Anatomy & Physiology · Chapter 28

Development & Inheritance

Every human life starts as a single cell. It is no bigger than the period at the end of this sentence. Yet folded inside it is a full instruction book for building a person. This chapter follows that cell forward. First comes the exciting meeting of sperm and egg. Next comes a busy first week of cell division, as the new embryo digs into the uterus. Then three germ layers appear. These are sheets of cells that quietly split up the job of making every organ. After that comes the long stretch of fetal growth that ends in birth.

Then we turn the story around. We ask how the instruction book itself is passed on. These are the rules of inheritance. With a simple square grid, they let us predict the odds that a child will have brown eyes or a certain disease. It is the most packed-in wonder in all of body science, and it follows rules you can learn.

By the end of this chapter you can

Describe the steps of fertilization. Start with the sperm crossing the egg's outer coats and end with the joining that forms a zygote. Explain how the egg stops more than one sperm from getting in.
Trace the first week of growth through cleavage, the morula, and the blastocyst. Describe how and where implantation happens.
Explain gastrulation. Name the three primary germ layers and the main tissues each one makes.
Tell the embryonic period apart from the fetal period. Outline the key events of the three trimesters.
Describe how the placenta is built and what it does. List the three stages of labor.
Define allele, genotype, and phenotype. Compare dominant with recessive inheritance.
Use a Punnett square to predict the genotype and phenotype ratios of children from two parents.
Explain why two healthy parents can have a child with a recessive condition.

Section 1Fertilization

Diagram of a sperm passing through the coats of the egg — **Reference.** The steps of fertilization — sperm crossing the corona radiata and zona pellucida to fuse with the oocyte — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A race with one finisher

Fertilization is the joining of a sperm and an oocyte (egg). Together they make a new cell with its own unique set of genes. It usually does not happen in the uterus. Instead it happens high up in the uterine tube, in a wide part called the ampulla. It takes place within about a day of ovulation. That is because the released egg can only be fertilized for about 12 to 24 hours. Sperm live a few days at most.

Hundreds of millions of sperm are released, but the trip is harsh. Most are lost to the acid in the vagina or the mucus of the cervix. Many just take wrong turns. Only a few hundred reach the egg. On the way, the survivors go through capacitation. This is a last chemical change in the female tract. It gets their outer covers ready to join with the egg.

The egg they find is not bare. It is wrapped in a halo of follicle cells called the corona radiata. Under that is a thick shell of sugar-coated protein called the zona pellucida.

Breaking in: the acrosome reaction

To reach the egg's cell surface, a sperm must get through both coats. A cap called the acrosome sits over the sperm's head. It is a sac full of enzymes that digest things. When the sperm touches the zona pellucida, the acrosome bursts and spills its enzymes. This is the acrosome reaction. The enzymes dig a narrow tunnel through the shell.

It takes many sperm chipping away at the corona and zona to get through. So fertilization is a team effort, but it has only one winner. The first sperm to reach the egg's surface grabs onto receptors there. Then the outer covers of the two cells join. The sperm's nucleus, with its 23 chromosomes, slides into the egg.

Barrier	What it is	How the sperm gets through
Corona radiata	Outer halo of granulosa (follicle) cells	Many sperm push and use enzymes to loosen the cells
Zona pellucida	Thick shell of sugar-coated protein around the egg	The acrosome reaction digests a path through it
Oocyte plasma membrane	The egg cell's own outer surface	The sperm grabs receptors; the two surfaces fuse

One sperm only: the blocks to polyspermy

What if two sperm got in? This is called polyspermy. The embryo would have the wrong number of chromosomes, and it would die. So the egg has two safety locks. They snap shut the moment one sperm joins.

The fast block comes first. The electric charge across the egg's surface changes right away. This stops other sperm from grabbing on. The slow block, or cortical reaction, comes seconds later. Tiny sacs just under the egg's surface spill their contents into the zona pellucida. This hardens the zona into a wall no sperm can cross.

Only now does the egg finish its own last division (meiosis II). The two sets of 23 chromosomes, one from each parent, come together. This brings back the full human count of 46. The fertilized egg is now a single diploid cell called the zygote. Its sex was set at this exact moment. It depends on whether the winning sperm carried an X or a Y chromosome.

Fertilization sets the clock and the sex. The zygote is the true start of a new person. It fixes the child's full set of chromosomes and its biological sex. It is also about the moment we count the baby's age from. But doctors count pregnancy from the last menstrual period instead. That date is about two weeks ahead of the true age from fertilization. Keep this in mind when you compare week numbers.

Step through the meeting of sperm and egg in the interactive below.

Interactive Of millions of sperm, only one joins the egg. The egg makes sure it is exactly one. Open full ↑

Key points

Fertilization usually happens in the ampulla of the uterine tube. The egg can be fertilized for only about 12–24 hours.
The sperm crosses the corona radiata and zona pellucida using the acrosome reaction. Then it joins with the egg's outer surface.
A fast block (a change in electric charge) and a slow block (the cortical reaction that hardens the zona) stop polyspermy.
The two nuclei join and bring back 46 chromosomes. This forms a diploid zygote. The sperm sets its sex.

Section 2The first week: cleavage & implantation

Diagram of the early embryo from two cells to a blastocyst — **Reference.** Cleavage from the two-cell stage to the blastocyst, marked by where each step occurs — uterine tube versus uterus — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Cleavage: dividing without growing

Within about a day, the zygote starts to divide again and again by mitosis. These fast divisions are called cleavage. Here is the surprise. The whole ball does not get bigger. Each division splits the same cytoplasm into smaller and smaller cells. So the cluster stays about the size of the first egg. But the number of cells climbs: 2, then 4, then 8, then 16. These early cells are called blastomeres.

All this happens while the tiny embryo drifts down the uterine tube. Tiny hairs called cilia and waves of muscle push it toward the uterus. By about day 4 it is a solid ball of about 16 cells. It looks like a mulberry, so it is called the morula. That is Latin for "little mulberry."

From morula to blastocyst

Around day 5, fluid seeps into the center of the morula. It hollows the ball out into a sphere filled with fluid, called the blastocyst. This is the first time the embryo's cells take on different jobs. They split into two groups you need to know.

The outer shell of cells is the trophoblast. It will become the embryo's part of the placenta and the membranes around the baby. It is meant to be support tissue, not the baby. Pressed against one inner wall is the inner cell mass (embryoblast). This small group of cells will become the embryo itself. The fluid space between them is the blastocoel. In just a few days, the embryo has answered its first big question. Which cells build the child, and which build its life-support system?

Day (about)	Stage	Where
0	Zygote (fertilized egg)	Ampulla of the uterine tube
1–3	Cleavage: 2-, 4-, 8-cell stages	Moving down the uterine tube
4	Morula (~16 cells)	Entering the uterus
5	Blastocyst (trophoblast + inner cell mass)	Floating free in the uterus
6–10	Implantation into the endometrium	Wall of the uterus

Implantation and the pregnancy signal

Implantation is when the embryo digs into the wall of the uterus. First the blastocyst must "hatch." It breaks free of the leftover zona pellucida. Now its sticky trophoblast can touch the lining of the uterus. Around day 6 or 7, the trophoblast sticks to the endometrium, the lining, and starts to burrow in. Its cells release enzymes that eat into the lining. By about day 10 the embryo is fully buried.

Right away, the trophoblast starts making human chorionic gonadotropin (hCG). This hormone tells the corpus luteum in the ovary to keep making progesterone. That way the lining is not shed. This one hormone keeps the pregnancy from ending in a normal period. It is also exactly what a pregnancy test looks for.

The wrong address is dangerous. The embryo is meant to implant in the upper wall of the uterus. Sometimes the blastocyst implants somewhere else, most often in the uterine tube. This is an ectopic pregnancy. The narrow tube cannot stretch as the embryo grows. So it can burst and bleed. That makes an ectopic pregnancy a true emergency. This is why hCG that rises in an unusual way, along with pain, leads doctors to search fast for where the embryo is.

Follow the zygote's first-week journey from division to implantation below.

Interactive The zygote divides again and again as it travels to the uterus. Then it burrows into the lining. Open full ↑

Key points

Cleavage splits the zygote into more and smaller cells, but the cluster does not grow. This makes the morula.
The blastocyst has an outer trophoblast (future placenta) and an inner cell mass (future embryo). A fluid space, the blastocoel, sits between them.
Implantation into the endometrium happens about days 6–10. First the blastocyst must hatch from the zona pellucida.
The trophoblast makes hCG. This hormone keeps the pregnancy going, and pregnancy tests look for it.

Section 3The germ layers

Diagram of cells moving to form the three germ layers — **Reference.** Gastrulation at the primitive streak — epiblast cells migrating to form ectoderm, mesoderm, and endoderm — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Gastrulation: one sheet becomes three

Around the third week, the inner cell mass makes one of the most important moves in all of growth. It is called gastrulation. A groove called the primitive streak forms down the middle of the flat embryo. Cells from the top layer (the epiblast) stream toward it and dive under. They rearrange into three primary germ layers, three sheets of cells.

Your body has more than 200 kinds of cells. Every one of them comes from one of these three sheets. Laying down these layers in the right order and the right place is the base for every organ that follows.

Three layers, three destinies

The three germ layers split up the whole body among them. The outer layer is the ectoderm. It becomes the parts that face the world and sense it. That means the epidermis, the outer skin, and the whole nervous system.

The middle layer is the mesoderm. It becomes the bulk and machinery of the body. That means muscle, bone, blood, heart, blood vessels, and kidneys. The inner layer is the endoderm. It becomes the linings of the body's hollow tubes and the glands that grow off them. That means the lining of the gut and the airways, plus the liver and pancreas.

Here is an easy way to hold it. Ectoderm is the outside and the wiring. Endoderm is the innermost linings. Mesoderm is everything in the middle.

Germ layer	Position	Main things it becomes
Ectoderm	Outer	Epidermis, hair, nails; the whole nervous system; parts of the eye
Mesoderm	Middle	Muscle; bone and cartilage; blood and heart; blood vessels; kidneys; dermis (deep skin)
Endoderm	Inner	Lining of the gut (GI tract) and airways; liver; pancreas

Folding into a body: organogenesis

With the three layers in place, organogenesis begins. This is the actual building of organs. It goes on through the eighth week. The flat disc folds into a shape like a tube inside a tube. Each layer starts shaping the parts it was given.

One of the first and most important steps is when ectoderm folds into the neural tube. This tube will become the brain and spinal cord. If it does not close all the way, the result is a neural tube defect, such as spina bifida. The neural tube closes very early, often before a person even knows they are pregnant. So getting enough folic acid, a B vitamin, around the time of conception greatly lowers the risk.

Nervous system from skin? Yes. Many students are surprised that the brain and the epidermis are cousins. Both come from ectoderm. It makes sense once you think about it. The nervous system's job is to sense and respond to the outside world. So it grows from the very layer that touches that world.

Tap each of the three layers to see what it becomes in the interactive below.

Interactive Around week 3 the embryo forms three layers. Every organ comes from one of them. Open full ↑

Key points

Gastrulation (week 3) forms three primary germ layers at the primitive streak.
Ectoderm → skin and nervous system. Mesoderm → muscle, bone, blood, heart, kidney. Endoderm → linings of the gut and airways, liver, pancreas.
Organogenesis builds the organs through week 8. The neural tube closes early, and closing well depends on enough folic acid.

Section 4Embryo to fetus: the timeline

Diagram of the path from released egg to implanted blastocyst — **Reference.** The pre-embryonic journey — from ovulated oocyte through cleavage to a blastocyst implanting in the uterus — OpenStax, CC BY 3.0 (via Wikimedia Commons)

The embryonic period versus the fetal period

Growth before birth is split into two named periods. The split comes at the end of week 8. From fertilization through week 8 is the embryonic period. In this time the germ layers form and every major organ is first laid down. From week 9 until birth is the fetal period. This time is mostly about growing and finishing organs that already exist.

This matters for health. Organs are being built in the embryonic weeks, so that is when they are easiest to harm. A teratogen is a drug, infection, or other thing that disrupts growth before birth. It does its worst harm in weeks 3 to 8, while body parts are forming. The same exposure later, in the fetal period, may cause little or no damage to body shape.

Three trimesters

A full-term pregnancy lasts about 38 weeks counted from fertilization. Doctors usually count from the last menstrual period (LMP) instead. Counted that way, it lasts about 40 weeks. The pregnancy is split into three roughly equal parts called trimesters.

The first trimester is where the action is. Implantation, gastrulation, and organ building all happen here. The embryo also becomes a fetus. By the end, the fetus looks clearly human, but it is only about the size of a plum.

The second trimester is a time of fast growth and movement. The mother usually feels the first kicks, called "quickening." The fetus grows toward the edge of viability, the point where it could live outside the uterus. That is about 23 to 24 weeks when doctors count from the LMP. It is about 21 to 22 weeks after fertilization. (Always check how weeks are counted. Doctors count from the LMP. Embryology counts from fertilization. The two differ by two weeks.)

The third trimester is mostly about gaining weight and final growth. The lungs and brain mature most of all. This gets the fetus ready to live outside the uterus.

Trimester	Weeks (from fertilization)	Main events
First	1–12	Implantation, germ layers, organ building; the embryo becomes a fetus at week 9
Second	13–27	Fast growth; first movements felt; nears viability (~21–22 weeks after fertilization; ~23–24 weeks counted from the LMP)
Third	28–38	Big weight gain; lungs and brain mature; ready for birth

Timing decides the damage. The riskiest time for major birth defects is weeks 3–8, when organs are being built. This is often before a pregnancy is confirmed. That is why advice about alcohol, medicines, and folic acid focuses on the time around conception, not just after a positive test.

Tap through the stages of the earliest journey in the interactive below.

Interactive Pregnancy runs about 38 weeks from fertilization, split into three trimesters. It goes from building organs to fast growth. Open full ↑

Key points

The embryonic period (weeks 1–8) builds the organs. The fetal period (week 9 to birth) grows and finishes them.
Teratogens do the most harm to body shape in weeks 3–8, when organs are forming.
Pregnancy is about 38 weeks from fertilization, split into three trimesters. Viability starts around 23–24 weeks counted from the LMP (~21–22 weeks after fertilization).

Section 5Pregnancy & birth

Diagram of the placenta and the membranes around the fetus — **Reference.** The placenta and fetal membranes — chorionic villi bathed in maternal blood, with the umbilical vessels — OpenStax, CC BY 3.0 (via Wikimedia Commons)

The placenta: a shared border organ

The placenta is a short-term organ. It is built together from the embryo's tissue (which comes from the trophoblast) and the mother's endometrium. It is the lifeline for the whole pregnancy. Finger-like chorionic villi reach out from the baby's side into pools of the mother's blood. This makes a huge surface for trading materials.

Across that thin barrier, O₂ and food pass from mother to fetus. CO₂ and wastes pass the other way. Yet the two bloodstreams normally never mix. This is key. The placenta also becomes a strong hormone maker. It takes over from the corpus luteum and makes the progesterone and estrogen that keep the pregnancy going. The fetus is tied to the placenta by the umbilical cord. It floats inside the amnion, a sac of fluid that cushions it.

A body remade for pregnancy

Carrying a fetus changes almost every system in the mother's body. Her blood volume rises by about 30–50 percent to supply the placenta. Her heart pumps more blood each minute. The growing uterus crowds the diaphragm and bladder. This changes her breathing and makes her need to pee more often.

Hormones loosen ligaments, including those of the pelvis, to get ready for birth. They also shift her metabolism to put the fetus's fuel first. These are normal changes that work together. But they also explain why pregnancy puts stress on problems like heart disease. It is also why doctors watch blood pressure and blood sugar so closely.

Labor: birth in three stages

Near the end of pregnancy, a shift in the balance of hormones starts labor. Once it begins, a classic positive feedback loop drives it. The baby's head pushes on the cervix. This causes the release of oxytocin. Oxytocin makes the uterus squeeze harder. That pushes the head harder against the cervix, and the loop repeats. The cycle keeps building on itself. It only ends when the baby is born. Labor happens in three stages.

Stage	Name	What happens
1	Dilation	The longest stage. Squeezes of the uterus thin (efface) and open the cervix all the way (~10 cm)
2	Expulsion	Strong squeezes and the mother's pushing move the baby out through the birth canal
3	Placental	The uterus squeezes out the placenta (the "afterbirth"), then clamps down to limit bleeding

The umbilical vein is the odd one out. By rule, arteries carry blood away from the heart. In the fetus, the umbilical vein carries oxygen-rich blood from the placenta to the fetus. The two umbilical arteries carry oxygen-poor blood back to the placenta. This reminds you that "artery" and "vein" describe which way blood flows compared with the heart. They do not describe how much oxygen the blood has.

Step through the placenta and the three stages of birth in the interactive below.

Interactive The placenta keeps the fetus alive for nine months. Then labor delivers the baby in three stages. Open full ↑

Key points

The placenta trades O₂, CO₂, food, and wastes without mixing the mother's and baby's blood. It also makes pregnancy hormones.
Pregnancy brings big changes. Blood volume rises 30–50%, the heart pumps more, and belly organs get pushed aside.
Labor is driven by oxytocin positive feedback. It has three stages: dilation → expulsion → placental.

Section 6Inheritance

A Punnett square for one gene — **Reference.** A Punnett square for a monohybrid cross (Nn × nn), predicting a 50:50 ratio of offspring — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Two copies of every gene

You get one set of chromosomes from each parent. So you carry two copies of nearly every gene, one from your mother and one from your father. The different versions of a gene are called alleles. For a gene with two alleles, a person can be homozygous, with two matching alleles. Or they can be heterozygous, with two different ones.

The pair of alleles a person carries is their genotype. The trait you can see is their phenotype. Examples are eye color, blood type, or whether a disease shows up. All of inheritance is about the link between these two. How does the genotype you cannot see make the phenotype you can?

Dominant and recessive

When the two alleles differ, they do not always count the same. A dominant allele hides the effect of a recessive one. So one copy is enough to show its trait. A recessive allele only shows its trait when both copies are recessive. By custom, the dominant allele gets a capital letter. The recessive one gets a small letter.

Take the example from the interactive. B = brown eyes (dominant) and b = blue (recessive). The genotypes BB and Bb both give brown eyes. Only bb gives blue. This is why a recessive trait can seem to "skip" generations. People with one copy, called carriers, can pass it on quietly for years. It shows up again only when two carriers each pass the recessive allele to the same child.

Genotype	Name	Phenotype (B = brown, b = blue)
BB	Homozygous dominant	Brown eyes
Bb	Heterozygous (carrier)	Brown eyes
bb	Homozygous recessive	Blue eyes

Predicting the odds: the Punnett square

A Punnett square is a simple grid. It predicts the chance of each genotype in the children. You list one parent's possible alleles across the top. You list the other parent's down the side. Then you fill each box by combining them.

Cross two brown-eyed carriers (Bb × Bb). The four boxes come out 1 BB : 2 Bb : 1 bb. That is a 3:1 ratio of brown to blue. So each child has a 25 percent chance of blue eyes, even though both parents have brown eyes. Read these as chances, not promises. A 1-in-4 chance starts fresh with every pregnancy, just like flipping a coin. Four children could all have brown eyes, or two could have blue.

Not every trait follows this neat dominant and recessive pattern. Some alleles are codominant, meaning both show, as in type AB blood. Genes on the X chromosome make special sex-linked patterns. But the two-allele Punnett square is the base that everything else builds on.

Two healthy parents, one affected child. This classic puzzle of genetics is solved by carriers. Say two people each carry one recessive disease allele. Both are healthy heterozygotes. In every pregnancy, they have a 1-in-4 chance of a child who gets two recessive copies and has the disease. This pattern is behind many inherited diseases, such as cystic fibrosis and sickle cell disease.

Set the two parents and watch the ratios change in the interactive below.

Interactive Each parent passes on one allele. A Punnett square predicts the odds for their children. Open full ↑

Key points

You carry two alleles of most genes. Your genotype is the allele pair. Your phenotype is the trait you can see.
A dominant allele shows with one copy. A recessive allele shows only when both copies are recessive.
A Punnett square predicts the ratios in children (for example, Bb × Bb → 3:1). The results are chances that start fresh with each pregnancy.
Two heterozygous carriers can have an affected child. This explains recessive diseases that skip generations.

The chapter in one breath

Fertilization in the uterine tube joins sperm and egg into a diploid zygote. Fast and slow blocks stop polyspermy. The sperm sets the child's sex.
Cleavage divides the zygote into a morula and then a blastocyst (trophoblast + inner cell mass). The blastocyst implants in the endometrium and makes hCG.
Gastrulation forms three germ layers. Ectoderm makes skin and nerves. Mesoderm makes muscle, bone, blood, and kidney. Endoderm makes the linings of the gut and airways, the liver, and the pancreas.
The embryonic period (weeks 1–8) builds organs and is most at risk from teratogens. The fetal period grows them. Pregnancy has three trimesters (~38 weeks in all).
The placenta trades gases and food without mixing blood, and it makes pregnancy hormones. Labor is oxytocin positive feedback in three stages: dilation, expulsion, placental.
The genotype (allele pair) makes the phenotype (the trait you see). Dominant alleles hide recessive ones.
A Punnett square predicts the chances for children. Two carriers can have a child with a recessive condition.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back when a word trips you up.

Allele: One of the different versions of a gene. You carry two alleles of most genes, one from each parent.
Blastocyst: The early hollow ball of cells, filled with fluid, that implants in the uterus. It has an outer trophoblast and an inner cell mass.
Cleavage: The fast cell divisions after fertilization. They make more cells, but the embryo as a whole does not get bigger.
Dominant allele: An allele that shows its trait with just one copy. It hides any recessive allele. It is written with a capital letter.
Ectoderm: The outer germ layer. It makes the epidermis and the whole nervous system.
Endoderm: The inner germ layer. It makes the linings of the gut and airways, plus the liver and pancreas.
Fertilization: The joining of a sperm and an oocyte to form a diploid zygote. It usually happens in the ampulla of the uterine tube.
Gastrulation: The reshaping of the embryo around week 3. It forms the three primary germ layers at the primitive streak.
Genotype: The exact pair of alleles a person carries for a gene.
Germ layer: One of the three main sheets of cells in the embryo: ectoderm, mesoderm, and endoderm. All tissues grow from them.
Heterozygous: Having two different alleles of a gene (for example, Bb).
Homozygous: Having two matching alleles of a gene (for example, BB or bb).
Human chorionic gonadotropin (hCG): The hormone made by the trophoblast. It keeps the corpus luteum, and so the pregnancy, going. Pregnancy tests look for it.
Implantation: When the blastocyst buries itself in the endometrium, the lining of the uterus. It happens about days 6–10 after fertilization.
Mesoderm: The middle germ layer. It makes muscle, bone, blood, the heart and blood vessels, and the kidneys.
Morula: The solid ball of about 16 cells made by cleavage. It looks like a mulberry and comes before the blastocyst.
Phenotype: The trait you can see that comes from a genotype. The environment can also shape it.
Placenta: The short-term organ built from the baby's and the mother's tissue. It trades gases, food, and wastes and makes pregnancy hormones.
Punnett square: A grid that predicts the chance of each genotype in the children of two parents.
Recessive allele: An allele whose trait shows only when both copies are recessive. It is written with a small letter.
Trophoblast: The outer layer of cells of the blastocyst. It forms the embryo's part of the placenta and makes hCG.
Zygote: The single diploid cell formed at fertilization. It carries the full set of 46 chromosomes.

Check yourself

1. Where in the female body does fertilization usually happen? About how long can the released egg be fertilized?

In the ampulla of the uterine tube. The egg can be fertilized for about 12–24 hours. Sperm must reach the egg high in the tube within about a day of ovulation. That is why the timing is so tight.

2. What is the acrosome reaction? Which two coats of the egg must a sperm cross before the cells join?

The acrosome reaction is when the sperm's acrosome releases enzymes that dig through the egg's coats. The coats are the corona radiata and the zona pellucida. Only after crossing both can a sperm reach and join with the egg's outer surface.

3. Name the two blocks to polyspermy. Why does it matter to block a second sperm?

The fast block (a change in electric charge on the egg's surface) and the slow block (the cortical reaction, which hardens the zona pellucida). A second sperm would give the embryo the wrong number of chromosomes, and it would die. So the egg shuts out all but one.

4. During cleavage, the number of cells rises from 2 to 4 to 8. Yet the cluster does not get bigger. Why?

Cleavage splits the same cytoplasm into smaller cells. The cells do not grow between divisions. Each blastomere is smaller than the last. So the whole ball stays about the size of the first egg.

5. In the blastocyst, what do the trophoblast and the inner cell mass each become?

The trophoblast becomes the embryo's part of the placenta and membranes. The inner cell mass becomes the embryo itself. This is the first time the embryo's cells take on different jobs.

6. Which hormone does the implanting trophoblast make? What does it do, and why is it useful to doctors?

Human chorionic gonadotropin (hCG). It keeps the corpus luteum making progesterone, so the lining is not shed. It is also what pregnancy tests look for. Rising hCG is the earliest chemical sign that a pregnancy has started.

7. List the three primary germ layers. Give one main thing each one becomes.

Ectoderm → skin/nervous system; mesoderm → muscle/bone/blood/heart/kidney; endoderm → linings of the gut and airways, liver, pancreas. Every tissue in the body comes from one of these three layers, formed at gastrulation.

8. Why is a teratogen most dangerous in weeks 3 to 8? How does this shape advice before and during pregnancy?

That is the embryonic period, when organs are being formed and are easiest to damage. This window can pass before a pregnancy is confirmed. So advice on alcohol, medicines, and folic acid focuses on the time around conception.

9. Across the placenta, what passes from mother to fetus, and what passes the other way? Do the two bloodstreams mix?

O₂ and food pass to the fetus. CO₂ and wastes pass to the mother. The two bloodstreams normally do not mix. The trading happens across the thin wall of the chorionic villi, which sit in the mother's blood.

10. Name the three stages of labor in order. What positive feedback loop drives labor?

Dilation → expulsion → placental. The loop is stretch of the cervix → oxytocin → stronger squeezes → more stretch. The oxytocin cycle keeps building until the birth ends it.

11. Use B (brown, dominant) and b (blue, recessive). Give the phenotype for each genotype: BB, Bb, bb.

BB = brown; Bb = brown; bb = blue. One dominant allele is enough for brown eyes. So blue shows up only when both alleles are recessive (bb).

12. Two brown-eyed parents have a blue-eyed child. Explain how. What is the chance of a blue-eyed child in each pregnancy?

Both parents are heterozygous carriers (Bb). A Bb × Bb cross gives a 3:1 ratio of brown to blue. So each child has a 25% chance of blue eyes (bb). Each parent carried the recessive allele quietly. The chance starts fresh with every pregnancy.

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