Lecture 26: Sex Determination Flashcards

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Basic Genetics Terms

Q: What is a gene?

A: A DNA sequence on a chromosome that encodes proteins or traits.

Q: What is an allele?

A: A version of a gene created by DNA mutations.

Q: What is a chromosome?

A: A long linear DNA molecule that contains many genes.

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Ploidy and Chromosomes

Q: What does it mean to be diploid?

A: A cell with two of each chromosome type (from both parents).

Q: What does it mean to be haploid?

A: A cell with one of each chromosome type (as in gametes).

Q: What are homologous chromosomes?

A: Chromosome pairs with the same genes, one from each parent.

Q: Are most human cells diploid?

A: Yes.

Q: Are gametes (sex cells) haploid?

A: Yes.

Q: How many autosomal chromosome pairs do humans have?

A: 22 pairs.

Q: How many sex chromosome pairs do humans have?

A: 1 pair.

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Sex Chromosomes & Sex Determination

Q: What are sex chromosomes?

A: A chromosome pair that determines biological sex.

Q: Which sex chromosomes indicate male?

A: XY.

Q: Which sex chromosomes indicate female?

A: XX.

Q: Which parent determines the offspring’s sex?

A: The father (sperm can carry X or Y).

Q: Do you need at least one X chromosome to live?

A: Yes.

Q: Do you need a Y chromosome to live?

A: No.

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SRY Gene and Male Development

Q: What is the SRY gene and where is it located?

A: A gene on the Y chromosome; it triggers male development.

Q: What does the SRY gene produce?

A: TDF (Testes Determining Factor), a DNA-binding protein.

Q: What does TDF do?

A: Activates male development genes; causes gonads to become testes.

Q: How do Sertoli and Leydig cells influence development?

A:

• Sertoli cells secrete anti-Müllerian hormone (destroys female ducts).

• Leydig cells secrete testosterone and DHT (form male structures).

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Meiosis and Disorders

Q: Do X and Y chromosomes behave like homologous chromosomes in male meiosis?

A: Yes.

Q: What is nondisjunction?

A: Failure of chromosomes to separate during meiosis.

Q: What disorders result from nondisjunction?

A:

• Klinefelter’s Syndrome (XXY): Male with some female traits, often sterile.

• Turner’s Syndrome (XO): Female, sterile, with mild developmental issues.

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X Inactivation & Gene Dosage

Q: How is gene dosage handled in XX females?

A: One X becomes a Barr body (inactivated).

Q: What is a mosaic expression pattern in females?

A: Random X inactivation causes different cells to express different X-linked traits.

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Embryonic Development & Gonadal Differentiation

Q: When is the “sex indifference stage”?

A: Around 5 weeks of embryonic development.

Q: What is the default developmental pathway?

A: Female (XX).

Q: What are gonads?

A: Undifferentiated tissue that can develop into testes or ovaries.

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Duct Systems and Differentiation

Q: What are Müllerian ducts?

A: Potential female ducts; develop if no SRY/AMH is present.

Q: What are Wolffian ducts?

A: Potential male ducts; need testosterone to survive and develop.

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Hormones & Structure Formation

Q: What hormones do Sertoli cells secrete and what’s their function?

A: Anti-Müllerian hormone → degrades Müllerian ducts.

Q: What hormones do Leydig cells secrete and what’s their function?

A: Testosterone → promotes Wolffian ducts; DHT → develops external genitalia.

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Sex Organ Development from Shared Structures

Q: What does the genital tubercle become?

A: Penis (male) or clitoris (female).

Q: What do urethral folds become?

A:

• Male: Fuse to form spongy urethra

• Female: Form labia minora

Q: What does the urethral groove become?

A: The vestibule in females.

Q: What do labioscrotal swellings become?

A:

• Male: Scrotum

• Female: Labia majora

Q: What are examples of homologous sex structures?

A:

• Testes / Ovaries

• Penis / Clitoris

• Scrotum / Labia majora