Sex Determination and Development

Gene

DNA sequence on chromosomes that encodes proteins/traits

Allele

A different version of the same gene caused by DNA mutations

Chromosome

long, linear stretch of DNA with many genes

Diploid

A cell with two of each chromosome (from mom and dad)

Haploid

A cell with one of each chromsome, made during meiosis

• gametes

Homologous chromosomes

pairs of the same chromosome type with the same genes

Most body cells are…

diploid

Sex cells (gametes) are

haploid

How many autosomal chromosome pairs do humans have?

22 pairs

How many sex chromosome pairs do humans have?

1 pair

What are sex chromosomes?

Pair of homologous chromosomes that determine sex during development

What is the X chromosome like?

Large with 1000’s of genes

What is the Y chromosome like?

Small, contains the SRY gene

What combination determines male sex?

XY

What combination determines female sex?

XX

What is the SRY gene?

Gene on the Y chromosome that triggers TDF and starts male development

In males, do X and Y pair like homologous chromosomes in meiosis?

Yes, during spermatogenesis

Which parent determines the sex of the baby? Why?

The father

• Only he can provide an X or Y

Is one X chromosome required to live?

Yes

Is one Y chromosome required to live?

No

Nondisjunction

failure of chromosomes to separate during meiosis

What are two examples of nondisjunction syndromes?

• XO = Tuner’s Syndrome (female, sterile, minor symptoms)

• XXY = Klinefelter’s Syndrome (male, sometimes sterile)

How is gene dosage handled in XX females?

One X in each cell is inactivated (forms a Barr Body)

Why do XX females show mosaic expression?

Random X inactivation leads to different cells expressing different alleles

• Ex: normal vs hemophilia

When is the “Sex Indifference Stage”?

Around 6 weeks of embryonic development

What is the “default” developmental state?

XX (female)

Gonads

Strucutres that can become testes or ovaries

Mullerian ducts

Female potential ducts

• Not connected to gonads

Wolffian ducts

Male potential ducts

• Connected to gonads

How does the SRY gene activate male development?

SRY gene produces TDF → turns on genes that forms testes

What is Testes Determining Factor (TDF)?

A protein made by SRY that recruits transcription factors for male genes

What do Sertoli cells do?

Secrete Anti-Mullerian Hormone (AMH) → degrades Müllerian ducts

What do Leydig cells do?

Secrete testosterone & dihydrotestosterone → support male structures and Wolffian ducts

What structures become sex organs based on hormones?

• Testes/Ovaries

• Penis/Clitoris

• Scrotum/ Labia

How does testosterone affect development?

• Present → structures become male organs

• Absent → structures become female organs

When does sexual differentiation occur?

Around 8 weeks