1. symbol for broad heritability (also symbolized H2); h2 for narrow heritability. 2. symbol for hy- drogen. H1, H2A, H2B, H3, H4 See histones. H19 a gene that encodes one of the most abundant RNAs transcribed by the mouse embryo. The prod- uct of H19 is not a protein but an RNA particle with a sedimentation coefficient of 28S. H19 lies on chro- mosome 7, about 10 kb downstream from Igf2, the gene that encodes a protein called the insulin-like growth factor 2. The genes are oppositely imprinted, with H19 and Igf2 being expressed in the maternal and paternal homologs, respectively.
Imprinting is controlled by the imprinting control region (ICR). The mechanism for parental imprinting of H19 and Igf2 is illustrated here. Each embryonic cell has two homologs of chromosome 7, one maternally derived (M) and one paternally derived (P). ICR, lying be- tween the two genes, contains binding sites (s1-s4) for the CTCF protein (q.v.). Methylation (CH3) of these sites prevents attachment of CTCF to the ICR.
The enhancer (E) is free to loop over to the pro- moter of Igf2 and facilitate its transcription (II). When ICR is unmethylated (I), CTCF binds to it and insulates (i) Igf2 from E. So Igf2 is silenced, and H19 is switched on by its interaction with E. See Ap- pendix C, 2000, Bell and Felsenfeld; enhancers, insu- lator DNAs, insulin-like growth factors 1 and 2 (IGF-1 and IGF-2), parental imprinting. 3 H See tritium. habitat the natural abode of an organism. Habrobracon juglandis See Microbracon hebetor. Hadean the geologic eon beginning with the origin of the earth about 4.6 billion years ago and ending with the formation of the earliest rocks about 4 bil- lion years ago. See geologic time divisions. Haemanthus katherinae the African blood lily.
A favorite species used for the time-lapse photographic study of mitotic endosperm cells. haemoglobin See hemoglobin. Haemophilus influenzae a Gram-negative bacte- rium parasitic on the mucous membranes of the hu- man respiratory tract. It is the most common cause of middle ear infections in children. This bacterium is the first free-living organism to have its entire ge- nome sequenced. Therefore, L 42023, which repre- sents its Genome Sequence Database accession number (q.v.), is of historic significance. The Haemophilus
genome consists of a circular chromosome containing 1,830,173 base pairs. The number of genes it con- tains is estimated to be 1,743, and 60% of these have sequences similar to genes previously described in other bacteria. The remainder have unknown func- tions. See Appendix A, Bacteria, Proteobacteria; Ap- pendix C, 1995, Fleischmann, Venter et al.; Appendix E; shotgun sequencing, TIGR.
hairpin loops any double-helical regions of DNA or RNA formed by base pairing between adjacent inverted complementary sequences on the same strand. See attenuator, palindrome, terminators. hairpin ribozyme the catalytic center of the 359- base, negative strand of the satellite RNA (q.v.) of the tobacco ringspot virus; it consists of a catalytic segment 50 bases long and a 14-base substrate. The catalytic RNA forms a closed loop during the cleav- age reaction, hence the term hairpin ribozyme (HR). The HR has been engineered to bind to and cleave specific foreign RNAs. One of these is the transcript from a gene of the HIV-I virus that is essential for its replication. Suitably engineered ribozymes may someday play an important role in AIDS therapy. See AIDS, HIV, plus (+) and minus (−) viral strands, ribozyme.
Haldane rule the generalization that when one sex is absent, rare, or sterile, in the offspring of two dif- ferent animal races or species, that sex is the hetero- gametic sex. The Haldane rule is known to apply for various species of mammals, birds, and insects. In Drosophila and Mus, the X and Y chromosomes in- teract during spermatogenesis, with the Y repressing the transcription of certain X-linked loci.
Presum- ably, when the X and Y chromosomes are from dif- ferent species, such regulation does not take place and sterility results. Thus, the Haldane rule may be explained by the nonharmonious interaction of X- and Y-linked fertility genes in the hybrid. See Ap- pendix C, 1922, Haldane. half-chromatid conversion See chromatic conver- sion. half-life 1. biological the time required for the body to eliminate one-half of the dose of a given substance. This time is approximately the same for both stable and radioactive isotopes of any element. 2. radioactive the time required for half the mass of
a radioactive substance to decay into another sub- stance. Each radionuclide has a unique half-life. half-sib mating mating between half brother and half sister. Such individuals have one parent in common. half-tetrad analysis recombinational analysis where two of the four chromatids of a given tetrad can be recovered, as in the case of attached X chromosomes in Drosophila.
half-value layer the thickness of a specified mate- rial that reduces the flux of radiation by one-half. halide a fluoride, chloride, bromide, or iodide. Halobacterium species NRC-1 cies that can be easily cultured. It grows readily at 40°C-50°C on a well aerated tryptone-yeast extract- salt medium. Its genome has been recently se- quenced and shown to consist of a large chromo- some (2,014,239 bp) and two minichromosomes (191,346 bp and 365,425 bp). The chromosomes are a home for a total of 91 insertion sequences (q.v.) belonging to 12 different families. The ge- nome contains 2,682 coding genes, and 972 of these are URFs. Among the genes with known functions are many that encode proteins which control the ac- tive transport of anions and cations.
See Appendix A, Archaea, Euryarchaeota; Appendix C, 2000, Ng et al.; halophiles. halogen fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). halophiles bacteria that require high salt concen- trations in order to survive. They are found in salt lakes and evaporating brine. halteres paired club-shaped appendages that ex- tend from the metathorax of Dipterans. They serve as gyroscopic sense organs adapted to perceive devi- ations from the plane of their vibration. Halteres are evolutionarily equivalent to the hind pair of wings in other insects. In Drosophila, certain homeotic mu- tations (q.v.) convert halteres into wings and vice versa. See bithorax. Hamilton genetical theory of social behavior a theory put forth by W. D. Hamilton to explain how altruism can evolve when it increases the fitness of relatives. The theory proposes that a social act is fa-
vored by natural selection if it increases the inclusive fitness of the performer. Inclusive fitness consists of the individual’s own fitness as well as his effects on the fitness of any genetically related neighbors. The idea is that genetic alleles change in frequency in a population owing to effects on the reproduction of relatives of the individual in which the character is expressed, rather than on the personal reproduction of the individual. According to Hamilton, altruism is favored when k is greater than 1 divided by r, where r is the relatedness between individuals and k is the ratio of gain to loss of the behavior being stud- ied. The Hamilton theory is often referred to as kin selection. For example, a mutation that affected the behavior of a sterile worker bee so that she fed her fertile queen but starved herself would increase the inclusive fitness of that worker because, while her own fitness decreased, her actions increased the fit- ness of a close relative. See Appendix C, 1954, Ham- ilton. hammerhead ribozyme a folded RNA structure found in the genome of the satellite tobacco ring spot virus and other RNA viruses of plants. The vi- rus replicates by a rolling circle mechanism to pro- duce a concatomeric molecule. This is self-cleaved by the hammerhead to generate single genomic mol- ecules.
See hairpin ribozyme, rolling circle, viroid. hamster common laboratory rodent. See Cricetu- lus griseus, Mesocricetus auratus. hanging drop technique a method for micro- scopic examination of organisms suspended in a drop on a special concave microscope slide. The technique was invented by Robert Koch in 1878. Hansen disease the preferred term for leprosy (q.v.). The eponym honors the scientist who discov- ered the leprosy bacterium. Hansenula wingei a yeast that has provided infor- mation concerning the genetic control of mating type (q.v.). H antigens erned by histocompatibility genes (q.v.). 2. flagellar protein antigens of motile Gram-negative enterobac- teria. Compare with O antigens. haplo- the prefix haplo-, when followed by a sym- bol designating a particular chromosome, indicates an individual whose somatic cells lack one member of the designated chromosome pair. Thus, in Dro- sophila, haplo-IV means a fly that is monosomic for chromosome 4. haplodiploidy a genetic system found in some ani- mals (such as the honey bee) in which males de- velop from unfertilized eggs and are haploid, where- as the females develop from fertilized eggs and are diploid. haplodiplomeiosis See meiosis.
haploid cell culture See anther culture. haploidization a phenomenon taking place during the parasexual cycle in certain fungi during which a diploid cell is transformed into a haploid cell by the progressive loss of one chromosome after another by nondisjunction. haploid or haploidy referring to the situation or stage in the life cycle where a cell or organism has a single set of chromosomes. N refers to the normal haploid chromosome number for a species, while C is the haploid amount of DNA. Prokaryotic cells are characterized by a single large chromosome and are therefore haploid. However, Deinococcus radiourans (q.v.) is an exception. It has multiple chromosomes each of which is present in quadruplicate in each cell during the stationary phase (q.v.). See autosome, chromosome set, diploid or diploidy, merozygote, sex chromosome. haploid parthenogenesis the situation in which a haploid egg develops without fertilization, as in the honey bee.
haploid sporophytes flowering plants have a re- productive cycle in which a diploid sporophytic phase alternates with a haploid gametophytic phase. The plant represents the sporophyte, and the ga- metophytes are microscopic. As illustrated in the double fertilization entry on page 131, the male ga- metophyte is the pollen grain and the female ga- metophyte is the embryo sac. However, parthenoge- netic development of unfertilized eggs can occur, but it is very rare.
Haploid sporocytes were first re- ported in Datura stramonium (q.v.). The frequency of haploid plants can be greatly increased using an- ther culture (q.v.). Homozygous diploid plants can then be generated by treatment with colchicine (q.v.). See Appendix C, 1922, Blakeslee et al.; alter- nation of generations. haploinsufficiency the situation in which a single copy of a normal gene is not enough to ensure the normal phenotype. Therefore the heterozygote or an individual carrying a deletion of the gene in one ho- molog is detectably abnormal. haplont an organism in which only the zygote is diploid (as in the algae, protozoa, and fungi). It im- mediately undergoes meiosis to give rise to the haplophase. See diplo-haplont, diplont.