whom one quarter of one’s genes are shared. Con- trast with first-degree relative. second division segregation ascus pattern in as- comycetes, a 2-2-2-2 or 2-4-2 linear order of spore phenotypes within an ascus. These patterns indicate that a pair of alleles (e.g., those controlling spore pigmentation) separated in the second meiotic divi- sion because crossing over occurred between that lo- cus and the centromere. See ordered tetrad. second law of thermodynamics See thermody- namics. second messenger small molecules or ions gener- ated in the cytoplasm in response to binding of a signal molecule to its receptor on the outer surface of the cell membrane.
Two major classes of second messengers are known: one involves cyclic adenosine monophosphate and the other employs a combina- tion of calcium ions and either inositol triphosphate or diacylglycerol. See G proteins. second site mutation See suppressor mutation. secretin a hormone that stimulates the secretion of pancreatic juice.
The epithelial cells of the duode- num release secretin when activated by the acidic contents of the stomach. secretion the passage out of a cell or gland of com- pounds synthesized within it. Secretor gene a dominant autosomal gene in hu- mans that permits the secretion of the water-soluble forms of the A and B blood-group antigens into sa- liva and other body fluids. The Se gene is not linked to the I locus. See A, B antigens. secretory vesicle a vesicle that contains a secre- tory product. Secretory vesicles in the acinar cells of the pancreas, for example, contain precursors of di- gestive enzymes in a highly concentrated form (200 times as high as their concentration in the ER). Pro- teins destined for secretion contain sorting signals (q.v.) that target them to secretory vesicles.
See Golgi apparatus. sedimentation coefficient (S) the rate at which a given solute molecule suspended in a less dense sol- vent sediments in a field of centrifugal force. The sedimentation coefficient is a rate per unit centrifu- gal field. The S values for most proteins range be- tween 1 × 10−13 sec and 2 × 10−11 sec. A sedimenta- tion coefficient of 1×10−13 sec is defined as one Svedberg unit (S). Thus a value of 2 × 10−11 sec would be denoted by 200S.
For a given solvent and temperature, S is determined by the weight, shape, and degree of hydration of the molecule.
See Appen- dix C, 1923, Svedberg. seed a mature ovule containing an embryo in an arrested state of development, generally with a food reserve. See kernel. seeding efflciency in cell culture, the percentage of the cells in an inoculum that attach to the culture vessel within a specific length of time; synonymous with attachment efficiency. segmental alloploid See allosyndesis. segmental interchange a translocation. segmentation See metamerism. segmented genome a viral genome fragmented into two or more nucleic acid molecules. For exam- ple, the alfalfa mosaic virus has four different RNA segments, each packaged in a different virion. Suc- cessful infection requires that at least one RNA of each type enters the cell. Such a virus is said to be heterocapsidic.
If all fragments of a segmented ge- nome are present in the same virion (e.g., influenza virus), the virus is said to be isocapsidic. segment identity genes genes that determine the type of differentiation the cells in a specific Drosoph- ila segment will undergo.
These genes express them- selves later in development than the zygotic segmen- tation genes. While mutations in these genes cause the deletion of certain body parts and are generally lethal, mutations of segment identity genes allow the mutants to survive but with inappropriate structures developing in specific segments. For example, a bi- zarre four-winged fly results if the mutation converts halteres (q.v.) to wings. The segment identity genes are located in two clusters on the right arm of chro- mosome 3 (the Antp complex and the bx complex).
See Antennapedia, bithorax, homeotic mutations, Hox genes, metamerism, pattern specification, zygotic segmentation mutations. segment polarity genes See zygotic segmentation mutants. segregational lag delayed phenotypic expression of an induced mutation in one nucleoid of a multi- nucleoid bacterium. The lag period is the time re- quired for the fission of the parent to produce a cell containing only the mutant chromosome. segregational load the genetic disability sustained by a population due to genes segregating from ad- vantageous heterozygotes to less fit homozygotes.
segregational petites See petites. segregation distortion a distortion of expected Mendelian ratios in a cross due to dysfunction or le- thality in gametes bearing certain alleles. This form of meiotic drive (q.v.) is represented in Drosophila melanogaster by the segregation distorter (SD) muta- tion on chromosome 2. SD/sd+ heterozygous males produce both SD and sd+ spermatozoa, but only those carrying SD are functional. SD cannot achieve fixation, however, because it is lethal when homozy- gous.
segregation of chromosomes See disjunction. segregation of genes See Mendel’s laws. segregation ratio distortion the distortion of the 1:1 segregation ratio produced by a heterozygote. Such distortions may arise because of abnormalities of meiosis that result in an Aa individual’s producing unequal numbers of A- and a-bearing gametes, or it may arise from A- and a-bearing gametes, being unequally effective in producing zygotes. selectins a family of structurally related lectins (q.v.), so named because they mediate the selective contact between cells. Selectins are glycoproteins that form an integral part of the membrane of cer- tain cells. At the N-terminal end of each molecule is a lectin domain followed by an EGF domain.
Next comes a short consensus sequence, which is repeated several times, and finally a transmembrane domain. Selectins are distinguished by capital letters that in- dicate the specific cells synthesizing the molecules (E,L,P; endothelial cells, lymphocytes, platelets). See epidermal growth factor. selection the process determining the relative share allotted individuals of different genotypes in the propagation of a population. The selective effect of a gene can be defined by the probability that car- riers of the gene will reproduce. See alloprocoptic selection, artificial selection, balanced selection, direc- tional selection, disruptive selection, frequency-depen- dent selection, group selection, indirect selection, kin selection, normalizing selection, r and K selection, sexual selection, stabilizing selection theory. selection coefficient (s) the proportionate reduc- tion in the average gametic contribution to the next generation made by individuals of one genotype rel- ative to those of another genotype (usually the most fit).
For example, if the best adapted genotypes are AA and Aa, and they are not being selected against, s=0, and their fitness=(1−s)=1. If individuals of genotype aa leave only on the average 80% as many progeny (proportionate to their numbers in the pop- ulation) as the other genotypes, then the selection coefficient against aa individuals is 0.2 or 20%, and their fitness is (1 − 0.2) = 80%. selection differential the difference between the average value of a quantitative character in the whole population and the average value of those se- lected to be parents of the next generation.
See rec- ord of performance. selection pressure the effectiveness of natural se- lection in altering the genetic composition of a pop- ulation over a series of generations. selective advance the increment in the average value (measured for a quantitative character being selected in a population) from one generation to the next, usually a fraction of the selection differential (q.v.). selective medium a medium designed to allow growth of only those cells of a specific genotype. Compare with nonselective medium. selective neutrality a situation in which the phe- notypic manifestations of certain mutant alleles are equivalent to that of the wild-type allele in terms of their fitness values. See neutral gene theory, silent mutations. selective plating a method for selectively isolating recombinants. Two different auxotropic mutants are plated upon a minimal medium.
Only the recombi- nant class receiving the normal allele of each mutant can multiply under these conditions. selective silencing any mechanism that consis- tently eliminates plasmagenes of one parent from the zygote, such as the destruction of chloroplasts or chloroplast DNA in some algae and plants or of sperm mitochondria in some animals. selective system any experimental technique that aids in the detection and isolation of a specific (usu- ally rare) genotype.
See penicillin selection technique. selective variant in microbial genetics, a mutation that confers upon the organism the ability to exist under conditions that kill off all organisms not pos- sessing the mutation. Examples of selective variants are mutations conferring resistance to antibacterial agents or the ability to synthesize some essential me- tabolite lacking in the medium. selector genes a subset of regulatory genes that control choices between alternative developmental pathways. For example, a cell may continue along pathway A, unless it receives a signal to enter path- way B. The signal is often a protein encoded by a
selector gene that binds to specific genomic sites, ac- tivating one or more of the genes necessary for the new pattern of development. See Antennapedia, apetala-2, bicoid, bithorax, caudal, compartmental- ization, decapentaplegic, developmental control genes, dorsal, downstream genes, engrailed, eyeless, fushi tarazu, goosecoid, gurken, hedgehog, Hox genes, hunchback, metamerism, nanos, oskar, paired, Polycomb, sevenless, SRY. self to undergo self-pollination (q.v.) or self-fertil- ization (q.v.). self-assembly the spontaneous aggregation of multimeric biological structures involving formation of weak chemical bonds between surfaces with com- plementary shapes. For example, most of the com- ponents of the phage T4 capsid (head, tail, base plate, and tail fibers) are self-assembled. self-compatible said of a plant that can be self- fertilized. self-fertilization the fusion of male and female gametes from the same individual. self-incompatibility self-sterility (q.v.). selfish DNA (also called junk DNA or parasitic DNA) 1. functionless segments of DNA that are replicated along with the rest of the chromosomal regions that serve vital functions. Examples would be pseudogenes (q.v.) and tandemly repeated and dispersed repetitive DNA segments that appear to serve no function, yet accumulate by unequal cross- ing over (q.v.). 2. the term is also used to refer to a parasitic DNA that has the ability to engineer its host genetically so that the host cell is better able to survive in nature. Examples would be R (resistance) plasmids and Ti plasmids (both of which see).
See Ap- pendix C, 1980, Doolittle and Sapienza, Orgel and Crick; 1997, Yoder, Walsh, and Bestor; C value para- dox, DNA methylation, repetitious DNA. selfish operon a model to explain the origin in bacteria of clusters of genes that have similar func- tions and are conditionally expressed. The idea is that genes whose products are used only during con- ditions that happen infrequently are continually mu- tating to inactive alleles. When conditions that require the missing gene products reoccur, the bac- teria die off unless horizontal transfer of active alleles can take place. But only small segments of DNA are transferred by episomes. Therefore, when two or more genes are needed simultaneously, selec- tion will favor genes that are near one another. Also, cotranscribed genes are more likely to function adaptively in a new host, since the host needs to be able to recognize only one promoter. Thus the trans- mission of DNA by horizontal mobile elements (q.v.) tends to select the stepwise formation of gene clusters that function as operons.
See Appendix C, 1996, Lawrence and Roth; sympatric speciation. self-pollination the transfer of pollen to the stig- mas of the same plant. self-splicing rRNA See ribozyme. self-sterility the inability of some hermaphrodites to form viable offspring by self-fertilization. self-sterility genes genes that prevent the delete- rious effects of inbreeding in monoecious plants by controlling the rate of growth of the pollen tube down the style. Self-incompatibility is controlled by a highly polymorphic S locus. Growth of a pollen tube in the style is arrested when the S allele carried by the pollen matches one of the two S alleles car- ried by the pistil. S genes have been cloned and shown to encode glycoproteins with RNase activity. In self-pollinations, the RNase is internalized by re- ceptors on the pollen tube surface. Once inside the pollen tube, the enzyme degrades RNAs essential to its further growth. SEM scanning electron microscope. See electron microscope. semelparity reproduction that occurs only once in the life of an individual (e.g., annual plants, Pacific salmon). Compare with iteroparity. semen a biochemically complex nutrient fluid containing spermatozoa which is transferred to the female during copulation. semiconservative replication the method of rep- lication of DNA in which the molecule divides lon- gitudinally, each half being conserved and acting as a template for the formation of a new strand. See Appendix C, 1953, Watson and Crick; 1957, Taylor et al.; 1958, Meselson and Stahl; 1963, Cairns; 1964, Luck and Reich. Compare with conservative replica- tion. semidiscontinuous replication a mode of DNA replication in which one new strand is synthesized continuously, while the other is synthesized discon- tinuously as Okazaki fragments. See replication of DNA. semidominance the production of an intermedi- ate phenotype in individuals heterozygous for the gene concerned; also known as partial dominance. See incomplete dominance.
semidwarf a term used to distinguish mutant strains of wheat that are of agricultural importance from the extremely short dwarfs of purely genetic interest. Semidwarfs grow from half to two-thirds the height of and have greater yields than those of standard varieties. semigeographic speciation See parapatric specia- tion. semilethal mutation a mutation causing death of more than 50%, but not of all individuals, of mutant genotype. seminiferous tubule dysgenesis Klinefelter syn- drome (q.v.). semiochemistry the study of the chemical signals that mediate interactions between members of dif- ferent species.
See pheromone. semipermeable membrane any membrane that permits passage of molecules selectively. semispecies incipient species. semisterility a situation in which half or more of all zygotes are inviable (as in Oenothera crosses that maintain only heterozygotes). See balanced lethal system. semisynthetic antibiotic a natural antibiotic that has been chemically modified in the laboratory to enhance its stability. Sendai virus a virus, first isolated in Japan, that causes an important and widespread infection of lab- oratory mice; it belongs to the Paramyxoviridae. The virus is widely used in cell fusion studies.
The viruses so modify the surfaces of infected cells that they tend to fuse. Even UV-killed viruses adsorb on host cells and promote their fusion. See Appendix C, 1965, Harris and Watkins; enveloped viruses. senescence the process of aging (q.v.). sense codon any of the 61 triplet codons in mRNA that specify an amino acid. sense strand See strand terminologies. sensitive developmental period a period during development when there is an enhanced chance that genetic malfunction will bring development to a standstill.
In Drosophila these sensitive periods corre- spond to the onset of embryonic, larval, pupal, or adult development, and it is during such periods that many new systems are differentiated and put to im- mediate test. Gastrulation (q.v.) is a sensitive period for amphibians. sensitive volume 1. that portion of an ionization chamber that responds to radiation passing through it. 2. that biological volume in which an ionization must occur to produce a given effect (such as a mu- tation). See target theory. sensitizing agent an agent which, when added to a biological system, increases the amount of damage done by a subsequent dose of radiation. sepal See floral organ primordia. separase a cysteine protease related to the cas- pases (q.v.) that cleaves cohesin complexes and allows sister chromatids to separate and be drawn to opposite poles during the anaphase stage of mitosis (q.v.). During meiosis (q.v.) separase can only cleave those cohesin complexes that are not protected by Sgo (q.v.). See cohesin, sister chromatid cohesion. sepiapterin See Drosophila eye pigments. septal fission cell division that characterizes yeasts like Schizosaccharomyces pombe (q.v.), where mitosis is followed by the formation of a septum that segre- gates the sibling nuclei into two cells of roughly equal size (contrast with bud). sequence the order in which amino-acid or nucle- otide residues are arranged in a protein, DNA, and so forth; to ascertain the sequence of amino-acid or nucleotide residues in (a protein, DNA, etc.).
See DNA sequencing techniques, protein structure. sequencer an apparatus for determining the se- quence of amino acids or other monomers in a bio- logical polymer. sequence similarity estimates conclusions made from nucleotide by nucleotide comparison of DNA segments from species that have diverged only re- cently. For example, when the class 1 segments of the major histocompatibility complexes (MHC) (q.v.) of man and chimpanzee were compared, nucleotide substitutions were found less often than insertions and deletions (indels). This was unexpected, since it was always assumed that indels were more difficult to generate than base substitutions. See Appendix C, 2003, Anzai et al. sequence tagged site (STS) a short DNA se- quence, readily located and amplified by the poly- merase chain reaction, that uniquely identifies a physical genomic location. Expressed sequence tags (ESTs) are STSs obtained from cDNAs (q.v.). See physical map.