cally dissimilar parents, even different species. See Raphanobrassica, mullato, mule. hybrid arrested translation a method for identify- ing the cDNA corresponding to an mRNA that de- pends upon the ability of cDNA to hybridize with its mRNA and thereby to inhibit its translation in an in vitro system; the disappearance of the translation product from the system indicates the presence of the cDNA. hybrid breakdown the reduction in fitness of F2 and/or backcross populations from fertile hybrids produced by intercrossing genetically disparate pop- ulations or species; a postzygotic reproductive isolat- ing mechanism.
hybrid corn commercial corn grown from seed produced by the “double cross” (q.v.) procedure. Such corn is characterized by its vigor and unifor- mity. See Appendix C, 1909, Shull. hybrid DNA model a model used to explain both crossing over and gene conversion by postulating that a short segment of heteroduplex (hybrid) DNA is produced from both parental DNAs in the neigh- borhood of a chiasma. See Holliday model. hybrid duplex molecule an experimentally recon- stituted molecule containing a segment of single- stranded DNA hydrogen bonded to a second RNA or DNA molecule of complementary base sequence.
hybrid dysgenesis a syndrome of correlated ge- netic abnormalities that occurs spontaneously when hybrids are formed between certain strains of Dro- sophila. The hybrids show germ line defects includ- ing chromosomal aberrations, high frequencies of le- thal and visible mutations, and in most extreme cases, sterility. The cause of PM hybrid dysgenesis is a transposable element named P. Long-established laboratory strains lack P elements.
Those strains sus- ceptible to P elements are called M strains. P elements are often present in strains established from newly captured wild flies. The cross P male × M female generates dysgenic F1 individuals; the reciprocal cross does not. The P elements do not produce dysgenesis within P strains. P elements are dis- persed over all the chromosome arms, and in P strains their transposition is repressed.
When chromosomes carrying P elements are placed in M cytoplasm by suitable crosses, the P elements become derepressed and transpose at high rates, disrupting genetic loci and causing the dysgenic syndrome. See Appendix C, 1982, Bingham et al.; dysgenesis, P elements. hybrid inviability a postmating (postzygotic) re- productive isolating mechanism in which hybrids between disparate populations fail to survive to re- productive age. hybridization 1. the mating of individuals belong- ing to genetically disparate populations or to differ- ent species. 2. in Mendelian terms, the mating of any two unlike genotypes or phenotypes. 3. the pairing of complementary RNA and DNA strands to pro- duce an RNA-DNA hybrid, or the pairing of com- plementary DNA single strands to produce a DNA- DNA hybrid.
See introgressive hybridization. hybridization competition a technique for distin- guishing different mRNA molecules, using a varia- tion of the basic filter hybridization (q.v.) technique. A specific DNA sequence trapped on a nitrocellu- lose filter is exposed to a tritiated RNA known to be complementary to that DNA. An unlabeled RNA of unknown specificity is added and, if complementary to the DNA, it will compete with the labeled mole- cules for hybridization to the DNA.
Any diminution of labeling in the hybrid after equilibrium is reached is attributed to displacement by the unlabeled mole- cules. hybridogenesis a form of clonal reproduction in species hybrids whose gametes carry only the nu- clear genome derived from one of the parental spe- cies. For example, a hybrid species of frog, Rana es- culenta, is derived from crosses between R. lessonae and R. ridibunda, but only ridibunda chromosomes (and lessonae mitochondria) are found in gametes of esculenta. hybridoma a cell resulting from the fusion of an antibody-producing plasma cell (a B lymphocyte) and a myeloma (a bone-marrow cancer) cell.
Such a hybrid cell produces a clone that can be maintained in tissue culture or as an animal tumor, and the clone may secrete only a single kind of antibody. Such monoclonal antibodies are used as probes in western blot experiments or in the histochemical lo- calization of antigens of interest. See Appendix C, 1975, Kohler and Milstein; 1980, Olsson and Kaplan; immunofluorescence, transfectoma. hybrid resistance the phenomenon whereby tu- mors may grow more readily in homozygous recipi- ents than in heterozygous recipients even though the tumor may be genetically histocompatible with both types of recipients. hybrid sterility the failure of hybrids between dif- ferent species to produce viable offspring. See Hal- dane rule for hybrids.
hybrid swarm a continuous series of morphologi- cally distinct hybrids resulting from hybridization of two species followed by crossing and backcrossing of subsequent generations. hybrid vigor heterosis (q.v.). hybrid zone a geographical zone where hybrids between two geographical races are observed. See Appendix C, 1973, Hunt and Selander. hydrocarbon an organic compound composed only of carbon and hydrogen atoms. hydrogen the most abundant of the biologically important elements. Atomic number 1; atomic weight 1.00797; valence 1+, most abundant isotope 1H, heavy isotope 2H (deuterium); radioisotope 3H (tritium, q.v.).
hydrogen bond a weak (relative to a covalent bond) electrostatic attraction between an electro- negative atom (such as O or N) in one chemical group and a hydrogen atom covalently bound to an electronegative atom in another group. Hydrogen bonds between purines and pyrimidines are impor- tant because they are the basis for molecular recog- nition during replication and transcription. Accord- ing to the DNA model proposed by Watson and Crick, adenosine (A) pairs with two hydrogen bonds to thymine (T) and guanine (G) pairs with three hy- drogen bonds to cytosine (C).
In the diagram below, the hydrogen bonds are dashed lines, open circles represent oxygens, and closed circles represent nitro- gens. The amino groups of adenine and cytosine serve as hydrogen donors, and nitrogen atoms (N-1 in adenine and N-3 in cytosine) serve as hydrogen acceptors. The oxygen of the C-2 of cytosine is also a hydrogen acceptor. In the case of thymine, hydro- gen is donated by N-3 and accepted by the oxygen of C-4. Guanine has two hydrogen donors (N1 and the amino group of C-2) and one hydrogen acceptor (the oxygen of C-6). In RNA the hydrogen-bonding abilities of uracil mimic those of thymine. K. Hoog- steen proved in 1963 that the bases can adopt a dif- ferent conformation, as shown in the upper part of the diagram.
Hoogsteen base pairs do not occur in DNA, except in the regions of the telomeres (q.v.). See Appendix C, 1973, Rosenberg et al.; alpha helix, bases of nucleic acids, beta pleated sheet, deoxyribo- nucleic acid, guanine quartet model. hydrogen ion concentration expressed as the log- arithm of the reciprocal of the concentration of hy- drogen ions in grams per liter of solution; abbrevi- ated as pH, the scale runs from 0 to 14, with values above 7 basic; those below, acidic. hydrogenosomes organelles that resemble mito- chondria, but generally lack genomic DNA.
They generate hydrogen and ATP and are found in various unrelated protoctists, such as anaerobic ciliates and flagellates. However, one ciliate (Nyctotherus ovalis), living in the hindgut of cockroaches, has a rudimen- tary genome which encodes components of the mi- tochondrial electron transport chain. This finding supports the idea that most hydrogenosomes have evolved from mitochondria. See adenosine phos- phate.
hydrogen peroxide H2O2. See catalase, horserad- ish peroxidase, peroxisomes. hydrolases enzymes that catalyze the transfer of water between donor and receptor molecules. Pro- teolytic enzymes are a special class of hydrolases. hydrolysis the splitting of a molecule into two or more smaller molecules with the addition of the ele- ments of water. hydroperoxyl radical HO2, an oxidizing agent formed during the interaction of ionizing radiation with oxygenated water. See free radical. hydrophilic water attracting; referring to mole- cules or functional groups in molecules that readily associate with water.
The carboxyl, hydroxyl, and amino groups are hydrophilic. hydrophobic water repelling; referring to mole- cules or functional groups in molecules (such as al- kyl groups) that are poorly soluble in water. Popula- tions of hydrophobic groups form the surface of water-repellent membranes. hydrophobic bonding the tendency of nonpolar groups to associate with each other in aqueous solu- tion, thereby excluding water molecules. hydrops fetalis a fetal syndrome characterized by the accumulation of fluids in the tissues, jaundice, and erythroblastosis due to hypoxia.
It occurs in in- dividuals homozygous for deletions of alpha hemo- globin genes (αo/αo). The mothers are generally of Southeast Asian or Mediterranean origin. Affected infants die in utero between 20 and 40 weeks of ges- tation or soon after birth. When obstetric assistance is not available, 20-50% of mothers with a hydropic fetus suffer lethal complications. See thalassemias. hydroquinone See quinone. hydrothermal vent a fissure in the ocean floor out of which hot water flows. See hyperthermophile, un- dersea vent communities. hydroxykynurenine See Drosophila eye pigments.
hydroxylamine NH2OH, a mutagen that converts the NH2 of cytosine to NHOH, which pairs only with adenine. hydroxylapatite a hydrated form of calcium phos- phate that is naturally found in bone and tooth enamel. A synthetic form of this compound is used by molecular biologists to separate single-stranded nucleic acids from double-stranded ones. The latter bind to a column of hydroxylapatite, while single- stranded nucleic acids pass through it. Also called hydroxyapatite. 5-hydroxymethyl cytosine a pyrimidine found in- stead of cytosine in the DNA of T-even coliphages. 5-Hydroxymethyl cytosine pairs with guanine. It is postulated that the phage-specific DNase, which breaks down the host DNA, attacks DNA molecules containing cytosine. See T phages.
hydroxyurea a compound that inhibits semicon- servative DNA replication, but not repair synthesis. Hylobates a primate genus that contains nine spe- cies of gibbons. H. concolor is the best known from the genetic standpoint. Its haploid chromosome number is 26, and 20 genes have been assigned to 10 syntenic groups. hymenopteran an insect belonging to the order Hymenoptera (which includes bees, ants, wasps, etc.). See Appendix A. hyparchic genes See autarchic genes. hyperammonemia a hereditary disease in man arising from a deficiency of the enzyme ornithine carbamoyl transferase.
hypercholesterolemia See familial hypercholester- olemia. hyperchromic shift an increase in the absorbtion of ultraviolet light by a solution of DNA as these molecules are subjected to heat, alkaline conditions, etc. The shift is caused by the disruption of the hy- drogen bonds of each DNA duplex to yield single- stranded structures. hyperdontia the hereditary presence of one or more additional teeth. hyperglycemia an increased glucose content in the blood. hyperlipemia an increased concentration of neu- tral fat in the blood serum. hypermorph a mutant gene whose effect is similar to, but greater than, that of the standard or wild- type gene.