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Chapter 5: Problem 15

Suppose that a mutation occurred in the \(S R Y\) gene on the human Y chromosome, knocking out its ability to produce the testis-determining factor. Predict the phenotype of an individual who carried this mutation and a normal X chromosome.

Short Answer

Expert verified
The individual will develop female characteristics.

Step by step solution

01

Understand SRY gene function

The SRY gene is located on the Y chromosome and is responsible for producing the testis-determining factor (TDF), which triggers the development of male characteristics by initiating the formation of testes.
02

Consequence of SRY gene mutation

A mutation knocking out the SRY gene's ability means that TDF is not produced. Without TDF, the development of testes is not initiated, and the individual will not develop male characteristics.
03

Evaluate the role of X chromosome

The individual also possesses a normal X chromosome. Since the X chromosome is not altered, it does not affect the development pathways that depend on the presence of the Y chromosome's signals for male differentiation.
04

Predict the Phenotype

Owing to the absence of TDF, the individual will develop default female characteristics despite the presence of a Y chromosome. The phenotype will be female, resembling conditions like Swyer Syndrome.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

testis-determining factor
The testis-determining factor (TDF) plays a crucial role in the sexual development of males. It is a protein product of the SRY gene, located on the Y chromosome. Think of TDF as the conductor in the orchestra of sex determination; it orchestrates the development of the embryonic gonads into testes. Without TDF, the gonads do not develop into testes but instead follow a default path to become ovaries. This protein essentially acts as a switch which directs the developmental program towards forming male characteristics. The presence of TDF leads to the production of testosterone and other male hormones that support the development of male secondary sexual characteristics.
Y chromosome
The Y chromosome is a sex chromosome that is vital for determining male biological characteristics. It holds a set of genes crucial for male development, with the SRY gene being one of the most pivotal.
  • The Y chromosome pairs with an X chromosome to form an XY pattern, which is typically associated with a male genetic makeup.
  • It carries genetic information that cannot be found on the X chromosome, making it unique and essential for processes that lead to male differentiation.
  • Unlike most chromosomes, the Y chromosome is passed from father to son, and it carries determinants of male sexual development, like the SRY gene.
Without a functional Y chromosome, an individual would not develop male characteristics. Instead, they would likely follow a female development pathway.
phenotypic sex determination
Phenotypic sex determination refers to the process by which an organism develops external and internal sexual characteristics associated with either female or male sexes. It's influenced by genetic factors, primarily the presence of sex chromosomes, namely X and Y. For most mammals, the presence of the Y chromosome and specifically the SRY gene determines the development of male phenotypic traits.
  • If TDF is present via a normal SRY gene, the pathway for male characteristics is activated.
  • In the absence of TDF, as in cases of SRY mutation, the default pathway leads to female characteristics.
This systematic process of sex determination is pivotal for understanding disorders where these processes are disrupted, leading to variations from typical male or female phenotypes.
Swyer Syndrome
Swyer Syndrome is a rare condition where an individual who biologically is male (has XY chromosomes) exhibits female external characteristics. This occurs due to the inability of the SRY gene to produce the testis-determining factor, thus no testes are formed, and as a result, the body develops female traits by default. People with Swyer Syndrome have:
  • Normal female external genitalia but lack functional ovaries or testes.
  • They typically do not produce sex hormones that come from ovaries or testes.
  • They usually require hormone replacement therapy to develop secondary sexual characteristics and to maintain overall health.
Despite having a Y chromosome, the lack of TDF means these individuals develop as females and often come to medical attention due to absence of menstruation or delayed puberty.

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Most popular questions from this chapter

A Drosophila female heterozygous for the recessive X-linked mutation \(w\) (for white eyes) and its wild-type allele \(w^{+}\) is mated to a wild-type male with red eyes. Among the sons, half have white eyes and half have red eyes. Among the daughters, nearly all have red eyes; however, a few have white eyes. Explain the origin of these white-eyed daughters.

A male with singed bristles appeared in a culture of Drosopbila. How would you determine if this unusual phenotype was due to an X-linked mutation?

A normal woman, whose father had hemophilia, marries a normal man. What is the chance that their first child will have hemophilia?

In Drosophila, vermilion eye color is due to a recessive allele \((v)\) located on the \(\mathrm{X}\) chromosome. Curved wings is due to a recessive allele ( \(c u\) ) located on one autosome, and ebony body is due to a recessive allele ( \(e\) ) located on another autosome. A vermilion male is mated to a curved, ebony female, and the \(\mathrm{F}_{1}\) males are phenotypically wild-type. If these males were backcrossed to curved, ebony females, what proportion of the \(\mathrm{F}_{2}\) offspring will be wild-type males?

A Drosopbila female homozygous for a recessive X-linked mutation that causes vermilion eyes is mated to a wild-type male with red eyes. Among their progeny, all the sons have vermilion eyes, and nearly all the daughters have red eyes; however, a few daughters have vermilion eyes. Explain the origin of these vermilion-eyed daughters.
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