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Ch. 3 - Cell Division and Chromosome Heredity
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. 3 - Cell Division and Chromosome HeredityProblem 25c
Chapter 3, Problem 25c

Lesch–Nyhan syndrome (OMIM 300322) is a rare X-linked recessive disorder that produces severe mental retardation, spastic cerebral palsy, and self-mutilation.


What is the probability that the first son of a man whose brother has Lesch–Nyhan syndrome will be affected?

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1
Understand the inheritance pattern: Lesch–Nyhan syndrome is an X-linked recessive disorder. This means the gene responsible for the condition is located on the X chromosome, and males (XY) are more likely to be affected because they have only one X chromosome. Females (XX) are typically carriers if they inherit one affected X chromosome.
Analyze the family history: The man in question has a brother with Lesch–Nyhan syndrome. This indicates that their mother is likely a carrier of the mutation on one of her X chromosomes, as she passed the affected X chromosome to the affected brother.
Determine the genetic status of the man: Since the man does not have Lesch–Nyhan syndrome, he must have inherited the unaffected X chromosome from his mother. Therefore, he is not a carrier and cannot pass the mutation to his children.
Consider the inheritance of the X chromosome: A father passes his X chromosome only to his daughters and his Y chromosome to his sons. Since the man does not carry the mutation on his X chromosome, he cannot pass it to his children, including his sons.
Conclude the probability: Based on the above analysis, the probability that the first son of this man will be affected by Lesch–Nyhan syndrome is 0% because the father does not carry the mutation on his X chromosome.

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Key Concepts

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

X-linked Recessive Inheritance

Lesch–Nyhan syndrome is an X-linked recessive disorder, meaning the gene responsible for the condition is located on the X chromosome. Males, having one X and one Y chromosome, are more likely to express the disorder if they inherit the affected X. In contrast, females have two X chromosomes, so they would need two copies of the mutated gene to be affected, making them carriers if they have only one.
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X-Inactivation

Carrier Status

In X-linked recessive disorders, females can be carriers if they possess one normal and one mutated X chromosome. A carrier female has a 50% chance of passing the affected X chromosome to her sons, who would then express the disorder. Understanding the carrier status of the mother is crucial in determining the probability of the son being affected.
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Sex-Linked Pedigrees

Probability Calculation

To determine the probability that the first son of a man whose brother has Lesch–Nyhan syndrome will be affected, we must consider the genetic background of the parents. Since the man is unaffected but has an affected brother, he does not carry the mutation. If the mother is a carrier, the son has a 50% chance of being affected. Thus, the overall probability depends on the mother's carrier status.
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Related Practice
Textbook Question

In a species of fish, a black spot on the dorsal fin is observed in males and females. A fish breeder carries out a pair of reciprocal crosses and observes the following results.

Identify which sex is heterogametic. Give genotypes for the parents in each cross, and explain the progeny proportions in each cross.

Textbook Question

Lesch–Nyhan syndrome (OMIM 300322) is a rare X-linked recessive disorder that produces severe mental retardation, spastic cerebral palsy, and self-mutilation.


What is the probability that the first son of a woman whose brother has Lesch–Nyhan syndrome will be affected?

Textbook Question

Lesch–Nyhan syndrome (OMIM 300322) is a rare X-linked recessive disorder that produces severe mental retardation, spastic cerebral palsy, and self-mutilation.


If the first son of the woman described in (a) is affected, what is the probability that her second son is affected?

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Textbook Question

In humans, SRY is located near a pseudoautosomal region (PAR) of the Y chromosome, a region of homology between the X and Y chromosomes that allows them to synapse during meiosis in males and is a region of crossover between the chromosomes. The diagram below shows SRY in relation to the pseudoautosomal region.

About 1 in every 25,000 newborn infants is born with sex reversal; the infant is either an apparent male but with two X chromosomes or an apparent female but with an X and a Y chromosome. Explain the origin of sex reversal in human males and females involving the SRY gene. (Hint: See Experimental Insight 3.1 for a clue about the mutational mechanism.) 

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Textbook Question

In an 1889 book titled Natural Inheritance (Macmillan, New York), Francis Galton, who investigated the inheritance of measurable (quantitative) traits, formulated a law of 'ancestral inheritance.' The law stated that individuals inherit approximately one-half of their genetic traits from each parent, about one-quarter of the traits from each grandparent, one-eighth from each great grandparent, and so on. In light of the chromosome theory of heredity, argue either in favor of Galton's law or against it.

Textbook Question

In Drosophila, the X-linked echinus eye phenotype disrupts formation of facets and is recessive to wild-type eye. Autosomal recessive traits vestigial wing and ebony body assort independently of one another. Examine the progeny from the three crosses shown below, and identify the genotype of parents in each cross.