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Ch. 10 - Eukaryotic Chromosome Abnormalities and Molecular Organization
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. 10 - Eukaryotic Chromosome Abnormalities and Molecular OrganizationProblem 24c
Chapter 10, Problem 24c

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.
What term best describes this kind of chromosome abnormality?

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1
Understand that Down syndrome (trisomy 21) is caused by the presence of an extra copy of chromosome 21, resulting in three copies of this chromosome instead of the usual two.
Note that the boy has 46 chromosomes, which is unusual for someone with trisomy 21, as individuals with Down syndrome typically have 47 chromosomes.
Observe that the parents and sisters have 45 chromosomes, which is fewer than the normal 46 chromosomes in humans. This suggests a structural chromosomal abnormality rather than a numerical one.
Recognize that the most likely explanation for this situation is a Robertsonian translocation, a type of chromosomal rearrangement where two acrocentric chromosomes fuse at their centromeres, leading to a reduction in chromosome number without loss of genetic material.
Conclude that the term 'Robertsonian translocation' best describes this chromosomal abnormality, as it explains both the reduced chromosome number in the parents and sisters and the inheritance of an unbalanced form leading to trisomy 21 in the boy.

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Key Concepts

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

Chromosome Number Abnormalities

Chromosome number abnormalities occur when there is a deviation from the normal diploid number of chromosomes, which is 46 in humans. These abnormalities can manifest as aneuploidy, where there is an extra or missing chromosome, or polyploidy, where there are multiple sets of chromosomes. In the case of Down syndrome, the presence of an extra chromosome 21 results in trisomy 21, leading to the characteristic features of the condition.
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Aneuploidy

Aneuploidy is a specific type of chromosome number abnormality characterized by the presence of an abnormal number of chromosomes in a cell. This can include conditions such as monosomy (one less chromosome) or trisomy (one extra chromosome). In the context of the question, the boy with Down syndrome has trisomy 21, indicating he has three copies of chromosome 21 instead of the usual two, while his family members have a normal chromosome count.
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Phenotype vs. Genotype

Phenotype refers to the observable physical and biochemical characteristics of an organism, which result from the interaction of its genotype with the environment. In this scenario, the boy with Down syndrome has a distinct phenotype due to his genetic makeup, while his parents and sisters, despite having a normal phenotype, have an unusual chromosome count of 45, suggesting a potential genetic anomaly that may not be phenotypically expressed.
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Related Practice
Textbook Question

In the tomato, Solanum esculentum, tall (D−)(D−) is dominant to dwarf (dd) plant height, smooth fruit (P−) is dominant to peach fruit (pp), and round fruit shape (O−) is dominant to oblate fruit shape (oo). These three genes are linked on chromosome 1 of tomato in the order dwarf–peach–oblate. There are 12 map units between dwarf and peach and 17 map units between peach and oblate. A trihybrid plant (DPO/dpo) is test-crossed to a plant that is homozygous recessive at the three loci (dpo/dpo). The accompanying table shows the progeny plants. Identify the mechanism responsible for the resulting data that do not agree with the established genetic map.

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Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

Explain how this is possible.

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Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

How many chromosomes do you expect to see in karyotypes of the parents?

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Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

What is the probability the next child of this couple will have a normal phenotype and have 46 chromosomes? Explain your answer.

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Textbook Question

Experimental evidence demonstrates that the nucleosomes present in a cell after the completion of S phase are composed of some 'old' histone dimers and some newly synthesized histone dimers. Describe the general design for an experiment that uses a protein label such as ³⁵S to show that nucleosomes are often a mixture of old and new histone dimers following DNA replication.

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Textbook Question

DNase I cuts DNA that is not protected by bound proteins but is unable to cut DNA that is complexed with proteins. Human DNA is isolated, stripped of its nonhistone proteins, and mixed with DNase I. Samples are removed after 30 minutes, 1 hour, and 4 hours and run separately in gel electrophoresis. The resulting gel is stained to make all DNA fragments in it visible, and the results are shown in the figure. DNA fragment sizes in base pairs (bp) are estimated by the scale to the left of the gel. Examine the gel results and speculate why longer DNase I treatment produces different results.

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