Chapter 05.1: Linkage, Recombination, And The Mapping Of Genes On Chromosomes

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linkage

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the proximity of two or more markers on a chromosome; the closer together the markers are, the lower the probability that they will be separated by recombination. Genes are linked when the frequency of parental type progeny exceeds that of recombinant progeny.

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recombination

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the process by which offspring derive a combination of genes different from that of either parent; the generation of new allelic combinations. In higher organisms, this can occur by crossing-over.

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syntenic

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relationship of two or more loci found to be located on the same chromosome.

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parental types

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phenotypes that reflect a previously existing parental combination of genes that is retained during gamete formation

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recombinant types

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phenotypes reflecting a new combination of genes that occurs during gamete formation

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parental classes

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combinations of alleles present in the original parental generation

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recombinant classes

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reshuffled combinations of alleles that were not present in the parental generation

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linked

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describes genes whose alleles are inherited together more often than not; linked genes are usually located close together on the same chromosome

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chi-square test

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a statistical test to determine the probability that an observed deviation from an expected outcome occurs solely by chance

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null hypothesis

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a statistical hypothesis to be tested and either accepted or rejected in favor of an alternative

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How is the Greek letter chi written?

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χ

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chiasmata

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observable regions in which nonsister chromatids of homologous chromosomes cross over

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physical markers

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cytologically visible abnormalities that make it possible to keep track of specific chromsome parts from one generation to the next

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genetic markers

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genes identifiable through phenotypic variants that can serve as points of reference in determining whether particular progeny are the result of recombination

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terminalization

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shifting of the chiasmata from their original position at the centromere toward the chromosome end or telomere

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What does RF stand for?

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recombination frequency

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recombination frequency

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the percentage of recombinant progeny; can be used as an indication of the physical distance separating any two genes on a chromosome

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what does cM stand for?

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centimorgan

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centimorgan

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a unit of measure of recombination frequency. One cM is equal to 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing-over in a single generation

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map unit

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synonym for centimorgan

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What does m.u. stand for?

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map unit

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locus

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a designated location on a chromosome.

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mapping

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the process of determining the locus of a gene on a particular chromosome

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red-green colorblindness is inherited on which chromosome?

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X-linked

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hemophilia is inherited on which chromosome?

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X-linked

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If two genes have a tendency to be inherited together, what does this imply?

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The two genes are close together

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How can recombinant progeny arise?

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Can result from:
1.) The recombination of genes on the same chromosome during gamete formation.
2.) The independent assortment of genes on nonhomologus chromosomes

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The farther apart two genes are, the greater the probability of:

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separation through recombination

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Two genes located on the same chromosome are said to be:

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syntenic

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In dihybrid crosses, departures from a 1:1:1:1 ratio F1 gametes indicates:

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that the two genes are on the same chromosome

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Can linkage ever be 100%

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No! No matter how tightly two genes are linked, if you observe enough individuals, you will find some recombinants.

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What's the Chi-Square equation?

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χ^2 = Σ (# observed - # expected)^2/(# expected)

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degree of freedom (df)

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In Chi-Square tests, this is a measure of the number of independently varying parameters in the experiment.

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p value

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Determined by the degree of freedom (df); the probability that a deviation from the predicted numbers at least as large as that observed in the experiment will occur by chance.