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COGNITIVE SCIENCE 17
Why Sex is Necessary
Part 1 Jaime A. Pineda, Ph.D.
Why is Sex Necessary?
• Sexually dimorphic behaviors– Anatomical,
physiological, and behavioral differences between males and females of the same species.
• Courting
• Mating
• Parental behavior
• Aggressive behavior
Without these behaviors most species would not survive
Is Sexual Identity Learned?• The case of Bruce who
became Brenda who became David
– Socialization over biology?
• XX (female) and XY (male) genes– A person’s genetic sex is
determined by the father– SRY gene on short arm of Y
chromosome- dictates the development of the fetal testis (testis-determining factor) and triggers action of receptors to receive chemical message hormone brings
Is Sexual Identity Learned?• The Y chromosome controls
the development of the glands that produce the male sex hormones
• The default sex (if the Y chromosome isn’t present or doesn’t work) is female
Initially, there are more males than females
Sexual Development
• Three categories of sex organs (primary sex characteristics)
– Gonads (testes or ovaries)• Develop first (Sry testes;
otherwise ovaries)• Produce ova/sperm and
secret hormones• Critical Period of sexual
development: Gestational weeks 7-12
– Internal sex organs– External sex organs
Sexual Development
Gonads can become either Testes or Ovaries
Develop as a function of the presence or absence of SRY gene
In males, at 7th week, outer portions of gonads degenerate and inner portions develop into testes
In females, at 11th week, inner portions of gonads degenerate and outer portions develop into ovaries
Sexual Development
Gonads become testes in males
Testes - endocrine glands that produce
• male hormones (androgens)
• Mullerian Inhibiting Substance
Most changes reflect the presence or absence of androgen
Sexual Development
Gonads become ovaries in females
Ovaries- endocrine glands that produce
• female hormones (e.g.estrogen)
Sexual Development
Internal sex organ precursors• Mullerian system female
– Develops into fimbriae, fallopian tubes, uterus, and vagina
• Wolffian system male– Develops into epididymis, vas deferens, seminal vesicles
• Develop as a function of hormones released by testes
– Mullerian inhibiting hormone or anti Mullerian hormone(defeminizing effect)
– Androgens (masculinizing effect): testosterone/dihydrotestosterone
Sexual Development
Wolffian Ducts - later become
• epididymis• vas deferens• seminal vesicles
Mullerian Ducts - later become
• uterus• fallopian tubes• upper part of vagina
Sexual Development
• External sex organs
– Penis and scrotum in males– Labia, clitoris, and outer vagina in females
– Female development is the default • “Nature’s impulse is to create a female”
– Male development requires androgens
Undifferentiated External Genitalia Undifferentiated External Genitalia (<7 weeks)(<7 weeks)
Undifferentiated External Genitalia Undifferentiated External Genitalia (<7 weeks)(<7 weeks)
Partially differentiated external Partially differentiated external male & female genitaliamale & female genitalia
Partially differentiated external Partially differentiated external male & female genitaliamale & female genitalia
Fully developed male & female external genitalia
Fully developed male & female external genitalia
Fetal External Genitals: FemaleFetal External Genitals: Female
11-12 weeks 19 weeks
Fetal External Genitals: FemaleFetal External Genitals: Female
5 months
Fetal External Genitals: MaleFetal External Genitals: Male
15 weeks 16 weeks
Sexual Development Dysfunctions
• Androgen insensitivity syndrome– Genetic mutation that prevents the formation of
androgen receptors• Gonads become testes (normal process)• Defeminization (normal process)• Lack of masculinization (abnormal process)
An XY male with female external genitalia;
have a woman’s body but not internal female sex organs
Sexual Development Dysfunctions
• Persistent Mullerian duct syndrome– Failure to produce anti-Mullerian hormone– Absence of receptors for this hormone
• In an XY male, defeminization does not occur but masculanization does
• Person is born with both sets of internal sex organs
• Turner’s syndrome– Individuals have only one sex hormone: an X
chromosome• Essentially develop into females (normal internal/external sex
organs)• However, no ovaries since two XX chromosomes needed
PUBERTY - development of secondary sex characteristics
• Puberty occurs when hypothalamus begins to secrete gonadotropin-releasing hormones (GnRH) causing pituitary to release
– Follicle-stimulating hormone (FSH)– Luteinizing hormone (LH)
In males, these hormones stimulate testes to produce sperms and secrete testosterone (androgens)
In females, they stimulate the ovaries to produce estradiol (estrogens)
Puberty – cont.
• Nutrition affects age of puberty– Reduced in developing countries– Thin girls reach puberty later
• Due to the presence of leptin
HypothalamusHypothalamus
Anterior Pituitary
FSH-RF
LH-RF
HormonalChanges in Puberty
HypothalamusHypothalamus
OVARIES TESTES
Anterior Pituitary
FSH-RF
LH-RF
FSH FSHLH ICSH(LH)
HormonalChanges inPuberty
HypothalamusHypothalamusHypothalamusHypothalamus
OVARIES TESTES
Anterior Pituitary
FSH-RF
LH-RF
FSH FSHLH ICSH*(LH)
OvulationCorpus Luteum
Progesterone
OvumGrowth
Estrogen (estradiol)
Testosterone
Spermatogenesis
HormonalChanges inPuberty
* Interstitial-cell stimulating hormone
Sexual Maturation
• Secondary sex characteristics (onset of puberty)– Females (estradiol)
• Enlarged breasts• Growth of the lining of the uterus• Widened hips• Maturation of genitalia
– Females (androgens)• Underarm and pubic hair (face as well)
– Males (androgens)• Facial, underarm, and pubic hair • Deep voice• Alter hairline (baldness)• Muscle development• Maturation of genitalia
– Males (estradiol)• Enlarged breasts
Hormonal Control of Reproductive Cycle
• Menstrual cycle– Begins with secretion of FSH to stimulate growth of ovarian
follicles (epithelial cells surrounding each ovum)– As ovarian follicles mature they secrete estradiol causing the
growth of the lining of the uterus (preparation for fertilization)– Increasing levels of estradiol triggers the release of LH causing
ovulation (release of ovum)– Ovum enters a Fallopian tube and starts migrating towards
uterus.– If it meets sperm and becomes fertilized it begins to divide and
then attaches itself to uterus wall– If it is not fertilized, the ruptured ovarian follicle (corpus luteum)
and the lining of the uterine wall will be expelled – menstruation commences