Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement

Problema 27a

Capitolo 8, Problema 27a

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?

Guida verificata passo dopo passo

Step 1: Understand the genetic condition and inheritance pattern. Klinefelter syndrome (47,XXY) means the boy has two X chromosomes and one Y chromosome. The father has an X-linked skin condition (anhidrotic ectodermal dysplasia), which is inherited through the X chromosome.

Step 2: Analyze the mother's genotype and phenotype. The mother is phenotypically normal with no signs of the skin condition, indicating she likely does not carry the mutant allele on either of her X chromosomes.

Step 3: Consider the son's phenotype of patches of normal and abnormal skin. This mosaic pattern suggests X-chromosome inactivation (lyonization) in a female or in this case, a 47,XXY male, where one X chromosome is randomly inactivated in different cells, leading to patches expressing either the normal or mutant allele.

Step 4: Determine which parent contributed the abnormal X chromosome. Since the mother is normal and the son has the condition, the abnormal X chromosome must have come from the father, who has the X-linked skin condition.

Step 5: Conclude that the father contributed the abnormal X chromosome carrying the mutation, while the mother contributed a normal X chromosome, and the Y chromosome came from the father as well, resulting in the 47,XXY karyotype.

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Klinefelter Syndrome (47,XXY)

Klinefelter syndrome is a chromosomal condition where a male has an extra X chromosome, resulting in a 47,XXY karyotype. This affects male development and can cause physical and reproductive abnormalities. Understanding this helps explain the genetic background of the boy in the question.

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Percorso guidato

10:35

Human Sex Chromosomes

X-linked Inheritance and Mosaicism

X-linked conditions are caused by mutations on the X chromosome. In males (XY), a single mutated X causes the disease, while females (XX) may be carriers without symptoms due to X-inactivation. Mosaicism, such as patches of normal and abnormal skin, occurs when some cells express the mutated gene and others do not, often due to random X-inactivation.

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09:30

X-Inactivation

Parental Origin of Chromosomes and Gametes

Determining which parent contributed the abnormal X chromosome involves understanding gamete formation. The mother provides one X chromosome, and the father provides either an X or a Y. Since the father has the X-linked condition, his X chromosome carries the mutation, but the son’s extra X could come from either parent, affecting the phenotype.

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Percorso guidato

21:21

Gamete Development

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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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