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centimorgan (cM) another name for a map unit in honor of Thomas Hunt Morgan chromosome map a representation of all chromosomes in the genome as lines, marked with the positions of genes known from their mutant phenotypes, plus molecular markers. based on analysis of recombinant frequency cis conformation in a heterozygote having two mutant sites within a gene or within a gene cluster, the arrangement A1A2/a1a2 coefficient of coincidence the ratio of the observed number of double recombinants to the expected number crossing over the exchange of corresponding chromosome parts between homologs by breakage and reunion crossover product meiotic product cells with chromosomes that have engaged in a crossover DNA fingerprint the autoradiographic banding pattern produces when DNA is digested with a restriction enzyme that cuts outside a family of VNTRs and a Southern blot of the electrophoretic gel is probed with a VNTR-specific probe double-stranded break a DNA break cleaving the sugar-phosphate backbones of both strands of the DNA double helix first-division segregation pattern (M1 pattern) a linear pattern of spore phenotypes within an ascus for a particular allele pair, produced when the alleles go into separate nuclei at the first meiotic division, showing that no crossover has taken place between the allele pair and the centromere genetic map unit a distance on the chromosome map corresponding to 1 percent recombinant frequency heteroduplex DNA DNA in which there is one or more mismatched nucleotide pairs in a gene under study interference a measure of the independence of crossovers from each other, calculated by subtracting the coefficient of coincidence from 1 linkage map a chromosome map; an abstract map of chromosomal loci that is based on recombinant frequencies linked genes the loci of two genes are on the same chromosome, and the alleles on any one homolog are physically jaoined by the DNA between them locus gene positions mapping function a formula expressing the relation between distance in a linkage map and recombinant frequency microsatellite marker a difference in DNA at the same locus in two genomes that is due to different repeat lengths of a microsatellite minisatellite marker heterozygous locus representing a variable number of tandem repeats of a unit 15 to 100 nucleotides long molecular marker a site of DNA heterozygosity, not necessarily associated with phenotypic variation, used as a tag for a particular chromosomal locus null hypothesis a hypothesis that proposes no difference between two or more data sets octad an ascus containing eight ascospores, produced in species in which the tetrad normally undergoes a postmeiotic mitotic division physical map the ordered and oriented map of cloned DNA fragments on the genome Poisson distribution a mathematical distribution giving the probability of observing various numbers of a particular event in a sample when the mean probability of an event on any one trial is very small recombinant frequency the proportion (or percentage) of recombinant cells or individuals recombination map a chromosome map in which the positions of loci shown are based on recombinant frequencies restriction fragment length polymorphism (RELP) a difference in DNA sequence between individuals or haplotypes that is recognized as different restriction fragment lengths second-division segregation pattern (MII) a pattern of ascospore genotypes for a gene pair showing that the two alleles separate into different nuclei only at the second meiotic division, as a result of a crossover between that gene pair and its centromere; can be detected only in a linear ascus simple sequence length polymorphism (SSLP) the existence in the population of individuals showing diferent numbers of copies of a short simple DNA sequence at one chromosomal locus single nucleotide polymorphism (SNP) a nucleotide pair difference at a given location in the genomes of two or more naturally occurring individuals three-point testcross a testcross in which one parent has three heterozygous gene pairs trans conformation in a heterozygote with two mutant sites within a gene or gene cluster, the arrangement a1 +/+ a2 variable number tandem repeat (VNTR) a chromosomal locus at which a particular repetitive sequence is present in different numbers in different individuals or in the two different homologs in one diploid individual