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Development & Sex Determination
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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
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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
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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
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Male Reproductive System
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Spermatogenesis
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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
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Female Reproductive System
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Oogenesis
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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
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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
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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
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Implantation
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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!
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Embryo
The embryo undergoes mitosis many times to form all the starter tissues of the body early
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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
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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?
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Teratogens
Alcohol Cigarettes X-rays Viruses
Rubella, HIV, Herpes Parasites
Toxoplasma gondii
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Development
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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
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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)
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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
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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!
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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!
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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
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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
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See you tomorrow!