Genetics Key Terms 3

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Ascospore

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A meiotic spore produced in certain fungi.

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Ascus

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In fungi, the sac enclosing four or eight ascospores.

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Centimorgan(cM)

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A unit of distance between genes on chromosomes representing 1 percent crossing over between two genes. Equivalent to 1 map unit (mu).

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Crossover

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the exchange of chromosomal material (part of chromosomal arms) between homologous chromosomes by breakage and reunion. The exchange of material between nonsister chromatids during meiosis is the basis of genetic recombination.

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Four-strand double crossover

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The four-strand double crossover is a single crossover between two chromatids and then a second single crossover between the remaining chromatids. The result is that all four products are recombinant.

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Genetic map(chromosome map)

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A diagram showing the location of genes on chromosomes

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Interference (positive interference)

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A measure of the degree to which one crossover affects the incidence of another crossover in an adjacent region of the same chromatid. Negative interference increases the chance of another crossover; positive interference reduces the possibility of a second crossover event.

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linkage

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The condition in which genes have their loci present on the same chromosome, causing them to be inherited as a unit, provided that they are not seperated by crossing over during meiosis.

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lod score(LOD)

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A statistical method used to determine whether two loci are linked or unlinked. A lod (log of the odds) score of 4 indicates that linkage is 10,000 times more likely than nonlinkage. By convention, scores of 3-4 are signs of linkage.

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mapping functions

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A mathetical formula that relates map distances to recombination frequencies.

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Non-parental ditypes

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A non-parental ditype (NPD) is a spore arrangement in that contains only the two recombinant-type ascospores (assuming two segregating loci). A tetrad type containing two different genotypes, both of which are recombinant.

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Parental ditypes

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A tetrad type containing two different genotypes, both of which are parental. A spore arrangement in Ascomycetes that contains only the two non-recombinant-type ascospores

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linkage map

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A linkage map is a genetic map of a species or experimental population that shows the position of its known genes or genetic markers relative to each other in terms of recombination frequency, rather than as specific physical distance along each chromosome. Linkage mapping is critical for identifying the location of genes that cause genetic diseases.

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noncrossover gamete

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A gamete whose chromosomes have undergone no genetic recombination

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recombinant frequency (% recombinant)

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Recombination frequency is a measure of genetic linkage and is used in the creation of a genetic linkage map. Recombination frequency (θ) is the frequency with which a single chromosomal crossover will take place between two genes during meiosis.

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crossover gamete

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A gamete whose chromosomes have undergone genetic recombination

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syntenic genes

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In somatic cell genetics, a method for determining whether two genes are on the same chromosome

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tetrad

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The four chromatids that make up paired homologs in the prophase of the first meiotic division. In eukaryotes with a predominant haploid stage (some algae and fungi), tetrad denotes the four haploid cells produced by a single meiotic division.

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three-strand double crossover

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Three-strand double crossovers result when one chromatid crosses over with two other chromatids. The result is two parental and two recombinant progeny, the same as a single crossover event.

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two-strand double crossover

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A two-strand double crossover is the result of two chiasma between the two loci involving the same two sister chromatids. These two events cancel each other out and result in parental or non-recombinant progeny.

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Linkage group

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Genes that are inherited as a single unit because they are near one another on the same chromosome. ex.: in fruit flies the genes affecting eye color and wing length are inherited together because they appear on the same chromosome.