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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 25a
Chapter 3, Problem 25a

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?

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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.
Determine the genetic status of the woman: Since her brother has Lesch–Nyhan syndrome, her mother must be a carrier (heterozygous for the mutation). The woman has a 50% chance of inheriting the affected X chromosome from her mother, making her a carrier.
Consider the genetic contribution to the son: If the woman is a carrier, there is a 50% chance she will pass on the affected X chromosome to her child. Sons inherit their X chromosome from their mother and their Y chromosome from their father.
Combine probabilities: The probability that the woman is a carrier (50%) and the probability that she passes on the affected X chromosome to her son (50%) must be multiplied to determine the overall probability that her son will be affected.
Express the final probability: Multiply the probabilities from the previous step (0.5 × 0.5) to calculate the likelihood that her first son will inherit Lesch–Nyhan syndrome. This step provides the final probability but does not calculate the numerical result explicitly.

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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. Females, with two X chromosomes, can be carriers if they have one affected X but typically do not show symptoms unless both X chromosomes are affected.
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Carrier Status

In the context of X-linked disorders, a carrier female has one normal X chromosome and one affected X chromosome. If her brother has Lesch–Nyhan syndrome, it indicates that he inherited the affected X from their mother, making her a carrier. The probability of her passing the affected X to her son is 50%, as sons inherit their X chromosome from their mother.
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Punnett Square

A Punnett square is a tool used in genetics to predict the probability of offspring inheriting particular traits from their parents. In this case, it can be used to visualize the potential genetic combinations between a carrier mother (X^H X) and a normal father (X Y). This helps in determining the likelihood of their son inheriting the affected X chromosome and thus being affected by Lesch–Nyhan syndrome.
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Related Practice
Textbook Question

In fruit flies, yellow body (y) is recessive to gray body , and the trait of body color is inherited on the X chromosome. Vestigial wing (v) is recessive to full-sized wing (v⁺), and the trait has autosomal inheritance. A cross of a male with yellow body and full wings to a female with gray body and full wings is made. Based on an analysis of the progeny of the cross shown below, determine the genotypes of parental and progeny flies.

[Table below appears at this point containing crosses and results]

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.

  

Why does this evidence support the hypothesis that a black spot is sex linked?

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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.


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

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?

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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