Genetic load

29 Mar

Genetic load

genetic death death of an individual without re- producing. See reproductive death. genetic detasseling a breeding technique used in the commercial production of corn seed. The breed- ing scheme produces pollen abortion with the result that the plants are no longer hermaphroditic and can only be cross-fertilized. genetic differentiation the accumulation of dif- ferences in allelic frequencies between isolated or semi-isolated populations due to various evolution- ary forces such as selection, genetic drift, gene flow, assortative mating, etc. genetic dissection analysis of the genetic basis of a biological phenomenon through the study of mu- tations that affect that phenomenon. For example, spermatogenesis can be “genetically dissected” by in- ducing and then characterizing mutations that steril- ize male Drosophila. genetic distance 1. a measure of the numbers of allelic substitutions per locus that have occurred during the separate evolution of two populations or species. 2. the distance between linked genes in terms of recombination units or map units. genetic divergence See genetic differentiation.

genetic drift the random fluctuations of gene fre- quencies due to sampling errors. While drift occurs in all populations, its effects are most evident in very small populations. See Sewall Wright effect. genetic engineering an all-inclusive term to cover all laboratory or industrial techniques used to alter the genetic machinery of organisms so that they can subsequently synthesize increased yields of com- pounds already in their repertoire, or form entirely new compounds, adapt to drastically changed envi- ronments, etc. Often, the techniques involve manip- ulating genes in ways that bypass normal sexual or asexual transmission. The vector of choice in plant genetic engineering is the Ti plasmid of Agrobacterium tumefaciens (q.v.). Genes of commercial importance are inserted into Ti DNA under laboratory conditions, and they become integrated into the genomes of host plants when the Ti DNA is transfected. See biotech- nology, recombinant DNA technology. genetic equilibrium the situation reached in a population containing, as an example, the allelic genes A and a, where the frequencies of both alleles are maintained at the same values generation after generation. See Hardy-Weinberg law. genetic fine structure See fine-structure genetic mapping.

genetic fingerprinting See DNA fingerprint tech- nique. genetic fitness the contribution to the next gener- ation of a specified genotype in a population in rela- tion to the contributions of all other genotypes of that same population. genetic fixation the status of a locus in which all members of a population are homozygous or hemi- zygous for a given allele; the frequency of the fixed allele is 1.0; all other alleles at that locus have been lost, and therefore their frequencies are zero. See monomorphic population. genetic hitchhiking See hitchhiking. genetic homeostasis the tendency of a popula- tion to equilibrate its genetic composition and to re- sist sudden changes. genetic identity a measure of the proportion of genes that are identical in two populations. genetic induction the process of gene activation by an inducer molecule, resulting in transcription of one or more structural genes. See inducible system.

genetic information the information contained in a sequence of nucleotide bases in a nucleic acid mol- ecule. See exon, intron. genetic instability a term that generally refers to chromosomal or other wide-scale genetic alterations that vary from cell to cell. They often arise from an overall deficit in systems that control the replication or repair of DNA or the checkpoints (q.v.) of the cell cycle. Widespread instabilities may also be caused by transposable elements (q.v.) or breakage- fusion-bridge cycles (q.v.) due to chromosome aber- rations. See DNA repair, Dotted, helicase, mitotic ar- rest-deficient (mad) mutations, mutator genes, RAD9. Contrast with genomic instability. genetic load 1. the average number of lethal equivalents per individual in a population. 2. the rel- ative difference between the actual mean fitness of a population and the mean fitness that would exist if the fittest genotype presently in the population were to become ubiquitous.

The genetic load of a given species may contain several components. The mutational load is due to recurrent mutations occur- ring in beneficial loci. Most new mutations are reces- sive, hypomorphic, and slow to be eliminated. The segregational load is caused by genes segregating from favored heterozygotes that generate less fertile or less viable homozygotes. There may also be an input load due to migrant individuals with an average fit- ness less than that of the original population. See heterozygote advantage, migrant selection, substitu- tional load.

Genetic map

genetic map the linear arrangement of mutable sites on a chromosome as deduced from genetic re- combination experiments. See Appendix C, 1913, Sturtevant. Compare with physical map. genetic marker a gene, whose phenotypic expres- sion is usually easily discerned, used to identify an individual or a cell that carries it, or as a probe to mark a nucleus, chromosome, or locus. genetic polymorphism the long-term occurrence in a population of two or more genotypes in fre- quencies that cannot be accounted for by recurrent mutation. Such polymorphism may be due to muta- tions that are (a) advantageous at certain times and under certain conditions and (b) disadvantageous under other circumstances, and which exist in habi- tats where situations (a) and (b) are encountered frequently. Genetic polymorphism may also result if genotypes heterozygous at numerous loci are gener- ally superior to any homozygous genotype. See bal- anced polymorphism, transient polymorphism. genetic predisposition the increased susceptibil- ity to a specific pathological condition due to the presence of one or more mutated genes or a combi- nation of alleles. In some cases all that is known is that there is a family history which indicates a ge- netic susceptibility to the condition.

genetic recombination in eukaryotic organisms the occurrence of progeny with combinations of genes other than those that occurred in the parents, due to independent assortment (q.v.) or crossing over (q.v.). In bacteria recombination of genes may occur as a result of conjugation, sexduction, trans- duction, or transformation (all of which see). In bac- terial viruses an infection of a host by two or more genetically distinct bacteriophages may result in pro- duction of recombinant phage. See Appendix C, 1961, Meselson and Weigle; beads on a string, Holli- day model, Visconti-Delbru¨ck hypothesis. genetics the scientific discipline dealing with the study of inheritance and variation of biological traits, and 2. the study of genes, including their structure, function, variation, and transmission. See Appendix C, 1856, 1865, 1866, Mendel; 1900, Bate- son; biochemical genetics, gene, heredity, Mendelian genetics, molecular genetics, population genetics, trait, transmission genetics, variation. genetic sex determination genotypic sex determi- nation (q.v.). genetic surgery replacement of one or more genes of an organism with the aid of plasmid vectors, or the introduction of foreign genetic material into cells by microsyringes or micromanipulators.

genetic variance the phenotypic variance of a trait in a population attributed to genetic heterogeneity. genic balance a mechanism of sex determination, originally discovered in Drosophila, that depends upon the ratio of X chromosomes to sets of auto- somes (A). Males develop when the X/A ratio is 0.5 or less; females develop when the X/A ratio is 1.0 or greater; an intersex develops when the ratio is be- tween 0.5 and 1.0. See Appendix C, 1925, Bridges; metafemales, metamales, sex determination.

genital disc the imaginal disc from which the re- productive duct system and the external genitalia are derived in Drosophila. genome in prokaryotes and eukaryotes, the total DNA in a single chromosome and in a haploid chro- mosome set (q.v.), respectively, or all of the genes carried by this chromosome or chromosome set; in viruses, a single complement of DNA or RNA, or all of the genes encoded therein. See C value, eukaryote, haploid or haploidy, genome size, Human Genome Project, metabolic control levels, prokaryote, virus.

genome annotation conversion of raw sequence data into useful information that concerns the posi- tions of structural genes on each chromosome, the methods by which they are switched on and off, and the functions of their products. Genes whose end products are RNA molecules must also be anno- tated.

The sequence organization of specialized chro- mosomal regions, like centromeres, replicons, and telomeres, must be worked out. Finally, there is the puzzle of annotating chromosomal sequences that contain repetitive sequences that function somehow to facilitate the shortening of chromosomes during mitotic prophase, their pairing during synaptonemal complex formation, and the condensation of an X chromosome in the somatic cells of mammalian fe- males. See centromere, gene, heterochromatin, insu- lator DNAs, meiosis, mitosis, repetitious DNA, repli- con, selfish DNA, shotgun sequencing, telomere, XIST. Genome Sequence Database accession number See Appendix E, Haemophilus influenzae. genome size the amount of DNA in the haploid genome. It is often measured in picograms, kilo- bases, megabases, or gigabases when organisms the size range of eukaryotes are being considered. Pro- karyotic genomes are smaller and are sometimes measured in daltons (q.v.).

When one refers to viral genomes or those of mitochondria or chloroplasts, the size is generally given in kilobase pairs. One kbp of DNA equals 1.02 × 10−6 picograms or 618,000 daltons. One pg of double-stranded DNA equals

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