Genotypic sex determination
0.98 × 106 kbp or 6.02 × 1011 daltons. See Appendix F; C value, gigabase, kilobase, megabase, picogram. genomic blotting See Southern blotting. genomic equivalence the theory that all cells of an organism contain an equivalent complement of genetic information. Genomic equivalence has been confirmed for most cells, but exceptions occur in some animal cells where loss, gain, or rearrangement of nuclear DNA has been observed. Examples of such exceptions include chromatin diminution (q.v.) in somatic cells of some nematodes, selective ampli- fication of rRNA genes in Xenopus oocytes, and DNA excision and rearrangement during mamma- lian lymphocyte maturation that result in the gener- ation of antibody diversity. See gene amplification, immunoglobulin genes.
genomic exclusion an abnormal form of conjuga- tion occurring in Tetrahymena pyriformis between cells with defective micronuclei and normal cells. The progeny are heterokaryons; each has an old macronucleus but a new diploid micronucleus de- rived from one meiotic product of the normal mate. genomic formula a mathematical representation of the number of genomes (sets of genetic instruc- tions) in a cell or organism. Examples: N (haploid gamete or monoploid somatic cell), 2N (diploid), 3N (triploid), 4N (tetraploid), 2N − 1 (monosomic), 2N + 1 (trisomic), 2N − 2 (nullisomic), etc. genomic imprinting See parental imprinting.
genomic instability the term is generally used to refer to a localized instability due to a chromosomal site that contains a small number of nucleotides re- peated in tandem. As a result of slippage during rep- lication the number of repeated sites may be in- creased or decreased. If this repeated segment is in a structural gene, it may be converted to a poorly or nonfunctioning allele. The term microsatellite insta- bility is used synonymously. See Huntington disease (HD). Contrast with genetic instability. genomic library a collection of cloned DNA frag- ments representing all the nucleotide sequences in the genome (q.v.) of an organism. A genomic library is usually constructed by cutting genomic DNA into random fragments, ligating the resulting fragments into a suitable cloning vector, and transforming a host cell. The library can then be screened with a molecular probe (q.v.) to identify a clone of interest, or used for sequence analysis. Compare with cDNA library.
See Appendix C, 1978, Maniatis et al.; bacte- rial artificial chromosomes (BACs), cosmid, DNA vec- tor, lambda phage vector, plasmid cloning vector, P1 artificial chromosomes (PACs), restriction endonucle- ase, transformation, yeast artificial chromosomes (YACs). genomic RNA the genetic material of all viruses that do not use DNA as genetic material. All cells and the vast majority of viruses use DNA as genetic material, but some bacteriophages and a few plant and animal viruses use RNA as genetic material.
Ex- amples include tobacco mosaic virus (TMV) and all retroviruses such as the human immunodeficiency virus (HIV) responsible for AIDS (acquired immu- nodeficiency syndrome). genomics the scientific study of the structure and functioning of the genomes of species for which ex- tensive nucleotide sequences are available. The ge- nome includes both nuclear DNA and that of mito- chondria and chloroplasts. In the case of structural genomics, high-resolution genetic, physical, and transcript maps are constructed for each species. Functional genomics expands the scope of research to the simultaneous study of large numbers of struc- tural genes that respond to a suitable stimulus. Evolutionary genomics contrasts the genomes of dif- ferent species to follow evolutionary changes in ge- nome organization. A common exercise in genomics is the in silico (q.v.) investigation of orthologs (q.v.).
A recent study of comparative genomics showed that Drosophila has orthologs to 177 of 289 human disease genes. See Appendix C, 1997, Lawrence and Oschman; 1999, Galitski et al.; 2000, Rubin et al.; DNA chip, genomic RNA, genome annotation. genopathy a disease resulting from a genetic de- fect. genophore the chromosome equivalent in viruses, prokaryotes, and certain organelles (e.g., the dis- crete, ringlike structure occurring in some algal chlo- roplasts). Genophores contain nucleic acids but lack associated histones. genotype the genetic constitution of a cell or an organism, as distinguished from its physical and be- havioral characteristics, i.e., its phenotype (q.v.). See Appendix C, 1909, Johannsen.
genotype-environment interaction an inference drawn from the observation that the phenotypic ex- pression of a given genotype varies when measured under different environmental conditions. genotype frequency the proportion of individuals in a population that possess a given genotype. genotypic sex determination any mechanism of sex determination (q.v.) in which a genetic factor (such as the nature of the sex chromosomes in the
fertilized egg or the X:A ratio (q.v.) of the embryo) is the primary sex-determining signal. Also called ge- netic sex determination. Compare with environmental sex determination. genotypic variance the magnitude of the pheno- typic variance for a given trait in a population attrib- utable to differences in genotype among individuals. See heritability. genus (plural genera) a taxon that includes one or more species presumably related by descent from a common ancestor. See hierarchy. geochronology a science that deals with the mea- surement of time in relation to the earth’s evolution. See Appendix C, 1953, Patterson; 1954, Barghoorn and Tyler; 1980, Lowe. geographical isolate a population separated from the main body of the species by some geographical barrier.
geographic speciation the splitting of a parent species into two or more daughter species following geographic isolation of two or more parental popula- tions; allopatric speciation. geologic time divisions See page 181. geometric mean the square root of the product of two numbers; more generally, the nth root of the product of a set of n positive numbers. geotropism the response of plant parts to the stimulus of gravity. germarium the anterior, sausage-shaped portion of the insect ovariole.
It is in the germarium that cysto- cyte divisions occur and the clusters of cystocytes become enveloped by follicle cells. See insect ovary types, vitellarium. germ cell a sex cell or gamete; egg (ovum) or sper- matozoan; a reproductive cell that fuses with one from the opposite sex in fertilization to form a sin- gle-celled zygote. germinal cells cells that produce gametes by mei- osis: e.g., oocytes in females and spermatocytes in males. germinal choice the concept advocated by H. J. Muller of progressive human evolution by the volun- tary choice of germ cells. Germ cells donated by in- dividuals possessing recognized superior qualities would be frozen and stored in germ banks. In subse- quent generations these would be available for cou- ples who wished to utilize these rather than their own germ cells to generate a family.
Such couples are referred to as “preadoptive” parents. germinal mutations genetic alterations occurring in cells destined to develop into germ cells. germinal selection 1. selection by people of the germ cells to be used in producing a subsequent gen- eration of a domesticated species. Such selection has been suggested for human beings. 2. selection during gametogenesis against induced mutations that retard the proliferation of the mutated cells. Such selection introduces errors in estimating the frequency of mu- tations induced in gonial cells. germinal vesicle the diploid nucleus of a primary oocyte during vitellogenesis. The nucleus is generally arrested in a postsynaptic stage of meiotic prophase. See Appendix C, 1825, Purkinje. germination inhibitor any of the specific organic molecules present in seeds that block processes es- sential to germination and therefore are often the cause of dormancy. germ layers three primordial cell layers from which all tissues and organs arise.
See Appendix C, 1845, Remak; ectoderm, endoderm, mesoderm. germ line pertaining to the cells from which ga- metes are derived. When referring to species, the cells of the germ line, unlike somatic cells, bridge the gaps between generations. See Appendix C, 1883, Weismann. germinal granules polar granules (q.v.).
germ line sex determination the genetic and de- velopmental process that specifies sexual identity and sex-specific development of the germ line cells of an organism. Compare with somatic sex determi- nation. See sex determination. germ line transformation See transformation. germ plasm 1. a specialized cytoplasmic region of the egg or zygote (q.v.) in many vertebrate and in- vertebrate species where germ cell determinants are localized. Removal or destruction of the germ plasm results in the absence of germ cells in the embryo. The germ plasm was first identified by Theodore Boveri in the nematode Ascaris in the late 1800s. 2. The hereditary material transmitted to the next generation through the germ cells or used for plant propagation through seeds or other materials. See Appendix C, 1883, Weismann; pole plasm. gerontology the study of aging.
Geologic time divisions
Each number gives the date in millions of years before present. In the figure, the relative lengths of each time division are not proportional to the absolute time they represent. Solid circles mark the occurrence of mass extinctions (q.v.).