Human Fetal Development

Chicken Pig Turtle Human

Sperm and Egg After sexual intercourse, the sperm unites with the egg.

Only one sperm After one sperm fuses with the egg, it develops a protective coating so that other sperm cannot get inside. Since normally only one egg is present at a time, this means that only one zygote (2n) can begin to develop.

Uterus After fertilization, the newly formed zygote travels to the uterus and implants itself in the uterus wall and begins to develop.

Fetus/Placenta/Umbilical Cord The zygote develops into a fetus in the uterus. The walls of the uterus thicken and become a temporary organ called the placenta. The umbilical cord connects the fetus to the placenta to provide oxygen and nutrition for the fetus.

The Navel After birth, the baby is still connected to the placenta by the umbilical cord. The doctor will cut the cord. Eventually, the remainder of the cord falls off, leaving a scar that becomes the navel or belly button.

Twins 1.Fraternal 2.Identical 3.Co-joined or “Siamese”

Central Questions 1.How do fraternal twins occurs? 2.How do identical twins occur? 3.How do “Siamese” twins occur? 4.How similar are identical/fraternal/cojoined twins genetically? 5.How common is each type of twin?

The normal situation Normally, the woman’s ovaries are coordinated so that the ovaries will only release one egg per month. Therefore, at any intercourse, there is only one egg that is available to be fertilized. If that egg is fertilized, the woman’s body stops releasing eggs until that baby is born. This is why generally, one baby is born at a time and you cannot have siblings who are closer than 9 months apart.

Fraternal Twins Fraternal or dizygotic twins means that two eggs were released by the ovaries at the same time and both were fertilized by two different sperm. They are called dizygotic because they form from two separate zygotes (fertilized eggs). This occurs in around 2.4% of births in the USA. Rivka Yosef

Fraternal Twins These both implant in the uterus in separate places and formed two separate placentas. These twins are no more genetically related than any other siblings. They are basically siblings born at the same time.

Identical twins Identical twins, or monozygotic twins occur when a single zygote splits into two separate organisms. (They are called monozygotic because they form from a single zygote). Normally, the zygote will perform mitosis and remain as a single organism but in rare circumstances the zygote will split into two genetically identical fetuses that form two distinct organisms. This occurs in roughly.8% of births.

Identical Twins These both implant in the uterus in the same place and form a single placenta. Identical twins have exactly the same DNA (since they were formed from the same egg and sperm).

Comparison of Fraternal and Identical Twins

Thinking Question- HW Can you have fraternal twins that are a boy and a girl? If so, how? If not, why not? Can you have identical twins that are a boy and a girl? If so, how? If not, why not?

Siamese Twins Siamese or Cojoined twins are twins that are born connected to each other. This occurs through an incomplete splitting of the zygote, resulting in two fetuses forming that are connected to each other. This is extraordinarily rare and is estimated to occur in roughly 1 out of every 100,000 births. The overwhelming majority of cojoined twins do not both survive, depending on how much of their bodies they share.

What is the origin of the term “Siamese twins”?

The origin of the term "Siamese twins" is Chang and Eng Bunker, famous conjoined twins from who earned their living in the U.S. as a circus attraction in the Barnum and Baily Circus. Upon termination of their contract with their discoverer, they settled in the United States, where they married two sisters and had nearly two dozen children. They were successful businessman and farmers in Wilkes County, North Carolina, where they lived until They were 63 years old when they died.

Abigail and Brittany Hensel

Review: Development How many sperm can enter an egg? Where does the zygote develop into a fetus? How does the fetus get nutrients and oxygen? The umbilical cord connects the baby to a particular organ in the mother’s body called the…? How does the belly button form? Did Adam and Chava have a belly button? One The uterus Through the umbilical cord The placenta It is the scar from where the umbilical cord used to be ????

Twins: Review Is it possible to have: Male-Male fraternal twins? Male-Female fraternal twins? Female-Female fraternal twins? Male-Male identical twins? Male-Female identical twins? Female-Female identical twins? Male-Male cojoined twins? Male-Female cojoined twins? Female-Female cojoined twins? Yes No

Review: Twins In normal births, how many eggs are available to be fertilized at once? A dizygotic twin is the same as a… Fraternal twins come from the same egg but two different sperm. True or false? How is it possible to have male-female fraternal twins? How do identical twins form? How do cojoined twins form? What is the most common type of twin? One Fraternal False. They are formed from two sperm fertilizing two separate eggs. If one egg is fertilized by a sperm containing an X- chromosome and the other is fertilized by a sperm containing a y chromosome A single zygote splits completely A single zygote splits incompletely Fraternal

Short quiz Friday on development Will give back and discuss final on Monday