Meiosis

Essential Question

How does meiosis make gametes with half the chromosomes and new gene combinations?

Sexual reproduction begins with gametes, such as sperm and egg cells. Each gamete needs half the usual number of chromosomes, or the number would double every generation. In this lesson, you’ll learn how meiosis halves the chromosome number, why every gamete is different, and how to count the possible combinations.

Diploid and Haploid Cells

• Body cells are diploid (2n): they have two sets of chromosomes
• Gametes are haploid (n): they have one set
• Homologous chromosomes are a matching pair, one from each parent
• Human body cells: 2n = 46. Human gametes: n = 23

Cell type	Symbol	Human chromosomes	Example
Body cell (diploid)	2n	46	Skin cell
Gamete (haploid)	n	23	Egg or sperm
Zygote (diploid)	2n	46	Fertilized egg

Most cells in your body have 46 chromosomes, arranged as 23 pairs. In each pair, one chromosome came from each parent, and the two are called homologous chromosomes. They carry the same genes in the same order, but they may carry different alleles. A cell with two sets of chromosomes is diploid (2n). A cell with one set is haploid (n). Gametes are haploid. When a sperm (n = 23) fertilizes an egg (n = 23), the zygote is diploid again, with 46 chromosomes.

How Meiosis Works

• A cell copies its chromosomes once
• It then divides twice
• The first division separates the homologous chromosomes
• The second division separates the copies of each chromosome
• One diploid cell makes four haploid cells

Meiosis is the type of cell division that makes gametes. The cell copies its chromosomes once, so each chromosome has two identical copies. Then the cell divides twice. In the first division, the homologous chromosomes are pulled apart into two cells. In the second division, the two copies of each chromosome are separated. The result is four cells, each with half the number of chromosomes of the parent cell. Compare this with mitosis (Unit 7), where one division makes two cells that are identical to the parent.

Crossing Over

• Homologous chromosomes pair up and swap matching pieces
• This is called crossing over
• It makes chromosomes with new combinations of alleles
• Two of the four combinations in the diagram are new

Early in meiosis, homologous chromosomes line up side by side and exchange matching pieces. This is crossing over. In the diagram, one chromosome carries alleles A and B, and its partner carries a and b. After crossing over, the four copies carry AB, Ab, aB, and ab. Ab and aB did not exist on either original chromosome. Crossing over creates new combinations of alleles that were on the same chromosome.

Independent Assortment and Random Fertilization

• Each pair of homologous chromosomes separates independently of the others
• With n pairs, a gamete can have 2ⁿ different chromosome combinations
• In humans, 2²³ = 8,388,608 combinations, before crossing over
• Any sperm can fertilize any egg, which multiplies the possibilities

Which chromosome of each pair goes into a gamete is random, and each pair is sorted independently of the others. This is independent assortment. With 2 pairs there are 2 x 2 = 4 possible gametes, as in the diagram. With 23 pairs there are 2²³ = 8,388,608 possible combinations in one person’s gametes, before crossing over adds more. Fertilization adds even more variation. Any sperm can fertilize any egg, so two parents can make about 2²³ x 2²³ = 7.0 x 10¹³ different combinations of chromosomes in a child. Meiosis and fertilization explain why siblings, other than identical twins, are not genetically identical. Mutations, covered in Unit 9, are a third source of new alleles.

↑ Back to top

---

Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Counting Chromosomes

A dog has 78 chromosomes in each body cell. A fruit fly has 4 pairs of chromosomes in each body cell. (a) How many chromosomes are in a dog gamete? (b) How many different chromosome combinations can one fruit fly make in its gametes, ignoring crossing over? (c) Two dog gametes fuse in fertilization. How many chromosomes does the zygote have?

Show the solution

1. (a) Haploid number: A gamete has half the chromosomes of a body cell. n = 78 / 2 = 39.
2. (b) Combinations: With 4 pairs, each pair sorts independently into one of 2 ways, so there are 2⁴ = 16 combinations.
3. (c) Zygote: Two gametes with 39 chromosomes each make a zygote with 39 + 39 = 78 chromosomes, the diploid number.
4. Answer: (a) 39 chromosomes. (b) 16 combinations. (c) 78 chromosomes.

Example 2: DNA and Chromosomes in Meiosis

The graph and table show one cell with 2n = 6 during meiosis. (a) By how much does the DNA per cell change during copying? (b) At which stage is the chromosome number cut in half? (c) Write a claim, evidence, reasoning statement for this claim: meiosis makes haploid cells.

Stage	Chromosomes per cell
Before copying	6
After copying	6
After first division	3
After second division	3

Show the solution

1. (a) Copying: DNA rises from 2 to 4 relative units, so it doubles (an increase of 2 units).
2. (b) Halving: The chromosome number falls from 6 to 3 after the first division.
3. (c) Claim: Meiosis makes haploid cells.
4. Evidence: The chromosome number falls from 6 to 3 and stays at 3 after the second division. The DNA per cell ends at 1 unit, half of the 2 units before copying.
5. Reasoning: DNA is copied once but the cell divides twice, so the four resulting cells each have half the DNA and one set of chromosomes. That is what haploid means.
6. Answer: (a) It doubles, from 2 to 4 units. (b) After the first division. (c) Chromosome number halves from 6 to 3 and DNA ends at half the starting amount.

Example 3: Use the Meiosis Model

The diagram shows meiosis in a cell with 4 chromosomes. Blue chromosomes came from the mother and orange ones from the father. (a) How many chromosomes are in each final cell? (b) Is each final cell diploid or haploid? (c) Are all four final cells the same? Describe the chromosomes in each.

Show the solution

1. (a) Count: Each final cell has 2 chromosomes, one long and one short.
2. (b) Ploidy: The starting cell has 4 chromosomes (2 pairs), so it is diploid. Each final cell has 2 chromosomes, one of each pair, so it is haploid.
3. (c) Compare: Not all four are the same. The top two cells each have a long blue and a short orange chromosome. The bottom two each have a long orange and a short blue chromosome. Crossing over, which is not shown, would make the cells even more different.
4. Answer: (a) 2 chromosomes. (b) Haploid. (c) Two types of cell, so the four cells are not all the same.

↑ Back to top

---

Practice

Practice finding haploid and diploid numbers, counting chromosome combinations, reading a model and a graph of meiosis, and listing the results of crossing over. Try it on your own first, then check your work with the answer key.

• Meiosis Practice Worksheet (PDF)
• Meiosis Practice Answer Key (PDF)

↑ Back to top

---

Lesson Slides

• 10.2 Meiosis Slides (PDF)

↑ Back to top

---

Big Ideas

• Gametes are haploid (n) and body cells are diploid (2n)
• Meiosis copies chromosomes once and divides twice
• One diploid cell makes four haploid cells
• Crossing over and independent assortment make every gamete different
• Fertilization restores the diploid number and adds more variation

Back to the essential question: Meiosis halves the chromosome number because the chromosomes are copied once but the cell divides twice, so each gamete has one set. Crossing over, independent assortment, and random fertilization make new gene combinations. Next, you’ll see what happens when chromosomes do not separate correctly.

↑ Back to top

---

↑ Back to top