Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

4. Genetic Mapping and Linkage

Mapping Genes

Genetics

4. Genetic Mapping and Linkage

Mapping Genes

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3:39 minutes

Problem 8b

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%.

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.

Verified step by step guidance

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.

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.

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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Related Practice

Multiple Choice

Using the following data collected from a test cross, calculate the recombination frequency.

In a diploid species of plant, the genes for plant height and fruit shape are syntenic and separated by 18 m.u. Allele D produces tall plants and is dominant to d for short plants, and allele R produces round fruit and is dominant to r for oval fruit.

Give the same information for a plant with the genotype Dr/dR.

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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

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

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

Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green color, whereas the homozygous presence of the recessive y allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained:

colored, green: 88

colored, yellow: 12

colorless, green: 8

colorless, yellow: 92

Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.

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Textbook Question

In the cross shown here, involving two linked genes, ebony (e) and claret (ca), in Drosophila, where crossing over does not occur in males, offspring were produced in a 2 + : 1 ca : 1 e phenotypic ratio:

These genes are 30 units apart on chromosome III. What did crossing over in the female contribute to these phenotypes?

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