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of genetically different nuclei each from a different thallus. Contrast with homomixis. See Appendix C, 1904, Blakeslee. heteromorphic bivalent a bivalent made up of chromosomes that are structurally different and con- sequently are only partly homologous (the XY biva- lent is an example). Contrast with homomorphic bi- valent. heteromorphic chromosomes homologous chro- mosomes that differ morphologically. heteromorphosis (also homeosis) the formation, whether in embryonic development or in regenera- tion, of an organ or appendage inappropriate to its site (for example, an antenna instead of a leg).

heteromultimeric protein See heteropolymeric protein. heterophile antigen a substance that stimulates production in a vertebrate of antibodies capable of reacting with tissue components from other verte- brates or even from plants. heteroplasmy the phenomenon where two dis- tinct populations of mitochondria exist in the same invidiual. In those cases where a mutant in mtDNA is responsible for a disease, the larger the proportion of mutant mitochondria, the more likely the person will show symptoms of the disease. See Leber heredi- tary optic neuropathy (LHON), Leigh syndrome. heteroplastic transplantation a transplantation between individuals of different species within the same genus. heteroplastidy having two kinds of plastids, spe- cifically, chloroplasts and starch-storing leukoplasts.

heteroploid referring in a given species to the chromosome number differing from the characteris- tic diploid number (or haploid number, if the spe- cies has a predominating haplophase). heteropolymeric protein a protein made up of more than one kind of polypeptide (e.g., hemoglobin). heteropycnosis referring to the appearance of chromosomes or chromosomal regions that have a coiling cycle out of phase with the rest of the ge- nome. Positively heteropycnotic segments are more tightly coiled and negatively heteropycnotic seg- ments are more loosely coiled than the rest of the chromosomal complement. See allocycly, isopyc- notic. heterosis the greater vigor in terms of growth, sur- vival, and fertility of hybrids, usually from crosses between highly inbred lines. Heterosis is always as- sociated with increased heterozygosity. See gibber- ellin. heterosomal aberration See chromosomal aberra- tion. heterospory in plants, the existence (within the same species or within an individual organism) of two kinds of meiocytes (megasporocytes and micro- sporocytes) that produce two kinds of meiospores. Compare with homospory.

heterostyly a polymorphism of flowers that en- sures cross-fertilization by producing flowers having stamens and styles of unequal lengths. heterothallic fungus a fungal species producing a sexual spore that results from the fusion of geneti- cally different nuclei which arose in different thalli. Compare with homothallic fungus. heterotopic transplantation transplantation of tissue from one site to another on the same or- ganism. heterotrophs organisms that require complex or- ganic molecules such as glucose, amino acids, etc., from which to obtain energy and to build macromol- ecules. Contrast with autotrophs. heterozygosis heterozygosity. heterozygosity the condition of having one or more pairs of dissimilar alleles. heterozygote a diploid or polyploid individual that has inherited different alleles at one or more loci and therefore does not breed true. In diploids the heterozygote usually has a normal dominant al- lele and a mutant recessive allele and is sometimes called a carrier.

Contrast with homozygote. See Ap- pendix C, 1900, Bateson; compound heterozygote. heterozygote advantage the situation where the heterozygote has a greater fitness than either homo- zygote. See cystic fibrosis, glucose-6-phosphate dehy- drogenase deficiency, hemochromatosis, Indy, over- dominance, sickle-cell trait. HEXA See Tay-Sachs disease. hexaploid a polyploid possessing six sets (6N) of chromosomes or genomes; e.g., bread wheat is thought to have originated by hybridizations involv- ing three different species, each of which contrib- uted two genomes to the allohexaploid. See wheat. HEXB See Sandhoff disease. hexosaminidase an enzyme functioning in the ca- tabolism of gangliosides (q.v.). Hexosaminidase A is composed of alpha and beta subunits coded for by

Hexose monophosphate shunt

genes on human autosomes 15 and 5, respectively. Mutations at these loci result in Tay-Sachs and Sandhoff diseases (q.v.). hexose monophosphate shunt See pentose phos- phate pathway. Hfr strain a strain of Escherichia coli that shows high frequencies of recombination (hence the abbre- viation). In cells from such a strain, the F-factor is integrated into the bacterial chromosome. See Ap- pendix C, 1953, Hayes; circular linkage map. hGH human growth hormone (q.v.). hGHR human growth hormone receptor (q.v.). HGPRT hypoxanthine-guanine-phosphoribosyl- transferase (q.v.); an enzyme involved in the sal- vage pathway (q.v.) of nucleotide synthesis. See HAT medium. hibernate to be dormant during winter. Many mammals, reptiles, amphibians, and certain inverte- brates hibernate. See also aestivate. hierarchy an organization pattern involving groups within groups, as exemplified by the taxonomic hi- erarchy of organisms. See classification.

high-energy bond a covalent chemical bond (e.g., the terminal phosphodiester bond of adenosine tri- phosphate) that liberates at least 5 kcal/mol free en- ergy upon hydrolysis. high-energy phosphate compound a phosphory- lated molecule that upon hydrolysis yields a large amount of free energy. See ATP, phosphate bond en- ergy. high frequency of recombination cell See Hfr strain. highly repetitive DNA the fast component in reas- sociation kinetics, usually equated with satellite DNA. See repetitive DNA. high-resolution chromosome studies analysis of the banding pattern of human chromosomes at pro- metaphase, when they are longer than at metaphase. Such analyses reveal 700-1200 bands and allow a more detailed localization of adjacent genes than is possible for metaphase chromosomes, where 300- 600 bands are typically observed. See human chro- mosome band designations.

Himalayan mutant an allele at the albino locus (with known examples in the mouse, rat, rabbit, guinea pig, hamster, and cat) associated with a very lightly pigmented body and somewhat darker ex- tremities. The form of tyrosinase encoded by this gene is temperature sensitive and normally functions well only in the extremities where the body temper- ature is lower. Such animals raised in cold environ- ments show darker pigmentation. See albinism, tem- perature-sensitive mutation, tyrosinase. HindII See restriction endonuclease. hinge region See immunoglobulin. hinny See horse-donkey hybrids. his histidine. See amino acid. histidine See amino acid. histidinemia a hereditary disease in humans aris- ing from a deficiency of the enzyme histidase. histidine operon a polycistronic operon of Salmo- nella typhimurium containing nine genes involved in the synthesis of histidine. Fused genes from this op- eron encoded a fused enzyme.

See Appendix C, 1970, Yourno, Kohno, and Roth. histochemistry the study by specific staining methods of the distribution of particular molecules within sections of tissues. See Appendix C, 1825, Raspail. histocompatibility antigen genetically encoded cell-surface alloantigens that can cause the rejection of grafted tissues, cells, and tumors bearing them. See Appendix C, 1937, Gorer. histocompatibility gene a gene belonging to the major histocompatibility (MHC) system or to any of numerous minor histocompatibility systems respon- sible for the production of histocompatibility anti- gens (q.v.) See Appendix C, 1948, Snell. histocompatibility molecules genetically encoded cell-surface alloantigens that can cause the rejection of grafted tissues, cells, and tumors bearing them.

These cell-membrane glycoproteins are grouped into two classes. Class I molecules are found on the sur- faces of all mammalian cells (except trophoblasts and spermatozoa). T lymphocytes (q.v.) of the CD8+ subgroup recognize antigenic determinants of for- eign class I histocompatibility molecules. Class II histocompatibility molecules are abundant on the surfaces of B lymphocytes (q.v.). T lymphocytes of the CD4+ subgroup recognize antigenic determi- nants of foreign class II histocompatibility mole- cules. These T cells subsequently divide and secrete lymphokines (q.v.), which are important for B cell growth and differentiation. Class I histocompatibil- ity molecules are heterodimers made up of heavy

(alpha) and light (beta) polypeptide chains. The class I chains are encoded by genes residing in the right portion of the HLA complex (q.v.). The alpha chain contains regions showing sequence diversity, whereas the beta chain has an invariant amino acid composition. The class II histocompatibility mole- cules are also dimers composed of heavy alpha and light beta chains. These are encoded by genes in the left portion of the HLA complex.

Most of the se- quence diversity of class II histocompatibility mole- cules is localized within a segment of the beta chain. Class II histocompatibility dimers are associated with a third polypeptide chain that exhibits no polymor- phism. See Appendix C, 1937, Gorer; 1987, Wiley et al.; major histocompatibility complex. histogenesis the development of histologically detectable differentiation. histogenetic antigens or responses defined by means of cell-mediated immunity. histogram a bar graph. histoincompatibility intolerance to transplanted tissue.

histology the study of tissues. histolysis tissue destruction. histone genes in both the sea urchin and Drosoph- ila, these genes are repetitive and clustered. In Stron- gylocentrotus purpuratus, the genes for H4, H2B, H3, H2A, and H1 lie in a linear sequence. They are tran- scribed from the same DNA strand to form a poly- cistronic message, and are separated by DNA spacers of similar lengths. In Drosophila there are about 110 copies of the histone genes, and these are localized in a four-band region in the left arm of chromosome 2. The gene sequence is H3, H4, H2A, H2B, H1.

Two genes are transcribed from one DNA strand, three from the other. The gene order in Xenopus laevis is the same as in Drosophila. In humans, his- tone genes show less clustering, and they are located on a variety of chromosomes (1, 6, 12, and 22). No- tophthalmus viridescens (q.v.) has 600 to 800 copies of the histone gene repeat. Transcribing histone genes have been detected in loops at specific loci on lampbrush chromosomes 2 and 6.

Most histone genes lack introns (q.v.). See Appendix C, 1972, Par- due et al.; 1981, Gall et al.; Strongylocentrotus pur- puratus. histones small DNA-binding proteins. They are rich in basic amino acids and are classified according to the relative amounts of lysine and arginine they contain (see table below). Histones are conserved during evolution. For example, H4 of calf thymus and pea differ at only two sites. The nucleosome (q.v.) contains two molecules each of H2A, H2B, H3, and H4. A single H1 molecule is bound to the DNA segment that lies between nucleosomal cores.

The histone H1 molecules are responsible for pull- ing nucleosomes together to form a 30 nm fiber. It is this fiber that is seen when nuclei are lysed and their contents are viewed under the electron micro- scope. During the maturation of sperm heads, the histones break down and are replaced with prota- mines (q.v.). See Appendix C, 1884, Kossel; 1974, Kornberg; 1977, Leffak et al.; CENP-A, chromatin fibers, chromatosome, histone genes, nucleoprotein, nucleosome, Polycomb (Pc), solenoid structure, ubi- quitin. hitchhiking the spread of a neutral allele through a population because it is closely linked to a benefi- cial allele and therefore is carried along as the gene that is selected for increases in frequency.

HIV the human immunodeficiency virus, a human RNA retrovirus known to cause AIDS. HIV is now referred to as HIV-1, to distinguish it from a related but distinct retrovirus, HIV-2, which has been iso- lated from West African patients with a clinical syn- drome indistinguishable from HIV-induced AIDS. Each viron contains two molecules of single-stran- ded, positive sense RNA (held together by hydrogen bonds to form a dimer). Each linear molecule is made up of 9.3 × 103 nucleotides. It contains gag, pol, and env, the same genes first observed in the Rous sarcoma virus (q.v.). There are seven other


genes as well, and some encode multiple proteins. See Appendix C, 1983, Montaigner and Gallo; AIDS, enzyme-linked immunosorbant assay, hairpin ribo- zyme, retroviruses, RNA vector, simian immunodefi- ciency viruses (SIVs). HLA human leukocyte antigens concerned with the acceptance or rejection of tissue or organ grafts and transplants. These antigens are on the surface of most somatic cells except red blood cells, but are most easily studied on white blood cells (hence the name). See Appendix C, 1954, Dausset. HLA complex the major histocompatibility gene complex of humans. The complex occupies a DNA segment about 3,500 kilobase pairs long on the short arm of chromosome 6. The portion of the HLA complex closest to the telomere contains the genes that encode the class I histocompatibility molecules (HLA-B, -C, and -A). The portion closest to the centromere contains the genes encoding the class II histocompatibility molecules (DP, DQ, and DR).

Genes encoding components of the complement (q.v.) system lie in the midregion of the complex. See major histocompatibility complex (MHC). H locus in humans, a genetic locus that encodes a fucosyl transferase enzyme that is required during an early step in the biosynthesis of the antigens of the ABO blood group system. See A, B antigens, Bombay blood group. hnRNA heterogeneous nuclear RNA (q.v.). hog See swine. Hogness box a segment 19-27 base pairs up- stream from the startpoint of eukaryotic structural genes to which RNA polymerase II binds. The seg- ment is 7 base pairs long, and the nucleotides most commonly found are TATAAAA; named in honor of David Hogness. See canonical sequence, Pribnow box, promoter, TATA box-binding protein. holandric appearing only in males.

Said of a char- acter determined by a gene on the Y chromosome. See hologynic. holism a philosophy maintaining that the entirety is greater than the sum of its parts. In biology, a hol- ist is one who believes that an organism cannot be explained by studying its component parts in isola- tion. See reductionism. Holliday intermediate See Holliday model. Holliday model a model that describes a series of breakage and reunion events occurring during cross- ing over between two homologous chromosomes. The diagram on page 209 illustrates this model. In (a) the duplex DNA molecule from two nonsister strands of a tetrad have aligned themselves in regis- ter so that the subsequent exchange does not delete or duplicate any genetic information. In (b) strands of the same polarity are nicked at equivalent posi- tions.

In (c) each broken chain has detached from its partner and paired with the unbroken chain in the opposite duplex. In (d) ligases have joined the bro- ken strands to form an internal branch point. The branch is free to swivel to the right or left, and so it can change its position. This movement is called branch migration, and in (e) the branch is shown as having moved to the right. In (f) the molecule is drawn with its arms pulled apart, and in (g) the a b segment has been rotated 180° relative to the A B segment. The result is an x-shaped configuration, and in it one can readily see the four sectors of sin- gle-stranded DNA in the branch region. To separate the structure into two duplexes, cuts must be made across the single strands at the branch region. The cuts can occur horizontally (h) or vertically (i). In the horizontal case, the separated duplexes will ap- pear as shown in (j), and when the nicks are ligated, the duplexes will each contain a patch in one strand, as shown in (k).

In the vertical case, the separated duplexes will appear as shown in (l), and when the nicks are ligased, the duplexes will contain splices in every strand, as shown in (m). It is only in this case that single crossover chromatids (Ab and aB) will be detected. Structures similar to the molecule drawn in (g) have been observed under the electron micro- scope and have been named Holliday intermediates. See Appendix C, 1964, Holliday; chi structure, ho- mologous recombination.

holoblastic cleavage cleavage producing cells of approximately equivalent size. Holocene the epoch of the Quaternary period from the end of the Pleistocene to the present time. Neolithic to modern civilization. See geologic time divisions. holocentric referring to chromosomes with diffuse centromeres. See centromere. holoenzyme the functional complex formed by an apoenzyme (q.v.) and its appropriate coenzyme (q.v.). hologynic appearing only in females. Said of a trait passed from a P1 female to all daughters occur- ring, for example, in the case of a gene linked to the W chromosome (q.v.). See holandric, matrocliny. Holometabola the superorder of insects contain- ing species that pass through a complete metamor- phosis.

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