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Development & Sex Determination
Learning Objectives
By the end of this class you should
understand:

The key components of the male and female reproductive systems

The general processes of fertilization, gestation, and birth

The effects of teratogens and two commonly used teratogens

The process of sex determination in humans

Reasons a human's chromosomal sex may not match their physical sex

Effects of X inactivation
Reproduction



The process of
reproduction involves a
male gamete (sperm)
fertilizing a female
gamete (egg)
This chapter will cover
this process in more detail
Will also cover the
development of the
resulting zygote to a baby
Male Reproductive System

The male reproductive
system is designed to
constantly produce large
amounts of sperm

Sperm are the gametes
produced in the testes

Semen is the fluid that
contains sperm as well as
sugars, mucus, and alkaline
solution

Semen is expelled
(ejaculation) through the penis
Male Reproductive System
Spermatogenesis
Female Reproductive System

Female system hinges on
ovaries releasing one egg
per month into the oviduct

AKA fallopian tube

This is the site of
fertilization

If fertilized, zygote enters
uterus and implants

If unfertilized, ultimately
ejected along with uterine
lining during menstruation
Female Reproductive System
Oogenesis
Gamete Formation


Production of sperm is called spermatogenesis and
proceeds constantly in the seminiferous tubules of the
testes
Production of eggs is called oogenesis, where meiosis is
initiated in the ovaries when the female is still a fetus, but
does not complete until fertilization
Fertilization



When sperm enter the
vagina, they cross the
cervix and uterus and
enter both oviducts
If they encounter an
egg they will initiate
fertilization
Multiple sperm aid in
breaching the egg
barrier but only one
sperm nucleus enters
Embryo Formation

The instant the sperm and
egg fuse they cease to be
gametes and are termed a
zygote


You were a zygote once!
The zygote is nourished by
the fluid of the uterus and
eventually implants in the
uterine wall

Becomes an embryo once it
begins mitosis
Implantation
Placenta

The uterine wall creates the placenta, which
provides nutrients to the embryo, including oxygen

Fetuses do not need to breathe since the mother is
delivering oxygen through the placenta to the baby!
Embryo
The embryo
undergoes
mitosis
many
times to
form all the
starter
tissues of
the body
early
Fetal Development


The embryo is redefined as a fetus at week 9

End of first trimester

Organs have already formed

Sex is still undefined
Sex determination is initiated during second
trimester

If a Y chromosome is present, the SRY gene
activates and causes the gonads to become testes

If no Y chromosome is present, the gonads default
to become ovaries
What Can Go Wrong?




Exposure to certain chemicals
or other conditions may alter
and/or damage fetus
development
These environmental factors
are called teratogens
A wide variety are known and
have different deleterious
effects on the fetus
What are some well-known
teratogens?
Teratogens

Alcohol

Cigarettes

X-rays

Viruses


Rubella, HIV, Herpes
Parasites

Toxoplasma gondii
Development
Sex Determination

As previously mentioned, the
SRY gene on the Y
chromosome causes the
gonads to become testes


Any fetus with a Y
chromosome will become
male
Initiates production of male
hormones called androgens

Testosterone is the most
famous
Sexual Development



The analogous body parts
develop under the influence
of SRY or lack thereof
A rare mutation causes the
body to fail to detect the
SRY signal and the fetus
will become female despite
being XY
This is called Complete
Androgen Insensitivity
(CAIS)
Dosage Compensation


Many critical genes are on the X chromosome
including Factor XIII and Retinal red cones
Women have two X chromosomes yet they do
not express more than men


Even women with three or more X chromosomes
still have the same level of expression as a woman
with Turner Syndrome (monosomy X)
The level of X expression being equal is called
dosage compensation
Mechanism of Dosage
Compensation


The nucleus of women's cells
contains a strange dense
structure called a Barr body
Barr bodies are inactivated X
chromosomes and men do not
have one


Exception: men with Klinefelter's
Syndrome!
Women with three X
chromosomes have two Barr
bodies per cell!
Results of X Inactivation


At the embryonic stage,
one of the X
chromosomes in each
stem cell inactivates
Typically it is random
which one is inactivated

In cats, the X
chromosome has a
color gene, so female
cats can be a swirl of
colors but male cats
cannot!
X Inactivation

Since it is random
which X chromosome
inactivates, women
may sometimes
express X-linked
recessive disorders

Example:
colorblindness if the
cells that form the eye
all inactivated their
nonfaulty X
chromosome
X-linked Recessive Expression

In this pedigree,
identical female
twins have
discordant
expression of
colorblindness due
to X inactivation in
the eyes of one girl

Note this is NOT
an example of
multifactorial
expression
See you tomorrow!