Biology Chapter 6: Sex Determination & Gametogenesis

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Questions and Answers

What genetic combination leads to the formation of ovaries in mammals?

XX (correct)

Which mechanism of sex determination is NOT employed by mammals?

Chromosomal sex determination

Environmental sex determination

Gene regulatory networks

Temperature-dependent sex determination (correct)

What is the role of the Y chromosome in mammalian sex determination?

It promotes testis formation. (correct)

It has no role in sex determination.

It determines the number of X chromosomes.

It promotes ovary development.

In Drosophila, how is the sexual phenotype determined?

By the number of X chromosomes. Signup and view all the answers

What type of cells do mammalian gonads produce?

Sperm and eggs Signup and view all the answers

What chromosomal combination results in a genetically male mammal?

XY Signup and view all the answers

What is a bipotential gonad?

A gonad that can develop into either testes or ovaries. Signup and view all the answers

What could potentially result from errors in meiosis during gamete formation?

Gametes with abnormal numbers of sex chromosomes. Signup and view all the answers

What is the outcome for individuals with a single X chromosome?

They develop underdeveloped external female genitalia. Signup and view all the answers

What initiates the development of testis in mammals?

The presence of a Y chromosome. Signup and view all the answers

What is a characteristic of individuals with Klinefelter syndrome (XXY)?

They develop testis and male genitalia. Signup and view all the answers

How does the presence of XY or XX chromosomes affect the fate of bipotential cells in early gonads?

They differentiate according to the chromosome type. Signup and view all the answers

What determines secondary sex characteristics in mammals?

Hormones produced by the gonads. Signup and view all the answers

What is a potential consequence of having extra X chromosomes (e.g., XXX)?

Potential developmental issues or disorders. Signup and view all the answers

What hormonal type is typically elevated in individuals with Klinefelter syndrome?

Estrogens. Signup and view all the answers

Which of the following conditions is characterized by the absence of a sex chromosome (X0)?

Turner syndrome. Signup and view all the answers

What is the primary role of Sry in sex determination?

Activation of the Sox9 transcription factor Signup and view all the answers

Which hormone is secreted by testis to maintain the Wolffian duct?

Testosterone Signup and view all the answers

What can cause a 46,XX individual to develop male genitalia?

Duplication of the Sox9 gene Signup and view all the answers

What occurs during secondary sex determination?

Hormonal secretion that triggers further development Signup and view all the answers

What happens to the Wolffian duct in females?

It atrophies due to lack of testosterone Signup and view all the answers

How does Sox9 contribute to testis formation?

Inhibiting ovary formation Signup and view all the answers

What is the outcome of a deletion or mutation in the SRY gene in a 46,XY individual?

Development of female phenotypes Signup and view all the answers

What is the primary mechanism by which Sox9 is activated in birds, fish, and frogs?

Induction by Dmrt1 Signup and view all the answers

What roles do the genes Sf1, Wt1, Lhx9, and Gata4 play in sex determination?

They are required for the formation of the bipotential gonad. Signup and view all the answers

Which of the following is true regarding the Sry gene?

It acts as a transcription factor that activates Sox9. Signup and view all the answers

What occurs to germ stem cells during week 6 of development?

They migrate into the gonadal tissue. Signup and view all the answers

What is the primary role of β-catenin in sex determination?

It blocks the transcription of Sox9. Signup and view all the answers

What happens to individuals with Rspo1 mutations?

They have two X chromosomes but male external genitalia. Signup and view all the answers

How does a duplication of the region on chromosome 1 that includes Wnt4 and Rspo1 affect XY individuals?

It results in male-to-female sex reversal. Signup and view all the answers

Which process describes how the gonad influences germ cell fate?

The gonad defines whether germ cells develop into oocytes or sperm. Signup and view all the answers

What indicates the start of sexual differentiation in human gonadal tissue?

Week 8 of development. Signup and view all the answers

What is the role of estrogen in the development of the female reproductive tract?

To cause differentiation of the Mullerian duct into the female reproductive tract Signup and view all the answers

Why do male estrogen receptor knockout mice become sterile?

They develop very few sperm. Signup and view all the answers

What happens to the X chromosome that is inactivated in female mammals?

It forms tightly bound heterochromatin. Signup and view all the answers

What is a characteristic feature of calico cats related to genetics?

They exhibit patches of black and orange fur from X chromosome inactivation. Signup and view all the answers

Why are male cats typically not calico?

They only inherit one X chromosome and cannot be heterozygous. Signup and view all the answers

What hormone is secreted by Sertoli cells?

Anti-Mullerian hormone (AMH) Signup and view all the answers

What is the result of a mutation in the receptor that responds to testosterone in individuals with androgen insensitivity syndrome?

Inability to maintain Wolffian duct structures Signup and view all the answers

What is the primary effect of dihydrotestosterone (DHT) during prenatal development?

Masculinization of the urethra, prostate, penis, and scrotum Signup and view all the answers

Which of the following describes the genetic composition of individuals with androgen insensitivity syndrome?

XY chromosomes but unable to respond to testosterone Signup and view all the answers

In individuals with 5-alpha reductase deficiency, what happens at puberty?

Testosterone production leads to the appearance of male external genitalia Signup and view all the answers

What condition results from the secretion of testosterone but no response due to receptor mutation?

Androgen insensitivity syndrome Signup and view all the answers

Which cells are responsible for the degeneration of the Mullerian duct?

Sertoli cells Signup and view all the answers

What characterizes the external genitalia of individuals with androgen insensitivity syndrome?

Fully developed female external genitalia Signup and view all the answers

Study Notes

Lecture 9: Sex Determination and Gametogenesis

Textbook: Chapter 6 (pages 176-187)

Chapter Overview

Mammalian sex determination:
- How do chromosomes determine testis or ovary formation?
- What gene regulatory networks are activated by the Y and X chromosomes?
- Primary vs. secondary sex determination
- Effects of mutations in key sex determination genes
Drosophila sex determination: A cascade of alternative RNA splicing events dictates sex determination
Mammalian gametogenesis: spermatogenesis + oogenesis

Mechanisms of Sex Determination

Chromosomal sex determination:
- Examples: Mammals, birds, Drosophila
- Mammals: XY chromosomes; XY → testes; XX → ovaries
- Birds: males ZZ; females ZW
- Drosophila: X chromosome number determines sex phenotype, Y chromosome plays no role
Environmental sex determination:
- Example: incubation temperature determines sex

Mammalian Sex Determination

X-bearing sperm + X-bearing egg → female (XX)
Y-bearing sperm + X-bearing egg → male (XY)
How does the Y chromosome promote testis formation?
How do two X chromosomes promote ovary development and egg production?

Mammalian Sex Determination (Continued)

Gonad: reproductive organ producing gametes (reproductive cells)
Mammalian embryos have a bipotential gonad, meaning it can develop into either testes or ovaries
- XX → ovaries
- XY → testes

The Importance of the Y Chromosome in Mammalian Sex Determination

Each sperm has 22 autosomes + either an X or a Y chromosome
Each egg has 22 autosomes + an X chromosome
Errors in meiosis can lead to gametes with abnormal numbers of sex chromosomes

Importance of Chromosomes in Mammalian Sex Determination

Individuals with a single X chromosome (XO) have underdeveloped female external genitalia (Turner syndrome)
A second X is needed for normal ovarian development
The presence of a Y chromosome starts testis development
Klinefelter syndrome (XXY): Individuals have male genitalia, but have lower levels of androgens and higher estrogen levels; infertility commonly present

Mammalian Sex Determination (Summary)

XY or XX chromosomes determine the fate of early gonad cells
Primary sex determination (gonads): testes or ovaries
Secondary sex determination (external): male or female sex characteristics

Genes Involved in Primary Sex Determination

Genes required for differentiation of genital ridge into bipotential gonad
- Sf1, Wt1, Lhx9, Gata4
Mice lacking any of these genes have no gonads

X-Chromosome Inactivation (Dosage Compensation)

Female mammals have two X chromosomes → need to balance gene expression with males one X
Random inactivation of one X chromosome in each cell:
- Inactivation by forming heterochromatin through histone modifications
- Inactive X chromosome becomes a Barr body.

Calico Cats (Illustrative Example)

Calico coat color is determined by genes on the X chromosome
Females can have patches of different coat colors due to X chromosome inactivation
Males only inherit one X → cannot have a mix of colors

Secondary Sex Determination

Gonads secrete hormones that determine secondary sexual characteristics
Two phases: embryo development, puberty - Undifferentiated ducts (Mullerian and Wolffian) present in embryos. - Gonad hormones determine which ducts remain and which are removed
Testosterone in XY animals promotes Wolffian duct development
Sex hormones in XY animals degenerate Mullerian duct.
Estrogen promotes Mullerian duct development and external genitalia in XX animals

Secondary Sex Determination - XY

Leydig cells secrete testosterone (promotes Wolffian ducts and external genitalia)
Sertoli cells secrete AMH (causes degeneration of the Müllerian ducts)
These two pathways are independent

Androgen Insensitivity Syndrome

Individuals with XY chromosomes have a mutation in the androgen receptor
Testes develop normally (sertoli cells secrete AMH) but Leydig cells don’t cause Wolffian ducts to develop.
External female genitalia, lack internal female genitalia and internal testes

DHT

Testosterone must be converted to dihydrotestosterone (DHT) to masculinize structures that derive from the Wolffian duct
DHT is most active during prenatal and early childhood
Important for development of male genitalia

5-alpha Reductase Deficiency

Individuals lack the necessary enzyme to convert testosterone to DHT
At birth, they appear female but at puberty, testosterone production leads to development of male characteristics

Secondary Sex Determination - XX

Estrogen is needed for both males and females and from developing fetal ovaries
Causes differentiation of Mullerian ducts → female reproductive tract and external genitalia in XX animals

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Description

This quiz focuses on Chapter 6 of the textbook, covering key concepts of sex determination and gametogenesis in mammals and Drosophila. Learn about chromosomal and environmental mechanisms that drive sex differentiation, as well as the processes of spermatogenesis and oogenesis. Test your understanding of the genetic and environmental factors involved in these biological processes.