2. Mendel's Laws of Inheritance

Sex-Linked Genes

2. Mendel's Laws of Inheritance

Sex-Linked Genes

2:59 minutes

Problem 26

Textbook Question

In mice, the X-linked dominant mutation Testicular feminization (Tfm) eliminates the normal response to the testicular hormone testosterone during sexual differentiation. An XY mouse bearing the Tfm allele on the X chromosome develops testes, but no further male differentiation occurs—the external genitalia of such an animal are female. From this information, what might you conclude about the role of the Tfm gene product and the X and Y chromosomes in sex determination and sexual differentiation in mammals? Can you devise an experiment, assuming you can 'genetically engineer' the chromosomes of mice, to test and confirm your explanation?

Verified step by step guidance

Understand the role of the Tfm gene: The Tfm gene is located on the X chromosome and is responsible for producing a receptor that responds to testosterone. In the presence of the Tfm mutation, this receptor is nonfunctional, leading to a failure in male sexual differentiation despite the presence of testes. This suggests that the Tfm gene product is crucial for the downstream effects of testosterone during sexual differentiation.

Analyze the role of the X and Y chromosomes: The Y chromosome determines the development of testes through the SRY (Sex-determining Region Y) gene. However, the X chromosome, through the Tfm gene, plays a critical role in enabling the body to respond to testosterone. This indicates that sex determination (testes development) is Y-linked, but sexual differentiation (male phenotype development) requires functional genes on the X chromosome.

Devise an experiment: To test the role of the Tfm gene product, genetically engineer mice with the following genotypes: (1) XY with a functional Tfm gene, (2) XY with a Tfm mutation, (3) XX with a functional Tfm gene, and (4) XX with a Tfm mutation. Observe the sexual differentiation and external genitalia development in each case.

Include a control group: Ensure that you have a control group of normal XY and XX mice to compare the results. This will help confirm that the observed effects are specifically due to the Tfm mutation and not other genetic or environmental factors.

Interpret the results: If XY mice with a Tfm mutation develop female external genitalia despite having testes, it confirms that the Tfm gene product is essential for male sexual differentiation. If XX mice with a Tfm mutation show no significant changes, it further supports the idea that the Tfm gene product is specifically required for testosterone signaling in male differentiation.

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Key Concepts

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

Sex Determination

Sex determination refers to the biological process that dictates the development of sexual characteristics in an organism, primarily influenced by the presence of specific sex chromosomes. In mammals, the presence of the Y chromosome typically leads to male development, while its absence results in female development. The Tfm mutation in mice illustrates how genetic factors can disrupt this process, leading to atypical sexual differentiation despite the presence of male chromosomes.

Hormonal Influence on Sexual Differentiation

Hormonal influence on sexual differentiation involves the role of hormones, particularly androgens like testosterone, in the development of male characteristics. In normal male development, testosterone promotes the formation of male external genitalia and secondary sexual traits. The Tfm mutation disrupts this hormonal response, indicating that the gene product associated with Tfm is crucial for mediating testosterone's effects during sexual differentiation.

Genetic Engineering in Experimental Design

Genetic engineering involves manipulating an organism's DNA to study the effects of specific genes or mutations. In the context of the Tfm mutation, one could design an experiment to create genetically modified mice that either express or lack the Tfm gene. By observing the resulting sexual differentiation in these mice, researchers could confirm the role of the Tfm gene product in mediating testosterone's effects and further elucidate the mechanisms of sex determination.

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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.

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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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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