homopolymer tails a segment containing several of the same kind of deoxyribonucleotides arranged in tandem at the 3′ end of a DNA strand. See Appen- dix C, 1972, Lobban and Kaiser; terminal transferase. Homo sapiens the species to which human beings belong.
The scientific name was coined in 1758 by Linnaeus. The estimated genome size is 3.2 × 109 base pairs. See Appendix A, Chordata, Mammalia, Primates; Appendix C, 1735, Linne´; 1966, McKu- sick; 1991, Ijdo et al.; Appendix E, Appendix F; homi- nid, Hominoidea, human chromosome band designa- tions, human gene maps, human genetic databases, Human Genome Project, human mitotic chromo- somes, shotgun sequencing, symbols used in human cytogenetics. homosequential species species with identical karyotypes, as found in many species of Drosophila endemic to the Hawaiian Islands. homosomal aberration See chromosomal aberra- tion. homospory in plants of both mating types or sexes, the production of meiospores of equivalent size.
Compare with isogamy. homothallic fungus a fungal species producing a sexual spore that results from the fusion of geneti- cally different nuclei derived from the same thallus. Compare with heterothallic fungus. homozygosity the condition of having identical al- leles at one or more loci in homologous chromosome segments. homozygote an individual or cell characterized by homozygosity.
See Appendix C, 1900, Bateson. homozygous having identical rather than different alleles in the corresponding loci of homologous chromosomes and therefore breeding true. See het- erozygosity. homunculus a miniature individual imagined by early biologists to be present in a sperm. Hoogsteen base pairs See hydrogen bond. hopeful monster See saltation. Hordeum vulgare L. barley, a cereal grain domes- ticated about 10,000 years ago in the fertile crescent (q.v.). It is the fourth most important cereal crop (coming after wheat, maize, and rice). Its haploid chromosome number is 7, and its genome size is 4.8 gigabase pairs.
Malted barley is used in the brewing industry, and food-grade malts are components in the formulation of baked goods and breakfast cere- als. See Appendix A, Plantae, Tracheophyta, Angio- spermae, Monocotolyledoneae, Graminales. horizontal classification a system of evolutionary classification that tends to unite transitional forms with their ancestors; the opposite of vertical classifi- cation (q.v.). horizontal mobile elements (HMEs) genetic ele- ments capable of horizontal transmission Such HMEs enter host cells during conjugation or infection and subsequently insert their DNAs into the chromosomes of the hosts.
In prokaryotes, ex- amples of HMEs are F factors, R (resistance) plas- mids, and prophages (all of which see). The trans- posable elements (q.v.) first identified in corn are examples of eukaryotic HMEs. The Ti plasmid (q.v.) can accomplish transkingdom DNA exchanges. See Appendix C, 1977, Lawrence and Oschman; epi- some, selfish DNA, sympatric speciation. horizontal transmission the transfer of genetic in- formation from one cell or individual organism to contemporary cells or organisms by an infection-like process, in contrast to vertical transmission (q.v.). See Appendix C, 1999, Nelson et al.; Bacillus, mariner elements, P elements, Wolbachia. hormone an organic compound produced in one part of an organism and transported to other parts, where it exerts a profound effect.
Mammalian hor- mones include ACTH, endorphins, epinephrine, FSH, glucagon, GH, LH, insulin, intermedin, oxyto- cin, progesterone, prolactin, secretin, somatostatin, testosterone, thyroxin, and vasopressin. See also alla- tum hormones, ecdysterone, endocrine system, plant growth regulators. hormone receptors molecules located on the sur- face or within target cells to which hormones bind.
When such receptors are absent or defective, the hormones are without effect. Examples of genetic diseases caused by mutations of genes that encode hormone receptors are androgen-insensitivity syn- drome (q.v.) and vitamin D-resistant rickets (q.v.). See androgen receptor, steroid receptor, vitamin D re- ceptor. horotelic evolution See evolutionary rate. horse any of a number of domesticated breeds of the species Equus caballus.
Popular breeds include: DRAFT HORSES: Belgian, Clydesdale, Percheron, Shire, Suffolk. COACH HORSES: Cleveland Bay, French Coach, German Coach, Hackney. LIGHT HARNESS HORSES: American Trotter. SADDLE HORSES Ameri- can Saddle Horse, American Quarter Horse, Ap- paloosa, Arabian, Morgan, Palomino, Tennessee
Walking Horse, Thoroughbred, Lippizzaner. PONIES: Hackney Pony, Shetland Pony, Welsh Pony. See Equidae. horse bean broad bean (q.v.). horse-donkey hybrids the horse female X donkey male cross produces a mule; the reciprocal cross pro- duces a hinny. These hybrids have 63 chromosomes and their mitochondria are of maternal origin. Both mules and hinnies are sterile, but mules have been successfully cloned. The procedure involved im- planting a nucleus from a mule fetus into the enucle- ated egg from a horse. Therefore cloned mules also have horse mitochondria. See Appendix C, 1974, Hutchison; Equus, donkey.
horseradish peroxidase the most widely studied of the peroxidase enzymes that utilize hydrogen per- oxide as an oxidant in the dehydrogenation of vari- ous substrates. The reaction catalyzed is AH2 + H2O2 →A+2H2O,whereAisthesubstrate.Horseradish peroxidase reacts with a variety of organic reagents to produce a colored product. So when this enzyme is covalently linked to a reporter antibody, the posi- tion or amount of the complementary antigen can be determined by the location or amount of the dye generated. See enzyme-linked immunosorbant assay. host 1. an organism infected by a parasite. 2. the recipient of a graft from a donor individual. host-cell reactivation cut-and-patch repair (q.v.) of UV-induced lesions in the DNAs of bacterio- phages once they infect a host cell. Host-cell reacti- vation does not occur in the case of ssDNA or RNA viruses.
host-controlled restriction and modification See DNA restriction enzyme, restriction and modification model. host range the spectrum of strains of a species of bacterium that can be infected by a given strain of phage. The first mutations to be identified in phage involved host range. The term is used more generally to refer to the group of species that can be attacked by a given parasite. Some bacteria can parasitize cer- tain mammals, but only at specific developmental stages.
For example, the K99 strain of E. coli can in- fect calves, lambs, and piglets, but not adult cattle, sheep, and swine. The resistance shown by the adult farm animals is due to the replacement of cell sur- face receptors by molecules to which the bacterium does not bind. See Appendix C, 1945, Luria; Duffy blood group gene. host-range mutation a mutation of a phage that enables it to infect and lyse a previously resistant bacterium. hot spot 1. a site at which the frequencies of spon- taneous mutation or recombination are greatly in- creased with respect to other sites in the same cis- tron.
Examples are in the rII gene of phage T4 and in the lacZ and trpE genes of E. coli. 2. a chromo- somal site at which the frequencies of mutations are differentially increased in response to treatment with a specific mutagen. See Appendix C, 1961, Benzer; 5-methyl cytosine, muscular dystrophy. hot spot archipelago a chain of islands formed when a tectonic plate moves over a stationary plume of lava arising from the earth’s deep mantle. For ex- ample, the Hawaiian archipelago was formed as the Pacific tectonic plate moved northwestward (at a rate estimated to be about 9 cm per year) over just such a hot spot. This erupted periodically, perforat- ing the plate with volcanoes that grew until some of them rose above sea level to form an island chain. Kauai was the first of the large islands to be formed, 5 million years ago. Hawaii, the youngest island, was formed about 500,000 years ago, and its three active volcanoes sit above the hot spot.
An undersea vol- cano, Loihi, is also erupting 30 km off shore on the southeast flank of Mauna Loa. The hot spot hypothesis, put forth in 1963 by J. Tuzo Wilson, is now the main unifying theory that explains the origin of many groups of oceanic islands worldwide. The Galapagos islands (q.v.) are also a hot spot archipelago. They began as a group of sub- marine volcanoes that grew progressively from the ocean floor until about 4.5 million years ago, when they emerged above sea level.
The northwest islands of Fernandina and Isabela, which currently lie above the hot spot, have active volcanoes. See plate tec- tonics. housekeeping genes constitutive loci that are theoretically expressed in all cells in order to provide the maintenance activities required by all cells: e.g., genes coding for enzymes of glycolysis and the citric acid cycle. Hox genes genes that contain homeoboxes (q.v.).
The Hox genes of most animals have three proper- ties: (1) the genes are arranged along a chromosome in tandem arrays, (2) they are expressed sequentially along the chromosome, and (3) the sequence of acti- vation of the genes corresponds to the relative order in which the genes are expressed in tissues along the main anterior-posterior axis of the organism. The sit- uation is illustrated for some animal species that show bilateral symmetry (see Appendix A, Eumeta-
zoa, Bilateria). In the case of Drosophila melanogas- ter, where Hox genes were discovered, there are eight such genes, and they all reside on chromosome 3. The genes lab, pb, Dfd, Scr, and Antp belong to the Antennapedia complex, while farther down the chromosome are the genes of the bithorax complex (Ubx, abd-A, and Abd-B). The location of tissues along the anterior-posterior axis of the embryo where the genes are active is shown at the top of the figure. Several Hox genes also occur in a similar sequence on specific chromosomes of Tribolium, Branchiostoma, and Caenorhabditis (all of which see).
In mammals like the laboratory mouse and humans there are 38 or 39 Hox genes grouped in clusters on 4 different chromosomes. The locations of nine genes of the Hox-2 complex on mouse chromosome 11 are shown below the homologous genes from the fly. Moving from left to right in the mouse Hox-2 complex, each gene is expressed progressively later in development. Human chromosome 7 contains the Hox-1 cluster of nine genes, which are homologs of the aligned genes of the Hox-2 cluster of the mouse. See Appendix C, 1978, Lewis; 1983, Hafen, Levine, and Gehring; 1984, McGinnis et al.; Antennapedia, bicoid, bithorax, floral identity mutations, homeotic mutations, polycomb, segment identity genes, selec- tor genes. heterochromatin-associated protein 1 (q.v.).
HPHF hereditary persistence of hemoglobin F. HPRL human prolactin. See human growth hor- mone. HPRT hypoxanthine-guanine-phosphoribosyl transferase (q.v.). H substance a precursor polysaccharide for pro- duction of A and B antigens of the ABO blood group system. It is usually unmodified on group O cells, but is modified by the addition of different sugars to produce the A or B antigens. The molecule is speci- fied by a gene on human chromosome 19. See Bom- bay blood group. hts See hu-li tai shao (hts). hu-li tai shao (hts) a second chromosomal gene in Drosophila melanogaster which encodes isoforms of adducin-related protein. Some are expressed during pre-adult development (e.g., in the embryonic cen- tral nervous system and epidermis and in the diges- tive system of embryos and larvae). Two isoforms are expressed in the germ line. One localizes in the spectrosome (q.v.) and the fusome (q.v.) and is ho- mologous to mammalian adducin (q.v.), while the a
Human genetic diseases
other coats ring canals (q.v.). Female sterile muta- tions of hts eliminate the formation of spectrosomes and fusomes, and produce egg chambers with re- duced numbers of nurse cells, which often lack oo- cytes (hence the name hu-li tai shao, which means too little nursing in Chinese). See cystocyte divisions. human adenovirus 2 (HAdV-2) an adenovirus (q.v.) that causes respiratory infections in children.
The genome is a double-stranded DNA molecule that contains 35,937 bp and includes 11 transcrip- tion units. Split genes (q.v.) and alternative splicing (q.v.) were found in HAdV-2. human chromosome band designations quina- crine and Giemsa-stained human metaphase chro- mosomes show characteristic banding patterns, and standard methods have been adopted to designate the specific patterns displayed by each chromosome.
The X chromosome shown here illustrates the ter- minology. In the diagram, the dark bands represent those regions that fluoresce with quinacrine or are darkened by Giemsa. The short (p) arm and the longer (q) arm are each divided into two regions. In the case of longer autosomes, the q arm may be di- vided into three or four regions and the p arm into three regions. Within the major regions, the dark and light bands are numbered consecutively.
To give an example of the methods used for assigning loci, the G6PD gene is placed at q28, meaning it is in band 8 of region 2 of the q arm. The color-blindness genes are both assigned to q27-qter. This means they reside somewhere between the beginning of q27 and the terminus of the long arm. See Appendix C, 1970, Casperson, Zech, and Johansson; 1971, O’Riordan et al.; high resolution chromosome stud- ies, human mitotic chromosomes. human cytogenetics See human mitotic chromo- somes, symbols used in human cytogenetics.
human cytomegalovirus (HCMV) a giant virus (240 nm in diameter) belonging to the Herpesviri- dae. Most of the congenital viral infections in hu- mans are caused by this virus. The genome is a 230 kbp linear dsDNA in which reside at least 208 ORFs. HCMV is covered by a phospholipid enve- lope. Between this and the capsid is a compartment (the tegument) which contains 20 different proteins encoded by the viral genome.
Four different mRNAs are packaged inside the virion. See Appendix F, virus. human gene maps the total human genome con- tains 3.2 Gbp of DNA, and 2.95 Gbp of this repre- sents euchromatin (q.v.). There are about 31,000 genes that transcribe mRNAs and at least 750 genes that encode other RNAs.
For example, there are about 500 different tRNA genes. The genes are dis- tributed over 22 pairs of chromosomes that range in size from autosome 1 (263 Mbp) to autosome 21 (50 Mbp). The X and Y chromosomes are 164 and 59 Mbp, respectively. The most gene-rich chromo- some is autosome 19 (with 23 genes/Mbp), while the least gene-rich chromosomes are autosome 13 and the Y (each with 5 genes/Mbp). Genes (or at least their coding regions) make up only 1-2% of the genome. Just over 40% of the genes that encode pro- teins have orthologs in Drosophila and Caenorhab- ditis. Hundreds of human genes have resulted by ho- rizontal transfer from bacteria at some point during the evolution of vertebrates. More than half of the genome is composed of repetitious DNA (q.v.).
About 45% of the genome is derived from transpos- able elements (q.v.). Although the human genome has only about twice as many protein-coding genes as Drosophila or Caenorhabditis, human genes are more complex and often undergo alternative splicing (q.v.) to produce an array of different transcripts.
More than 1,000 genes that cause specific diseases when they mutate have been mapped and are listed in OMIM (q.v.). The human mitochondrial chromo- some is sometimes referred to as chromosome 25 or M. See Appendix C, 2001, Collins and Venter et al.; 2003, International Human Genome Sequencing Consortium; Down syndrome, gene, genome anno- tation, horizontal transmission, human mitochondrial DNA, repetitious DNA. human genetic databases See Appendix E.
human genetic diseases maladies caused by de- fects in the human genome. A single-gene disorder is caused by a single mutant gene (see the table on page 172). Chromosomal disorders arise from dis- ruptions in development produced by an excess or deficiency of whole chromosomes or chromosome segments (e.g., Down syndrome). Chromosomal dis-