PSY 267 Lecture Notes - Lecture 3: Paramesonephric Duct, Gonad, Zygote
15 Dec 2016
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Lecture: Diversity within Sex
Typical Development of Sex
• Most cells have 46 chromosomes
• The sperm and the egg both have 23 chromosomes each
• At fertilization, the egg (23) + the sperm (23) = zygote (46)
o Egg – X
o Sperm – X/Y
▪ Thus, the father determines the sex of the baby
• The X is bigger than the Y, and the Y is extremely fragile and cannot stand alone
• Two possible outcomes:
o XX = genetic female
o XY = genetic male
• In the first 6 weeks, the building blocks between males and females are identical
o The embryos have the makings of both male and
female reproductive systems
o The default is female
o The only difference is the chromosome pair
• At 6 weeks, differentiation begins in sex gland (gonad)
development
o Males develop testes
• At 3 months, the hormones kick in
o Hormones – differentiation in reproductive system and
sex organs
• Males produce 2 hormones
o Mullerian Regression Hormone
▪ Shrinks the Mullerian ducts, which produces the female internal
reproductive system
o Androgen
▪ Aka testosterone
▪ Encourages the growth of the Wolffian ducts, which produces the male
internal reproductive system
▪ Encourages the growth of the penis (external reproduction)
• Females produce estrogen
o Estrogen is believed to shrink the Wolffian ducts and encourage development of
the Mullerian ducts
o But this could also be due to the lack of testosterone produced
o The Mullerian ducts become the uterus, egg ducts and the vagina
o Sensory nerves that would have been the penis turn into the clitoris
Atypical Development of Sex
• Genotype – genetic composition of an individual (e.g. XX, XY)
• Phenotype – how the genotype is expressed in the
environment (the physical manifestation of the genotype)
• 2-4% of the population is ‘intersexed’
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