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

6. Chromosomal Variation

Chromosomal Mutations: Aneuploidy

Genetics

6. Chromosomal Variation

Chromosomal Mutations: Aneuploidy

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2:23 minutes

Problem 27c

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.

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Klinefelter syndrome (47,XXY) occurs when a male has an extra X chromosome, resulting in a karyotype of 47 chromosomes instead of the typical 46. This means the boy inherited one X chromosome from his mother, one X chromosome from his father, and a Y chromosome from his father.

Anhidrotic ectodermal dysplasia is an X-linked condition, meaning the gene responsible for the condition is located on the X chromosome. Since males typically have only one X chromosome, they either express the condition (if they inherit the affected X) or do not (if they inherit a normal X).

In this case, the boy has two X chromosomes due to Klinefelter syndrome. One X chromosome came from his mother (normal) and the other X chromosome came from his father (carrying the mutation for anhidrotic ectodermal dysplasia).

The patches of normal and abnormal skin in the boy are due to X-inactivation, a process in which one of the two X chromosomes in each cell is randomly inactivated during early embryonic development. This leads to a mosaic pattern where some cells express the genes from the maternal X chromosome (normal skin) and others express the genes from the paternal X chromosome (abnormal skin).

This phenomenon is called mosaicism, and it explains why the boy has patches of normal skin (where the paternal X chromosome is inactivated) and patches of abnormal skin (where the maternal X chromosome is inactivated).

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Key Concepts

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

Klinefelter Syndrome

Klinefelter syndrome is a genetic condition in males characterized by the presence of an extra X chromosome, resulting in a 47,XXY karyotype. This condition can lead to various physical and developmental features, including reduced testosterone levels, infertility, and sometimes, skin abnormalities. Understanding Klinefelter syndrome is crucial for analyzing the genetic background of the boy in the question.

X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. Conditions that are X-linked, such as anhidrotic ectodermal dysplasia, can manifest in males who have only one X chromosome. In this case, the boy's skin phenotype is influenced by the X-linked condition inherited from his father, which affects the development of sweat glands and can lead to patches of abnormal skin.

Phenotype

Phenotype refers to the observable physical and physiological traits of an organism, which result from the interaction of its genotype with the environment. In the context of the boy's skin phenotype, it describes the visible patches of normal and abnormal skin. This variation can be attributed to the combination of his Klinefelter syndrome and the X-linked condition inherited from his father, leading to a mosaic pattern of skin expression.

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Textbook Question

In humans that are XX/XO mosaics, the phenotype is highly variable, ranging from females who have classic Turner syndrome symptoms to females who are essentially normal. Likewise, XY/XO mosaics have phenotypes that range from Turner syndrome females to essentially normal males. How can the wide range of phenotypes be explained for these sex-chromosome mosaics?

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Textbook Question

A plant breeder would like to develop a seedless variety of cucumber from two existing lines. Line A is a tetraploid line, and line B is a diploid line. Describe the breeding strategy that will produce a seedless line, and support your strategy by describing the results of crosses.

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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.

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Textbook Question

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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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.

Most cases of Turner syndrome are attributed to nondisjunction of one or more of the sex chromosomes during gametogenesis, from either the male or female parent. However, some females possess a rare form of Turner syndrome in which some of the cells of the body (somatic cells) lack an X chromosome, while other cells have the normal two X chromosomes. Often detected in blood and/or skin cells, such individuals with mosaic Turner syndrome may exhibit relatively mild symptoms. An individual may be specified as 45,X(20)/46,XX(80) if, for example, 20 percent of the cells examined were X monosomic. How might mitotic events cause such mosaicism, and what parameter(s) would likely determine the percentages and distributions of X0 cells?

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Textbook Question

A 3-year-old child exhibited some early indication of Turner syndrome, which results from a 45,X chromosome composition. Karyotypic analysis demonstrated two cell types: 46,XX (normal) and 45,X. Propose a mechanism for the origin of this mosaicism.

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