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Ch. 5 - Genetic Linkage and Mapping in Eukaryotes
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 5 - Genetic Linkage and Mapping in EukaryotesProblem 8b
Chapter 5, Problem 8b

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.


Assuming that organisms with any desired genotype are available, propose a genetic cross whose result could be used to determine which of the proposed genetic maps is correct.

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1
Start by understanding that recombination frequency is directly proportional to the physical distance between genes on a chromosome. A recombination frequency of 1% corresponds to 1 map unit (or centimorgan, cM). Gene G recombines with gene T at 7% (7 cM), and gene G recombines with gene R at 4% (4 cM).
Propose two possible genetic maps based on the given recombination frequencies: (1) G is between T and R, with T-G = 7 cM and G-R = 4 cM, or (2) T and R are on opposite sides of G, with T-G = 7 cM and G-R = 4 cM. The total distance between T and R in the second case would be 7 cM + 4 cM = 11 cM.
Design a genetic cross to test these maps. Cross a double heterozygote (e.g., TtGgRr) with a triple homozygous recessive individual (e.g., ttggrr). This is a test cross, which allows you to observe the recombination events in the offspring.
Analyze the offspring phenotypes and their frequencies. The parental phenotypes will be the most frequent, while recombinant phenotypes will be less frequent. Identify the double recombinants (least frequent phenotypes) to determine the gene order.
Compare the observed recombination frequencies between the genes in the offspring to the proposed genetic maps. The correct map will match the observed recombination frequencies, confirming the gene order and distances.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Recombination Frequency

Recombination frequency is a measure of the likelihood that two genes will be separated during meiosis due to crossing over. It is expressed as a percentage, indicating the proportion of offspring that exhibit recombinant phenotypes. In this case, gene G's recombination frequencies with genes T and R are 7% and 4%, respectively, which suggests their relative positions on a genetic map.
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Recombination after Single Strand Breaks

Genetic Mapping

Genetic mapping is the process of determining the location of genes on a chromosome and the distances between them based on recombination frequencies. By analyzing the outcomes of genetic crosses, researchers can create maps that illustrate the arrangement of genes. The distances between genes can be inferred from the recombination frequencies, allowing for the construction of a genetic map that reflects their physical proximity.
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Mapping Overview

Test Cross

A test cross involves breeding an individual with a homozygous recessive organism to determine the genotype of the first individual based on the phenotypes of the offspring. This method is useful for revealing the genetic makeup of an organism, especially when assessing the linkage and arrangement of genes. In the context of the question, a test cross could help clarify the genetic relationships between genes G, T, and R by analyzing the resulting offspring's phenotypes.
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Trihybrid Cross
Related Practice
Textbook Question

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.


Draw two possible genetic maps for these three genes, and identify the recombination frequencies predicted for each map.

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Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.


Explain how each of the predicted progeny classes is produced.

Textbook Question

Genes A, B, and C are linked on a chromosome and found in the order A–B–C. Genes A and B recombine with a frequency of 8%, and genes B and C recombine at a frequency of 24%. For the cross a⁺b⁺c/abc⁺ × abc/abc, predict the frequency of progeny genotypes. Assume interference is zero.

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Textbook Question

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.


The test cross Ae/aE x ae/ae  indicates the genes recombine with a frequency of 28%. If 1000 progeny are produced by this test cross, determine the number of progeny in each outcome class.

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Textbook Question

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.


Previous genetic linkage crosses have determined that recombination frequencies are 6% for genes A and B, 4% for genes B and C, 10% for genes C and D, and 11% for genes D and E. The sum of these frequencies between genes A and E is 31%. Why does the recombination distance between these genes as determined by adding the intervals between adjacent linked genes differ from the distance determined by the test cross?

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Textbook Question

Syntenic genes can assort independently. Explain this observation.