Textbook Question
Predict the potential effect of the Lyon hypothesis on the retina of a human female heterozygous for the X-linked red-green color blindness trait.
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Ch. 7 - Sex Determination and Sex Chromosomes
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.
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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.
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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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Related Practice
Textbook Question
Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern are almost invariably females. Why are they certain of this?
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Textbook Question
In mice, the Sry gene (see Section 7.2) is located on the Y chromosome very close to one of the pseudoautosomal regions that pairs with the X chromosome during male meiosis. Given this information, propose a model to explain the generation of unusual males who have two X chromosomes (with an Sry-containing piece of the Y chromosome attached to one X chromosome).
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Textbook Question
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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Textbook Question
In chickens, a key gene involved in sex determination has recently been identified. Called DMRT1, it is located on the Z chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike SRY in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew Sinclair (a co-discoverer of the human SRY gene), Craig Smith and colleagues were able to 'knock down' expression of DMRT1 in ZZ embryos using RNA interference techniques (see Chapter 18). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?
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