sex
differences between males & females
– Anatomy (& its development)
– Physiology (e.g., hormones)
– Behavior (sexually dimorphic behavior)
'reclining hermaphrodite', a 1st-century BC sculpture
Anatomy: A hermaphrodite is an organism that posses both male
and female genitalia. Most common in invertebrates
– Simultaneous (slugs, earthworms)
– Sequential (clown fish)
Anatomy: Sex determination
Every 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)
Males
Chromosomes
Chromosomal sex
Y chromosome: Sry
Gonads (testes)
Gonadal sex
Prenatal Hormones
Internal
genitalia
Genital sex
Brain
structures
External
genitalia
Hormonal sex
Brain sex
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 of
Male internal and external genitalia
- Fallopian Tubes
- Uterus
- Inner Vagina
- Epididymis, vas deferens,
- seminal vesicles,
- prostate
- Clitoris,
- Labia,
- Outer vagina:
Internal Genitalia
Internal Genitalia:
- Penis
- Scrotum
External Genitalia:
Development of external genitalia in males
Testosterone
5 alpha-reductase
DHT
(deihydrotestosterone)
Development of penis and
scrotum
16-week
old fetus
- Clitoris,
- Labia,
- Outer vagina:
- Penis
- Scrotum
External Genitalia:
Sexual differentiation of the brain
TESTOSTERONE
aromatase
ESTROGEN
Masculinization
of the brain
• Sensitivity to
Test.osterone
• Spinal cord centers
• Hypothalamic
differentiation
• Defeminization
MALE
Sexual differentiation of the brain
TESTOSTERONE
aromatase
ESTROGEN
Masculinization
of the brain
• Sensitivity to Test.
• Spinal cord centers
• Hypothalamic
differentiation
• Defeminization
FEMALE
Circulating maternal estrogen? Attached to alpha-fetoprotein, does
not get into the fetus brain
Androgen Insensitivity Syndrome
Chromosomes
male
Y chromosome: Sry
Gonads (testes)
male
Prenatal Hormones
No Receptors for
Androgens
Internal
genitalia
External
genitalia
Brain
structures
male
FEMALE
Adrenogenital Syndrome
Chromosomes
female
Gonads (ovaries)
Prenatal Hormones
female
male
Androgen
Hormone
Internal
genitalia
External
genitalia
Brain
structures
MALE
Sex chromosomes abnormalities
Turner syndrome (X0)
Klinefelter syndrome (XXY