Bio 304 Topic 5 Key Terms

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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

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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

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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 alleles (nonparentals or recombinants) that are transmitted together significantly less often than expected

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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

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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 disequilibirum

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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, he 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

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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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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 or gamete

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Chromosomes in a gametes produced when crossing oer 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 or gamete

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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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Theta value

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Likelihood of genetic linkage for many different recombination frequencies

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Two-point test cross analysis

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A test cross designed to identify genetic linage between two genes and to provide data 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 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