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Ch. 5 - Chromosome Mapping in Eukaryotes
Chapter 5, Problem 24

Traditional gene mapping has been applied successfully to a variety of organisms including yeast, fungi, maize, and Drosophila. However, human gene mapping has only recently shared a similar spotlight. What factors have delayed the application of traditional gene-mapping techniques in humans?

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1
Identify the complexity of the human genome compared to other organisms.
Consider the ethical and practical challenges in experimenting with human subjects.
Discuss the longer generation time in humans, which complicates genetic studies.
Examine the genetic diversity and population size of humans, which adds complexity to mapping.
Reflect on the technological advancements needed to overcome these challenges in human gene mapping.

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Key Concepts

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

Complexity of Human Genetics

Human genetics is inherently more complex than that of model organisms like yeast or Drosophila. Humans have a larger genome, more genes, and greater genetic diversity, which complicates the identification of gene functions and interactions. This complexity makes traditional mapping techniques, which work well in simpler organisms, less effective in humans.
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Ethical Considerations

The study of human genetics raises significant ethical concerns that do not typically arise in research on non-human organisms. Issues such as informed consent, privacy, and the potential for genetic discrimination can hinder the application of traditional gene-mapping techniques in humans, as researchers must navigate these ethical landscapes carefully.
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Technological Limitations

Historically, the technological tools available for gene mapping in humans were less advanced than those used for model organisms. Techniques such as high-throughput sequencing and advanced bioinformatics have only recently become accessible, allowing for more effective mapping of human genes. The delay in technology adoption has significantly impacted the pace of human gene mapping research.
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Related Practice
Textbook Question
An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table. 20 AaBbCc 20 AaBbcc 20 aabbCc 20 aabbcc 5 AabbCc 5 Aabbcc 5 aaBbCc 5 aaBbcc What can you conclude from the actual data about the location of the three genes in relation to one another?

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Textbook Question
Based on our discussion of the potential inaccuracy of mapping (see Figure 5.12), would you revise your answer to Problem 22? If so, how?
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Textbook Question
How do we know that in humans the X chromosomes play no role in human sex determination, while the Y chromosome causes maleness and its absence causes femaleness? Why are many expected crossover phenotypes missing? Can any of these loci be mapped from the data given here? If so, determine map distances.
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Textbook Question
In a certain plant, fruit is either red or yellow, and fruit shape is either oval or long. Red and oval are the dominant traits. Two plants, both heterozygous for these traits, were testcrossed, with the following results. Determine the location of the genes relative to one another and the genotypes of the two parental plants.
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Textbook Question
Two plants in a cross were each heterozygous for two gene pairs (Ab/aB) whose loci are linked and 25 mu apart. Assuming that crossing over occurs during the formation of both male and female gametes and that the A and B alleles are dominant, determine the phenotypic ratio of their offspring.
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Textbook Question
A number of human–mouse somatic cell hybrid clones were examined for the expression of specific human genes and the presence of human chromosomes. The results are summarized in the following table. Assign each gene to the chromosome on which it is located.
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