Quantitative character

1 Apr

q See symbols used in human cytogenetics. Q10 temperature coefficient; the increase in a reac- tion or other process (expressed as a multiple of the initial rate) produced by raising the temperature 10°C. Qa in the mouse, a series of loci located very close to the major histocompatibility complex whose products are expressed on the surfaces of some lymphocyte classes and subclasses. Q bands See chromosome banding techniques. Q beta (Q ) phage an RNA virus that infects E. coli.

Its genome consists of a circular, positive sense single-stranded RNA molecule. This strand acts both as a template for the replication of a complementary strand and as an mRNA molecule that directs the translation of viral proteins. Q beta phage is one of the smallest known viruses, measuring 24 nm in di- ameter. Its icosahedral capsid is composed of 180 copies of a single coat protein. See Appendix C, 1965, 1967, Spiegelman et al.; 1973, Mills et al.; 1983, Miele et al.; Appendix F; androphages, bacterio- phage, in vitro evolution, Q beta replicase, virus. Q beta replicase the enzyme that catalyzes the replication of Qβ phage. See RNA-dependent RNA polymerase. QTLs quantitative trait loci; genes that control the expression of traits (such as height or skin color in humans, pesticide resistance in insects, and ear length in corn) that show quantitative inheritance (q.v.). quadrivalent a meiotic association of four homol- ogous chromosomes; synonymous with tetravalent.

quadruplex See autotetraploidy. quail See Coturnix coturnix japonica. quantasome a photosynthetically active particle found in the grana of chloroplasts. Each quantasome is an oblate ellipsoid with axes of about 100 and 200 A˚ngstroms. Chlorophyll (q.v.) is localized within quantasomes. quantitative character a character showing quan- titative inheritance (beef and milk production in cat- tle, egg production in hens, DDT resistance in Dro- sophila, stature, weight, and skin pigmentation in humans). quantitative inheritance phenotypes that are quantitative in nature and continuous in distribution are referred to as quantitative characters (q.v.). Dur- ing their genetic transmission, there is an absence of clear-cut segregation into readily recognizable classes showing typical Mendelian ratios.

An often-used ex- ample is ear length in maize, as illustrated by the histograms on page 366. When crosses are made be- tween individuals from lines showing large quantita- tive differences in ear length, the offspring are inter- mediate. When F1 individuals are crossed, the F2 population has a mean that is very similar to the F1 mean, but some individuals produce ears as long or as short as the grandparents. Such results are ex- plained by the “multiple factor hypothesis,” which assumes that the quantitative character depends on the cumulative action of multiple genes (or poly- genes), each on a separate chromosome, and each producing a unitary effect. In the corn example, a simple model would employ three genes, each exist- ing in two allelic forms.

Each capital-letter gene might be responsible for three units of “growth po- tential,” and each small-letter gene, for one unit. Thus the capital-letter genes are all interchangeable in the sense that each produces the same phenotypic effect, and the same is true for the small-letter genes. The long- and short-eared parental individu- als would be AABBCC and aabbcc, respectively, and their offspring would be AaBbCc. These would show little variability, because all plants would be genetically identical. The segregation of the alleles in the F2 population would produce 27 different geno- typic classes, and the cumulative action of the genes would generate 7 phenotypic classes. The most com- mon genotype (making up one-eighth of the total population) would be AaBbCc, genetically identical to the F1 plants. But there would also be plants of genotype AABBCC and aabbcc (each making up one- sixty-fourth of the population) and these would be phenotypically and genetically identical to the grand- parents. There would also be individuals with vari- ous intermediate ear lengths of genotypes (AABBCc,



Distribution of ear lengths in parents, F1 and F2 generations of a cross between Tom Thumb popcorn and Black Mexican sweet corn.
Quantitative inheritance in maize

aabbcC, AAbbCC, etc.) and the result would be an F2 population with a mean equivalent to the F1, but with a distribution whose width depended on the number of segregating alleles. By comparing the variances of the F1 and F2 populations, one can esti- mate the number of segregating gene pairs responsi- ble for the trait. See Appendix C, 1889, Galton; 1909, Nilsson Ehle; 1913, Emerson and East; Wright’s polygenic estimate. quantum according to the quantum theory, energy is radiated in discrete quantities of definite magni- tude called quanta and absorbed in a like manner.  quantum speciation the rapid evolution of new species, usually within small, peripheral isolates, with founder effects and genetic drift playing impor- tant roles. See evolution. quartet a group of four nuclei or of four cells aris- ing from the two meiotic divisions. Quaternary the most recent of the two geologic periods making up the Cenozoic era. See geologic time divisions. quaternary protein structure See protein structure.


Quercus the genus of oaks including: Q. alba, the white oak; Q. coccinea, the scarlet oak; Q. palustris, the pin oak; Q. suber, the cork oak. quick-stop mutants of E. coli that immediately cease replication when the temperature is increased to 42°C. quinacrine an acridine derivative used in the treat- ment of certain types of cancer and malaria. It is also used as a fluorochrome in chromosome cytology. See Appendix C, 1970, Caspersson et al.; 1971, O’Rior- dan et al. quinone a compound belonging to a class of mole- cules that function in biological oxidation-reduction systems.

q.v.  which see. An abbreviation for Latin, quod vide.

r 1. reproductive potential. 2. ring chromosome; see symbols used in human genetics. 3. roentgen. 4. correlation coefficient; see correlation. R 1. a chemical radical. Used to show the position of an unspecified radical in a generalized structural formula of a group of organic compounds. 2. a drug- resistant plasmid conferring resistance to one or more antibiotics on bacteria in which it resides. 3. the single-letter symbol for purine. Compare with Y. rII a segment on the chromosome of the T4 bacte- riophage that was the first to be subjected to fine structure mapping. Mutants at the rII locus failed to produce plaques or produced abnormal plaques, de- pending on the strain of E. coli used as hosts. Benzer mapped over 1,600 of these mutations. In the dia- gram each is represented by a box and they reside in adjacent genes (cistrons A and B).

The smallest unit of recombination (the recon) corresponded to a dis- tance of two nucleotide pairs. The mutational sites (mutons) were the equivalent of one or two nucleo- tides. Note that each cistron contains a mutational hot spot. See Appendix C, 1955, 1961, Benzer; 1978, Coulondre et al.; beads on a string, proflavin. rabbit See Oryctolagus cuniculus. rabies virus a virus (q.v.) belonging to the rhab- doviridae (q.v.). It can multiply in many species of mammals, and it induces aggressive-biting behavior by the infected host which maximizes the chances of spreading the viral infection.

Rabl orientation a chromosome orientation some- times observed in interphase nuclei where centro- meres are grouped near one pole and the telomeres all point toward the opposite pole. The arrangement has been interpreted to mean that centromeres and telomeres attach to opposite sides of the nuclear lamina. The orientation is named after C. Rabl, who first described the phenomenon in 1885. See Dro- sophila salivary gland chromosomes, lamins. race a phenotypically and/or geographically dis- tinctive subspecific group, composed of individuals inhabiting a defined geographical and/or ecological

region, and possessing characteristic phenotypic and gene frequencies that distinguish it from other such groups. The number of racial groups that one wishes to recognize within a species is usually arbitrary but suitable for the purposes under investigation. See ecotype, subspecies. raceme an inflorescence as in the hyacinth, in which the flowers are borne on pedicels arising from the rachis. A branched raceme, such as may be seen in oats, rice, wheat, and rye, is called a panicle. See spike. rachet a tool with teeth on the rim of a wheel which is prevented from moving backward by a ro- tating pivot. A rachet is sometimes used as a meta- phor for evolution moving relentlessly forward. See Dollo law, Muller rachet. rad abbreviation of radiation absorbed dose. A unit defining that energy absorbed from a dose of ionizing radiation equal to 0.01 joule per kilogram. 1 rad = 0.01 gray. RAD the symbol for a gene that repairs radiation damage to DNA.

In Schizosaccharomyces pombe there are four Rad genes that are involved at DNA damage checkpoints (q.v.). The wild-type allele of rad3 encodes a protein called a phosphatidylinositol 3-kinase (Pl 3-kinase). This enzyme transduces sig- nals from the environment that detect damage to DNA. The normal allele of a gene in humans that is responsible (when mutated) for the hereditary dis- ease ataxia-telangiectasia encodes a homologous Pl 3-kinase. See ATM kinase. radial cleavage a pattern of cell divisions seen in the developing embryos of deuterostomes, such as echinoderms and amphibians. The first two cleav- ages are vertical and the third is horizontal. As a re- sult, each of the blastomeres in the upper tier of four cells lies directly over the corresponding blastomeres in the lower tier. Compare with spiral cleavage.

Radiata a subdivision of the Eumetazoa containing animals, such as jelly fish and coral polyps, charac- terized by radial symmetry. See Appendix A. radiation the emission and propagation of energy through space or a medium in the form of waves. When unqualified, radiation usually refers to elec- tromagnetic radiations (radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays), and, by extension, ionizing particles. See microbeam irradia- tion, radiation units, recoil energy. radiation absorbed dose See rad. radiation chimera an experimentally produced animal containing hemopoeitic cells of a genotype different from that of the rest of the organism. Re- cipients receive a single dose of radiation that kills the stem cells of the bone marrow and much of the differentiated hemopoeitic tissue. Very shortly there- after, they receive an intravarious inoculation of bone marrow or fetal liver cells from nonirradiated donors.

The injected stem cells home to the recipi- ent’s bone marrow sites and begin repopulating them, and ultimately they replace the recipient’s hemopoeitic tissues. radiation dosage See dose, phantom. radiation genetics the scientific study of the ef- fects of radiation on genes and chromosomes. The science began with the demonstration for Drosophila and corn that x-rays produced deleterious muta- tions. See Appendix C, 1927, Muller; 1928, Stadler. radiation-induced chromosomal aberration a chromosomal aberration (q.v.) induced through breakage caused by ionizing radiation. In the figure on page 370 are shown the origin and mitotic behav- ior of a variety of radiation-induced aberrations. Original break positions are indicated by short diag- onal lines. radiation sickness a syndrome characterized by nausea, vomiting, diarrhea, psychic depression, and death following exposure to lethal doses of ionizing radiation.

The median lethal radiation dose for hu- mans is between 400 and 500 roentgens. Such a dose leaves only about 0.5% of the body’s reproduc- ing cells still able to undergo continued mitosis. Since each cell continues to function normally in the physiological sense, death is not immediate. Damage shows up first in tissues with a high mitotic rate (the blood cell-forming tissues of the bone marrow, for example). Death occurs when the surviving cells are unable to restore by mitosis the needed numbers in time to maintain the physiological functioning of the various vital tissues.

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