sex

differences between males & females

– Anatomy (& its development)– Physiology (e.g., hormones)– Behavior (sexually dimorphic behavior)

Anatomy: A hermaphrodite is an organism that posses both male and female genitalia. Most common in invertebrates

– Simultaneous (slugs, earthworms)

– Sequential (clown fish)

'reclining hermaphrodite', a 1st-century BC sculpture

Anatomy: Sex determinationEvery grandma wants to know: “Will it be a boy or a girl?”

What determines the sex of the baby varies across species

• Environmental– How hot it is (turtles, alligators)

• Social– Is there a mom here? (clown fish)

• Genetic – In humans & other mammals:

Sex determination systems

XX XY

Gamets (sperm & egg) are made by the gonads (testis, ovary)

A sperm fertilizing an egg (ovum)

Prenatal Hormones

Chromosomes

Gonads (testes)

Males

Internal genitalia

Chromosomal sex

Genital sex

Hormonal sex

Gonadal sex

Brain structures Brain sex

Y chromosome: Sry

External genitalia

Gonads: testis, ovary

produce gametes

(sperm, ovum)

secrete hormones(testosterone; estrogen,

progesterone).

Conception: Chromosomal sex is determined (XX, XY)

7-8 weeks embryo:

Undifferentiated gonads can develop into phenotypic male or female gonads

Sry (a gene in Y chromosome): leads to male gonad development (testes)

Hormones secreted by testes lead to development ofMale internal and external genitalia

Internal Genitalia:Internal Genitalia

- Fallopian Tubes- Uterus- Inner Vagina

- Epididymis, vas deferens, - seminal vesicles, - prostate

External Genitalia:

- Clitoris,- Labia,- Outer vagina:

- Penis- Scrotum

Testosterone

Development of external genitalia in males

16-week old fetus

DHT(deihydrotestosterone)

5 alpha-reductase

Development of penis and scrotum

External Genitalia:

- Clitoris,- Labia,- Outer vagina:

- Penis- Scrotum

Sexual differentiation of the brain

TESTOSTERONE

ESTROGEN

aromatase

Masculinization of the brain

• Sensitivity to Test.osterone

• Spinal cord centers• Hypothalamic

differentiation• Defeminization

MALE

Sexual differentiation of the brain

TESTOSTERONE

ESTROGEN

aromatase

Masculinization of the brain

• Sensitivity to Test.• Spinal cord centers• Hypothalamic

differentiation• Defeminization

Circulating maternal estrogen? Attached to alpha-fetoprotein, does not get into the fetus brain

FEMALE

Prenatal Hormones

Chromosomes

Gonads (testes)

Androgen Insensitivity Syndrome

Internal genitalia

male

FEMALE

male

male

Brain structures

Y chromosome: Sry

External genitalia

No Receptors for Androgens

Prenatal Hormones

Chromosomes

Gonads (ovaries)

Adrenogenital Syndrome

Internal genitalia

female

MALE

male

female

Brain structuresExternal

genitalia

Androgen Hormone

Sex chromosomes abnormalities

Turner syndrome (X0) Klinefelter syndrome (XXY)