Ch. 7 - Sex Determination and Sex Chromosomes

Klug - Concepts of Genetics 12th Edition

Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook

Klug 12th Edition

Ch. 7 - Sex Determination and Sex Chromosomes

Problem 23

Chapter 7, Problem 23

What is the role of the enzyme aromatase in sexual differentiation in reptiles?

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Understand that aromatase is an enzyme that converts androgens (male hormones) into estrogens (female hormones).

Recognize that in many reptiles, sexual differentiation is influenced by temperature-dependent sex determination (TSD), where the incubation temperature of the eggs determines the sex of the offspring.

Acknowledge that aromatase activity can be temperature-sensitive, meaning that at certain temperatures, the enzyme is more active, leading to higher estrogen levels and the development of female characteristics.

Consider that at lower temperatures, aromatase activity might be reduced, resulting in lower estrogen levels and the development of male characteristics.

Conclude that aromatase plays a crucial role in the balance of sex hormones during the critical period of sexual differentiation, influencing whether the developing embryo will become male or female based on the temperature conditions.

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Key Concepts

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

Aromatase Enzyme

Aromatase is an enzyme that converts androgens, such as testosterone, into estrogens, like estradiol. This conversion is crucial in various biological processes, including sexual differentiation, as it influences the development of sexual characteristics in many species, including reptiles.

Sexual Differentiation

Sexual differentiation refers to the process by which organisms develop male or female characteristics. In reptiles, this can be influenced by genetic factors, environmental conditions, and hormonal levels, with aromatase playing a key role in determining the sex by modulating estrogen levels during critical developmental periods.

Environmental Sex Determination

In some reptiles, sex is determined by environmental factors such as temperature during incubation rather than strictly by genetics. This phenomenon, known as temperature-dependent sex determination (TSD), is influenced by the activity of aromatase, which can be affected by temperature, thereby impacting the balance of sex hormones and ultimately the sex of the offspring.

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The Amami spiny rat (Tokudaia osimensis) lacks a Y chromosome, yet scientists at Hokkaido University in Japan have reported that key sex-determining genes continue to be expressed in this species. Provide possible explanations for why male differentiation can still occur in this mammalian species despite the absence of a Y chromosome.

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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?

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