Genetics Ch 5 – Flashcards
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allelic phase
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Describing the cis and trans arrangements of alleles of linked genes on homologous copies of a chromosome pair.
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ascus (ascospore)
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The spore sac formed by fungi containing four (tetrad) or eight (octad) haploid spores.
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coefficient of coincidence (c)
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The ratio of the observed number of double recombinants to the number of double recombinants expected to occur by chance.
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double recombinant (double crossover)
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The occurrence of two crossovers between homologous chromosomes in a particular region. May involve two, three, or all four chromatids.
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first-division segregation
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In Neurospora and other organisms forming an ascus, the separation of alleles at the first meiotic division due to no crossing over having occurred.
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genetic linkage mapping
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Process for creating maps of genes based on their linkage relationships to other genes.
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genetic linkage
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The result of genes being located so near one another on a chromosome that their alleles do not assort independently. Identified by detecting certain pairs of alleles (parentals) that are transmitted together significantly more often than expected by chance and of other pairs of allels (nonparentals or recombinants) that are transmitted together significantly less often than expected.
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incomplete genetic linkage
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The occurrence of crossing over between linked genes.
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complete genetic linkage
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The absence of crossing over between linked genes.
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haplotype
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The specific array of alleles encoded by linked genes in a segment of a single chromosome.
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interference (I)
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Measured on a zero to 1.0 scale, the measurement of the independence of crossovers. Expressed as 1.0 minus the coefficient of coincidence.
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intragenic recombination
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Crossing over within a gene.
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linkage equilibrium
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The random distribution into gametes of alleles of linked genes achieved by crossing over between the genes.
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linkage disequilibrium
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The nonrandom distribution into gametes of alleles of linked genes.
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linkage group
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A group of genes displaying genetic linkage.
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lod score (log of the odds ratio)
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Based on analysis of transmission in pedigrees, the statistic used to calculate the likelihood of genetic linkage between genes.
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mapping function
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Corrective calculations used to more accurately estimate recombination frequencies between linked genes. Mapping functions differ among certain species.
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map unit (m.u.), centiMorgan (cM)
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A theoretical unit of distance between linked genes on a chromosome.
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mitotic crossover
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Crossing over between homologous chromosomes during mitosis.
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negative interference
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Occurring when the coefficient of coincidence is greater than 1.0, the observation of more double crossovers than expected between a pair of genes.
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nonparental ditype (NPD)
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In an ascus, the occurrence of four haploid spores that are each recombinant.
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ordered ascus
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The linear sequence in an ascus of haploid spores whose arrangement allows determination of the chromatids participating in crossing over.
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parental (nonrecombinant) chromosome
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Chromosomes in gametes produced when crossing over does not take place between linked genes. Alleles marking each gene are retained in their initial (parental) configurations.
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parental ditype (PD)
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In an ascus, the occurrence of four haploid spores that are each nonrecombinant.
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recombinant (nonparental) chromosome
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Chromosomes in gametes produced by crossing over between linked genes. Alleles marking each gene are rearranged on chromatids by crossing over.
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recombination frequency (r)
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The rate of occurrence of recombination between a pair of linked genes. Expressed as the number of recombinants divided by the total number of meioses.
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second-division segregation
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Patterns of haploid spores in an ascus that indicate the alleles were separated at the second meiotic division as a result of crossing over between a gene and the centromere.
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syntenic genes
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Genes located on the same chromosome.
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tetrad
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An ascus containing four haploid spores.
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tetrad analysis
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The analysis of genetic linkage by analysis of different tetrad segregation types.
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tetratype (TT)
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In an ascus, the occurrence of both types of parentals and both types of recombinants among the spores.
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three-point test-cross analysis
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A test cross designed to identify genetic linkage between three genes and to provide data for determination of recombination frequency between linked genes.
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two-point test-cross analysis
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A test cross designed to identify genetic linkage between two genes and to provide date for determination of recombination frequency between linked genes.
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unordered tetrad
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Haploid spores in an ascus that are arranged in a random order.
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Zmax
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The most likely recombination distance (theta value) between genes as determined by lod score analysis.
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The results of the Stern experiment supported the general idea that...
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genetic recombination is a result of physical exchange between homologous chromosomes
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What is the definition of a map unit?
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One percent recombination (or more precisely, one percent recombinant products of meiosis)
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f two genes are 30 map units apart, what proportion of the gametes produced by a dihybrid individual (AB/ab) would be recombinant?
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30%
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Which of the following statements about genetic linkage is generally true?
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Genetic linkage results in the production of significantly more parental gametes, and significantly fewer nonparental (recombinant) gametes than expected by chance.
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In a two-point test cross involving unlinked genes, what phenotypic ratios would be expected from a cross between a dihybrid female and a homozygous recessive male?
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1:1:1:1; If the genes are unlinked, then they will assort independently in meiosis and test-cross progeny will be produced in a 1:1:1:1 ratio. If the two genes are linked, then greater than 50% of the progeny will display parental phenotypes and less than 50% of the progeny will display recombinant (nonparental) phenotypes.
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What is the name given to the specific array of alleles in a set of linked genes on a single chromosome?
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Haplotype
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Genes A, B, C, D, and E are linked on a chromosome and occur in the order given. The test cross Ae / aE ae / ae indicates the genes recombine with a frequency of 22%. If 1000 progeny are produced by the test cross, determine the number of progeny in each outcome class.
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Ae___, aE___, AE___, ae___ 390,390,110,110
