes, isotype exclusion, somatic recombination.
allelic frequency the percentage of all alleles at a given locus in a population gene pool represented by a particular allele. For example, in a population containing 20 AA, 10 Aa, and 5 aa, the frequency of the A allele is [2(20) + 1(10)]/2(35) = 5/7 = 0.714. See gene frequency.
allelism test complementation test (q.v.).
allelomorph commonly shortened to allele (q.v.). See Appendix C, 1900, Bateson.
allelopathy an interaction involving two different species in which chemicals introduced into the environment by one suppress the growth or reproduction of the other.
allelotype the frequency of alleles in a breeding population.
allergen a substance inducing hypersensitivity.
allergy an immune hypersensitivity response to an agent that is nonantigenic to most of the individuals in a population.
allesthetic trait any individual characteristic that has an adaptive function only via the nervous systems of other organisms, for example, odors, display of color patterns, mating calls, etc., which are important components of courtship in various species. See courtship ritual, pheromone.
Allium the genus that includes A. cepa, the onion; A. porrum, the leek; A. sativum, the garlic; and A. schoenoprasum, the chive—all classic subjects for cy-tological studies of mitotic chromosomes.
alloantigen an antigen (q.v.) that elicits an immune response (q.v.) when introduced into a genetically different individual of the same species. Antibodies produced in response to alloantigens are called alloantibodies. See histocompatibility molecules.
allochromacy the formation of other coloring agents from a given dye that is unstable in solution. Nile blue (q.v.) exhibits allochromacy.
allocycly a term referring to differences in the coiling behavior shown by chromosomal segments or whole chromosomes. Allocyclic behavior characterizes the pericentric heterochromatin, the nucleolus organizer, and in some species entire sex chromosomes. If a chromosome or chromosomal segment is tightly condensed in comparison with the rest of the chromosomal complement, the chromosome or chromosomal segment is said to show positive hetero-pycnosis (q.v.). Allocycly is also used to describe asynchronous separation of bivalents during the first anaphase in meiosis. In man, for example, the X and Y chromosomes segregate ahead of the autosomes and are said to show positive allocycly.
allogeneic disease See graft-versus-host reaction.
allogeneic graft a graft of tissue between genetically different members of the same species, especially with regard to alloantigens (q.v.). See allograft, heterograft. Compare with xenograft.
allograft a graft of tissue from a donor of one genotype to a host of a different genotype but of the same species.
allolactose See lactose.
allometry the relation between the rate of growth of a part of an individual and the growth rate of the whole or of another part. In the case of isometry, the relative proportions of the body parts remain constant as the individual grows; in all other cases, the relative proportions change as total body size increases. See heterauxesis.
allomone any chemical secreted by an organism that influences behavior in a member of another species, benefiting only the producer. If both species benefit, it is a synamone. If only the receiver benefits, it is a kairomone.
alloparapatric speciation a mode of gradual spe-ciation in which new species originate through populations that are initially allopatric, but later become parapatric before completely effective reproductive isolation has evolved. Natural selection may enhance incipient reproductive isolating mechanisms in the zone of contact by character displacement (q.v.), and other mechanisms. Compare with parapatric speciation.
allopatric speciation the development of distinct species through differentiation of populations in geographic isolation. Such populations are called al-lopatric.
allopatry referring to species living in different geographic locations and separated by distance alone or by some barrier to migration such as a mountain range, river, or desert. Compare with sympatry.
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allophene a phenotype not due to the mutant genetic constitution of the cells of the tissue in question. Such a tissue will develop a normal phenotype if transplanted to a wild-type host. See autophene.
allophenic mice chimeric mice produced by removing cleaving eggs from mice of different genotypes, 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 different species; for example, bread wheat (Triticum aestivum) chromosomes and rye (Secale cereale) cytoplasm. Compare with heteroplasmic, heteroplas-tidy.
allopolyploid (also alloploid) a polyploid organism arising from the combination of genetically distinct chromosome sets. See isosyndetic alloploid, segmental alloploid.
alloprocoptic selection a mode of selection in which association of opposites increases the fitness of the associates. An example involves the loci governing 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 molecule 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 catalytic activity is modified by the noncovalent attachment of a specific metabolite to a site on the enzyme other than the catalytic site.
allosteric protein a protein showing allosteric effects.
allosteric site a region on a protein other than its active site (q.v.), to which a specific effector molecule 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.
allosyndes is the pairing of homoeologous chromosomes (q.v.) in an allopolyploid (q.v.). Thus if the genetic composition of an alloploid is given by AABB, where AA represent the chromosomes derived from one parent species and BB the chromosomes 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 homologous, presumably because the two parent species 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 during 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 independent origin, as far as can be determined from pedigree 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 fetal 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 albumen arose in evolution as the result of a duplication of an ancestral gene (3-5) x 108 years ago. See albumen.
alpha galactosidase an enzyme that catalyzes the hydrolysis of substrates that contain a-galactosidic residues, including glycosphingolipids and glycopro-
teins. In humans, a-galactosidase exists in two forms, A and B. The A form is encoded by a gene on the X chromosome. Fabry disease (q.v.) is caused by mutations at this locus. The B form is encoded by a gene on chromosome 22.
alpha helix one of two common, regularly repeating structures seen in proteins (compare with beta pleated sheet). The alpha helix is a compact spiral with the side chains of the amino acids in the poly-peptide extending outward from the helix. The helix is stabilized by hydrogen bonds that form between the CO group of each amino acid and the NH group of the amino acid, which lies four residues ahead in the sequence. All main-chain CO and NH groups are hydrogen-bonded according to this pattern. One turn of the helix occurs for each 3.6 amino acid residues. Alpha helices are built from a continuous sequence that contains as few as 4 to as many as 50 amino acids. See Appendix C, 1951, Pauling and Corey; 1958, Kendrew et al; protein structure.
alpha particle a helium nucleus consisting of two protons and two neutrons, and having a double positive charge.
alpha tocopherol vitamin E (q.v.).
alphoid sequences a complex family of repetitive DNA sequences found in the centromeric hetero-chromatin of human chromosomes. The alphoid family is composed of tandem arrays of 170 base pair segments. The segments isolated from different chromosomes show a consensus sequence, but also differences with respect to individual bases, so that the 170 base pair units may vary in sequence by as much as 40%. The repeats are organized in turn into groups containing several units in tandem, and these groups are further organized into larger sequences 1 to 6 kilobases in length. These large segments are then repeated to generate segments 0.5 to 10 megabase pairs in size. Such larger, or "macro," DNA repeats are chromosome-specific. Since alphoid sequences are not transcribed, they play an as yet undefined structural role in the chromosome cycle. The variation in the sequences within the alphoid DNA results in a high frequency of RFLPs. These are inherited and can be used to characterize the DNAs of specific individuals and their relatives. See DNA fingerprint technique, restriction fragment length polymorphisms.
alteration enzyme a protein of phage T4 that is injected into a host bacterium along with the phage DNA; this protein modifies host RNA polymerase by linking it to ADP-ribose. RNA polymerase modified in this way renders it incapable of binding to sigma factor and thus unable to initiate transcription at host promoters. See RNA polymerase.
alternate disjunction, alternate segregation See translocation heterozygote.
alternation of generations reproductive cycles in which a haploid phase alternates with a diploid phase. In mosses and vascular plants, the haploid phase is the gametophyte, the diploid the sporo-phyte.
alternative splicing a mechanism for generating multiple protein isoforms from a single gene that involves the splicing together of nonconsecutive exons during the processing of some, but not all, transcripts of the gene. This is illustrated in the diagram, where a gene is made up of five exons joined by in-trons i1—i4. The exons may be spliced by the upper pathway shown by the dotted lines to generate a mature transcript containing all five exons. This type of splicing is termed constitutive. The alternative mode of splicing shown generates a mature transcript that lacks exon 4. If each exon encodes 20 amino acids, the constitutive splicing path would result in a poly-peptide made up of 100 amino acids. The alternative path would produce a polypeptide only 80 amino acids long. If the amino acid sequences of the two proteins were determined, the first 60 and the last 20 would be identical. The premessenger RNAs (q.v.) of at least 40% of all human genes undergo alternative splicing. This removes the intron RNAs and joins the adjacent exon RNAs by phosphodies-ter linkages. The splicing takes place in spliceosomes (q.v.) that reside within the nucleus. Therefore the number of proteins encoded by the human genome is many times larger than the number of structural genes it contains. See Appendix C, 1977, Weber et al.; adenovirus, DSCAM, fibronectin, Human Genome Project, isoforms, posttranscriptional processing, myosin genes, RNA splicing, tropomyosin.
altricial referring to the type of ontogeny seen in vertebrate species characterized by large litters, short gestations, and the birth of relatively undeveloped, helpless young. Compare with precocial.
altruism behavior of an individual that benefits others. To the extent that the "others" are related to the altruist (the one exhibiting altruistic behavior), such actions may actually be an expression of fitness. See inclusive fitness.
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