Fertilization and Development: Why Offspring Differ from Both Parents
How a sperm and egg fuse, how the zygote develops through the embryonic and fetal stages, and why a child is not genetically identical to either parent.
🎯 By the end of this lesson
- Students should be able to describe the steps of fertilization and state the chromosome numbers involved.
- Students should be able to explain how the egg prevents more than one sperm from entering.
- Students should be able to explain, using meiosis and fertilization, why offspring are not genetically identical to either parent.
- Students should be able to distinguish identical from fraternal twins.
- Students should be able to describe the first week of development from zygote to implantation.
- Students should be able to state what the embryonic and fetal stages are and give examples of development in each.
- Students should be able to describe the function of the placenta.
- Students should be able to calculate cell numbers during cleavage.
1From two cells to one
For all the effort that goes into making gametes, fertilization itself happens at a microscopic scale and in a tiny space. A single sperm and a single egg fuse, and from that moment a new, genetically distinct individual exists. This lesson follows the first steps: the meeting of the gametes, the first week of development, and the growth from embryo to fetus.
The BC curriculum states the key outcome of human sexual reproduction: because humans have two parents, offspring are not genetically identical to either parent. Understanding why requires putting together meiosis (which shuffles each parent's genes into the gametes) and fertilization (which combines two gametes at random).
2Fertilization: a journey with obstacles
Hundreds of millions of sperm are released during ejaculation, but acidic vaginal fluid, cervical mucus and immune cells in the uterus reduce them to a few thousand that reach the uterine tubes. Sperm that arrive must still pass a series of tests.
- Capacitation. Fluids in the female reproductive tract prime the sperm. They swim more strongly, and the membrane over the head becomes thinner. Sperm that reach an egg before this priming cannot get through its outer layers.
- The outer layers. The egg is surrounded by a thick, clear layer called the zona pellucida, plus supporting cells. When a sperm binds to the zona pellucida, its acrosome releases enzymes (the acrosomal reaction) that dissolve a path through. Many sperm release their enzymes, which together help to break down the cells around the egg.
- One sperm enters. When a sperm fuses with the egg, a fast change in the egg's membrane begins within about a minute, which stops other sperm from fusing. Then calcium triggers the cortical reaction, a slower block: granules release substances that detach extra sperm and form a protective fertilization membrane. Together these are the blocks to polyspermy.
- Finishing meiosis. Entry of the sperm triggers the egg to complete meiosis II. The extra set of chromosomes is expelled as a polar body.
- Fusion of nuclei. The egg nucleus and sperm nucleus (each now called a pronucleus) copy their DNA, move toward each other, lose their membranes and combine. The result is a diploid zygote with 46 chromosomes.
Fertilization restores the diploid number. The zygote receives 23 chromosomes from the egg and 23 from the sperm, so it has 23 pairs of homologous chromosomes, one chromosome of each pair from each parent.
“More than one sperm needs to enter the egg.” Many sperm help by breaking down the outer layers, but only one may enter. A second sperm would add an extra set of chromosomes, and the egg has two blocks to prevent this.
3Why a child is not identical to either parent
A child inherits half of its chromosomes from each parent. Yet the child is not a simple 50:50 copy of either, because several chance events combine.
- Each parent's gametes are unique. In meiosis, crossing over swaps segments between homologous chromosomes, and independent assortment shuffles which chromosome from each pair goes into each gamete. No two gametes from one person are likely to be the same.
- Fertilization is a matter of chance. Which sperm fertilizes the egg (out of an enormous number) is random, and the egg itself is one of many that could have been released.
- The two sets are combined. The zygote has a chromosome set from each parent. Each homologous pair contains one chromosome from the mother and one from the father, which may carry different alleles.
Independent assortment alone gives each person 223 = 8 388 608 different combinations of chromosomes in their gametes. Two parents together can produce 8 388 608 × 8 388 608 combinations of egg and sperm, which is about 70 trillion (7 × 1013), before even counting crossing over.
With a number like that, it is clear why two children of the same parents are very unlikely to have the same genetic makeup, and why neither is a copy of either parent. (Identical twins are the exception, as described below.)
Compare this with asexual reproduction, in which a single parent passes on a copy of all its DNA. In sexual reproduction, a mixture of two parents' DNA is made new each time. The consequences of this variation are the subject of the next lesson.
Look at any family: siblings share features with their parents and each other, but each has a unique mixture. The inheritance of blood type is one example (see What are blood types?). The whole set of genetic instructions is called a genome (see What is a genome?), and each child's genome is new.
Twins
Twins show the difference between “same parents” and “same genes”.
- Fraternal (non-identical) twins. In about 1% of cycles, two eggs are released and both are fertilized by different sperm. The result is two zygotes. The twins are no more alike genetically than any other siblings.
- Identical twins. One zygote splits into two embryos, most often around the blastocyst stage (about 70 to 100 cells). The two have nearly identical DNA, so identical twins are natural clones of each other. If the split happens at the two-cell stage, the twins have separate placentas, while a later split typically yields twins that share a placenta.
Even identical twins are not genetically identical to their parents. Each has the same new combination of chromosomes created at fertilization, not a copy of either parent's.
4The first week: from zygote to implantation
The zygote begins to divide by mitosis about a day after fertilization while it travels along the uterine tube toward the uterus. This early series of divisions is called cleavage. It divides the zygote 5 to 6 times, and the cells get smaller and more numerous, while the total size of the group stays about the same. Each cell is a blastomere.
- Around day 3: the morula. A ball of about 16 cells compacts into a solid cluster called a morula and reaches the uterus.
- Blastocyst. In the uterus, the ball of about 100 cells hollows out around a fluid-filled cavity. This hollow ball is the blastocyst. It has an inner cell mass that will form the embryo and an outer layer of trophoblast cells that will form part of the placenta.
- Hatching. The blastocyst sheds its protective outer coat (the zona pellucida) so that it can attach to the uterine wall.
- Implantation. At the end of week 1, the blastocyst attaches to the lining of the uterus and burrows in. Implantation is complete by the middle of week 2. The trophoblast starts to produce the hormone hCG, the hormone detected by pregnancy tests.
Many blastocysts (an estimated 50 to 75%) fail to implant and are lost with the menstrual flow, which is a reminder of how many steps must go right.
“The zygote grows bigger as it divides.” During cleavage, the cells do not grow between divisions. The same total amount of material is divided into more and more cells, so each cell becomes smaller. Growth in overall size comes later.
5The embryonic stage: weeks 3 to 8
From week 3 to week 8, the developing human is called an embryo. During this period, the three basic tissue layers form and the foundations of all the organs are laid down.
| Germ layer | Forms |
|---|---|
| Ectoderm (outer) | Nervous system, sense organs, skin, hair and nails |
| Mesoderm (middle) | Bone, muscle, connective tissue, the heart, blood vessels and kidneys |
| Endoderm (inner) | Lining of the digestive tract, liver, pancreas and lungs |
Cells of the early embryo all come from the zygote by mitosis, so all have the same DNA. Yet the three layers go on to form very different tissues and organs, so an embryo is a clear case of identical genetic instructions being used in different ways in different cells.
| Time | Development |
|---|---|
| Weeks 3 to 4 | The neural tube forms; it becomes the brain and spinal cord. A lack of folate can cause neural tube defects. |
| Week 4 | The heart begins to beat; it begins to pump blood about a week later. |
| Weeks 4 to 5 | Eye pits, limb buds and early lungs appear. |
| Week 6 | Limb movements begin; fingers and toes separate through programmed cell death. |
| Week 8 | Major brain structures are in place; the embryo is about 3 cm long and weighs about 8 g. |
Protective membranes form too. By the end of week 2, a fluid-filled sac, the amnion, surrounds the embryo; the amniotic fluid cushions it and allows it to move. The yolk sac and another small structure, the allantois, develop as well, and their stalks together form the umbilical cord.
The finger separation in week 6 is an example of cells being programmed to die as part of normal development. Cell death is part of normal development, working alongside cell division to shape the body. Another example from this stage: folate is important during the weeks when the neural tube forms, which is one reason health advice for pregnancy focuses on it.
The placenta: a meeting place
The placenta grows from the trophoblast and the mother's uterine lining. It begins as finger-like projections that anchor the embryo and reshape the mother's blood vessels. From about weeks 4 to 12, it takes over from the uterine lining the job of nourishing the embryo, and it is fully formed by about weeks 14 to 16. The placenta exchanges oxygen, nutrients and wastes between the mother's and the embryo's bloodstreams, which do not mix directly. It also produces hormones.
The fetus is connected to the placenta by the umbilical cord. The umbilical vein carries oxygen and nutrients from the placenta to the fetus, and the umbilical arteries carry low-oxygen blood and wastes back to the placenta. Because the lungs are not working before birth, the fetal circulation has three bypass channels that route blood around the immature liver and lungs. After birth, the cord is cut and the newborn's circulation reorganizes.
6The fetal stage: week 9 to birth
From week 9, the developing human is called a fetus. Fetal development is mostly about growth and the maturing of structures that formed in the embryo.
| Weeks | Developments |
|---|---|
| 9 to 12 | Brain keeps growing; bones begin to form; movements start but are jerky; bone marrow starts making red blood cells; about 9 cm crown to rump by week 12 |
| 13 to 16 | Sense organs develop; blinking and sucking motions begin; hair starts to grow; kidneys are well formed |
| 16 to 20 | Movements may be felt; protective waxy coating (vernix) forms and fine hair (lanugo) covers the body |
| 21 to 30 | Rapid weight gain; lungs begin to make surfactant; nerve fibres begin to be insulated (myelination); eyelids open; about 28 cm crown to rump at week 30 |
| 31 to birth | Fat builds up under the skin; just before birth, 35.5 to 40.5 cm crown to rump and roughly 2.5 to 4 kg |
Using the table, how much longer (crown to rump) is a fetus at week 30 than at week 12, and what is the average increase per week?
Length at week 12 is about 9 cm, and at week 30 it is about 28 cm. The difference is 28 − 9 = 19 cm over 18 weeks. The average is 19 ÷ 18, or just over 1 cm per week. The values are approximate averages, so this describes a typical pattern rather than any one pregnancy.
Counting cells and chromosomes in early development
Cleavage is mitosis, so the rules from the mitosis lesson apply: DNA is copied before each division and every new cell receives a full set of 46 chromosomes. The number of cells doubles with each division.
How many cells, and how many chromosomes in total, are there after 4 rounds of cleavage?
- Start: 1 cell (the zygote) with 46 chromosomes.
- Number of cells after 4 divisions: 24 = 16 cells. This matches the morula of about 16 cells at about day 3.
- Each cell has 46 chromosomes, so the total is 16 × 46 = 736 chromosomes.
After 6 divisions, 26 = 64 cells, which is of the same order as the roughly 100 cells of a blastocyst. (The real number of cells at a given moment depends on the timing of divisions in each cell, so the sequence 16, 32, 64 is only an approximation.)
The same arithmetic reveals something about identical twins. Because the embryo's cells all have the same 46 chromosomes from the same zygote, a split of the embryo gives two groups of cells that carry identical sets of DNA. This is why identical twins are genetically alike, while fraternal twins, who come from two separate zygotes, are not.
7Putting it all together
The human life cycle links all the cell processes in this strand.
- Meiosis in the testes and ovaries makes haploid sperm and eggs, each one different.
- Fertilization combines one sperm and one egg into a diploid zygote, with half of its chromosomes from each parent.
- The zygote divides by mitosis (cleavage) into a blastocyst, which implants in the uterus.
- Mitosis and cell specialization build the embryo and then the fetus, with the placenta providing exchange.
The result is a new person who is not genetically identical to either parent. This is the central point of the BC curriculum item on human sexual reproduction. The only people who share nearly identical DNA with each other are identical twins.
🔑Key terms
?Quick check
Try each question first, then reveal the answer.
1. How many chromosomes does a human zygote have and where do they come from?
46 chromosomes: 23 from the egg and 23 from the sperm, forming 23 homologous pairs.
2. Describe two ways the egg prevents polyspermy.
A fast change in the egg's membrane stops other sperm from fusing, and the slower cortical reaction releases substances that detach extra sperm and form a fertilization membrane.
3. Explain why a child is not genetically identical to either parent.
Each parent's gametes are unique because of crossing over and independent assortment. Chance decides which egg and sperm meet, and the child receives one chromosome of each pair from each parent, so the combination is new.
4. Compare the formation of identical twins with fraternal twins.
Identical twins form when one zygote splits into two embryos, so they have nearly identical DNA. Fraternal twins form from two eggs fertilized by two different sperm, so they are no more alike than other siblings.
5. Order these: implantation, morula, zygote, blastocyst.
Zygote, morula, blastocyst, implantation.
6. A zygote undergoes 5 rounds of cleavage. How many cells result and how many chromosomes does each have?
2 to the power 5 is 32 cells. Each cell has 46 chromosomes, because cleavage is mitosis.
7. State two jobs of the placenta.
It exchanges oxygen, nutrients and wastes between the mother's and the fetus's blood without the blood mixing, and it produces hormones.
8. Why does cleavage produce smaller and smaller cells, and what does this show about the size of the embryo at that stage?
The cells divide without growing in between, so the same amount of material is shared among more cells. The whole ball therefore stays about the same size.
BC curriculum content covered in this lesson
- Human sexual reproduction: the result of humans having two parents is that offspring are not genetically identical to either parent
- Fertilization and early human development (zygote, embryo, fetus)
Related articles
What Is a Genome, and What Was the Human Genome Project?
A genome is an organism's complete set of DNA instructions. What that means and how large the human genome is.
What Are Blood Types and Why Do They Matter?
Blood types come from inherited antigens on red cells. ABO and Rh groups decide which transfusions are safe and matter in pregnancy. How the groups were found and how common each is.
What Is DNA? The Molecule That Carries Your Instructions
DNA is the molecule that carries genetic information. A short guide to its structure and how its letters pair.
References
- BC Ministry of Education and Child Care. Science 9 curriculum (Big Ideas, Content, Curricular Competencies). Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 28.1 Fertilization. Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 28.2 Embryonic Development. Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 28.3 Fetal Development. Accessed October 7, 2026.
- OpenStax. Biology 2e, 11.1 The Process of Meiosis. Accessed October 7, 2026.
- OpenStax. Biology 2e, 11.2 Sexual Reproduction. Accessed October 7, 2026.
These lessons follow the content areas listed in the British Columbia curriculum. They are study material written for this site and are not an official document. The official curriculum is the authority on what each course requires. Lessons are general education, not medical advice.