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Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 8 - Chromosome Mutations: Variation in Number and ArrangementProblem 26
Chapter 8, Problem 26

In a recent cytogenetic study on 1021 cases of Down syndrome, 46 were the result of translocations, the most frequent of which was symbolized as t(14;21). What does this symbol represent, and how many chromosomes would you expect to be present in t(14;21) Down syndrome individuals?

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1
Understand that the symbol t(14;21) represents a chromosomal translocation between chromosome 14 and chromosome 21. This means a segment of chromosome 21 is abnormally attached to chromosome 14, or vice versa.
Recall that in Down syndrome caused by translocation, the extra genetic material from chromosome 21 is present due to this rearrangement, rather than having a full extra chromosome 21 as in trisomy 21.
Recognize that a normal human karyotype has 46 chromosomes, arranged in 23 pairs. In translocation Down syndrome, the total chromosome number can be different depending on whether the translocation is balanced or unbalanced.
Determine that in the common t(14;21) translocation causing Down syndrome, the individual typically has 45 chromosomes because two chromosomes have fused into one (the translocation chromosome), but there is still an extra copy of the critical part of chromosome 21.
Summarize that the t(14;21) symbol indicates a translocation between chromosomes 14 and 21, and individuals with this translocation Down syndrome usually have 45 chromosomes instead of the typical 46.

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Key Concepts

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

Chromosomal Translocations

A chromosomal translocation occurs when a segment from one chromosome breaks off and attaches to another chromosome. In Down syndrome, a common translocation involves chromosomes 14 and 21, denoted as t(14;21), where part of chromosome 21 is translocated to chromosome 14. This can lead to an extra copy of chromosome 21 material, causing the syndrome.
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Reciprocal Translocation

Karyotype and Chromosome Number

A normal human karyotype has 46 chromosomes arranged in 23 pairs. In translocation Down syndrome, the total chromosome count can remain 46 because the extra chromosome 21 material is attached to another chromosome rather than existing as a separate chromosome. Thus, individuals with t(14;21) typically have 46 chromosomes, but with an abnormal structure.
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Down Syndrome Genetic Mechanism

Down syndrome is caused by trisomy 21, meaning three copies of chromosome 21 material. In translocation cases like t(14;21), the extra chromosome 21 material is attached to chromosome 14, resulting in functional trisomy 21 despite a normal chromosome count. This explains the presence of Down syndrome traits without the typical 47 chromosomes.
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Related Practice
Textbook Question

A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed to the right.

Using a drawing, demonstrate how these chromosomes would pair during meiosis. Be sure to label the different segments of the chromosomes.

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Textbook Question

A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed to the right.

This woman is phenotypically normal. Does this surprise you? Why or why not? Under what circumstances might you expect a phenotypic effect of such a rearrangement?

627
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Textbook Question

The woman in Problem 24 has had two miscarriages. She has come to you, an established genetic counselor, with these questions:

Is there a genetic explanation of her frequent miscarriages?

Should she abandon her attempts to have a child of her own?

If not, what is the chance that she could have a normal child? Provide an informed response to her concerns.

614
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Textbook Question

A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X-linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin.

Which parent contributed the abnormal gamete?

2301
views
Textbook Question

A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X-linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin.

Using the appropriate genetic terminology, describe the meiotic mistake that occurred. Be sure to indicate in which division the mistake occurred.

1525
views
Textbook Question

A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X-linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin.

Using the appropriate genetic terminology, explain the son's skin phenotype.

2491
views