Week 10 · Chapter

Chapter 10 — Meiosis & Sexual Reproduction

Introduction to Biology Free sample edition

The module's primary reading · ~35–45 minutes · Objective 5 (describe the eukaryotic cell cycle and the ordered phases of mitosis, compute a mitotic index, contrast mitosis with meiosis, and explain how crossing over, independent assortment, and fertilization generate genetic variation, including the 2ⁿ count), with Objective 1 (describe the practice of science in biology — the characteristics of life as a set, the levels of biological organization, the scientific method and a controlled experiment's variables and control, and the difference between a hypothesis and a theory — and explain evolution by natural selection as the unifying framework of biology) carried forward · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Two children of the same parents are never copies of each other, and this week you learn why. You will meet meiosis (the two-step cell division that makes egg and sperm cells, each with half the usual number of chromosomes). You will see how fertilization (an egg and a sperm joining) puts the full number back. And you will find the shuffles inside meiosis that create genetic variation (the differences in DNA among family members and among the members of a species).

Why you already care about this

Think of two siblings at a family dinner. They have the same mother and the same father. One has curly hair and one has straight hair, one is tall and one is short, and the family argues every holiday about who looks like whom. Yet both children got all their chromosomes from the same two people. How can the same two parents make children who differ?

The explanation is a cell division you have never watched but that made you. A human body cell carries 46 chromosomes. An egg or a sperm carries 23, half the usual number. In this week's lab, a paper cell with only 3 pairs of chromosomes can already make 8 different kinds of sex cells. And a human, with 23 pairs, can make 8,388,608 different kinds from one shuffle alone, before a second shuffle adds even more.

So the differences at that dinner table are not an accident. They are built into the way your body makes eggs and sperm. This chapter shows you how.

By the end of this chapter you will be able to:

tell diploid from haploid, and homologous chromosomes from sister chromatids, using the human numbers 46 and 23;
explain why gametes must carry half the chromosomes, with the arithmetic shown;
name the two divisions of meiosis in order and say what separates in each;
explain crossing over and independent assortment, and say when in meiosis each one happens;
count the gametes a cell can make from independent assortment with the 2ⁿ rule, and list the eight for a cell with 3 pairs;
compare mitosis and meiosis on six features;
connect the variation meiosis makes to evolution by natural selection.

Carry this line with you: meiosis halves; fertilization restores.

Where we are

Already covered: Week 1 named evolution by natural selection, Week 3 introduced DNA, and Week 9 taught the chromosome, its sister chromatids, and mitosis.
This week: diploid and haploid, meiosis I and II in order, crossing over, independent assortment, and the 2ⁿ count.
How they connect: meiosis divides the same chromosomes as mitosis, but twice, and it supplies the variation natural selection acts on.
Where we're going next: Week 11 starts the inheritance unit with Mendel's rules, which follow single genes through these gametes.

Week 9 gave you the chromosome as one long DNA molecule with its packing proteins, and it showed that after the S phase each chromosome is two identical sister chromatids joined at the centromere. One X shape is one chromosome, not two. Week 9 also gave you mitosis, which makes two identical daughter cells. Week 1 gave you natural selection: individuals whose traits suit their surroundings survive and reproduce more. This week puts those pieces together. You will run a new kind of division on the chromosomes you already know, and you will see where the raw material for natural selection comes from.

1. Count the sets: diploid, haploid, and homologous pairs

Picture two decks of recipe cards, one from each grandparent. Each deck has a card for soup, a card for bread, and a card for salad, in the same order. The two soup cards cover the same dish, but the recipes on them can differ a little. That pair of matching decks is a good picture of the chromosomes in one of your body cells.

A human body cell carries 46 chromosomes, and they come as 23 pairs. One chromosome of each pair came from your mother and one from your father. Biologists have a word for a cell with two full sets like this.

A diploid cell carries two full sets of chromosomes, one set from each parent, written 2n; a human body cell is diploid, with 46 chromosomes.
A haploid cell carries one complete set of chromosomes, written n; a human egg or sperm is haploid, with 23 chromosomes.
A gamete is a sex cell, an egg or a sperm, that carries one set of chromosomes and joins with another gamete at fertilization.

The two chromosomes of a pair have their own name.

Homologous chromosomes are the two chromosomes of a matched pair, one from each parent, that carry the same genes in the same order, possibly as different versions of those genes.

Week 11 names those different versions of a gene. For now, the recipe cards are enough: same dish, same place in the deck, possibly a different recipe.

Here is where students most often trip. Homologous chromosomes are not sister chromatids. Sister chromatids are the two identical copies of one chromosome made in the S phase, joined at the centromere. Homologs are a matched pair that came from two different parents, so they are similar but not identical.

Worked example — reading the human numbers. You use the notation to describe a human body cell and a human gamete.

Body cell. It has two full sets, so it is diploid. 2n = 46.
Sets and pairs. Two sets of 23 make 46, so the cell holds 23 pairs of homologous chromosomes.
Gamete. It has one full set, so it is haploid. n = 23.
Check. 46 ÷ 2 = 23, so the gamete holds half the body cell's chromosomes and one chromosome from every pair.

In words: a human body cell is diploid with 46 chromosomes in 23 pairs, and a human gamete is haploid with 23 chromosomes, one from each pair.

Misconception check — two swaps. Students often think haploid means half a chromosome, because "half the number" sounds like cutting something in half. It means one complete set: a gamete has 23 whole chromosomes, one from each pair, and none of them is cut. Students also blur homologous chromosomes and sister chromatids. The cure is to ask where each one came from. Sisters are copies made from one chromosome in the S phase, so they match exactly. Homologs came from two different parents, so they carry the same genes but can carry different versions.

Memory hook: sisters are copies; homologs are a matched pair from two parents.

2. Solve the doubling problem: meiosis halves, fertilization restores

Imagine a recipe that says "add the whole bag of flour" and then tells you to use the finished bread as the starter for the next loaf. Every new loaf would carry the last loaf's flour plus a whole new bag. Within a few batches the bowl would overflow. Life would face the same overflow if eggs and sperm carried a full set of 46.

Worked example — the overflow you avoid. You follow the chromosome count across generations under two rules.

If gametes were diploid. An egg with 46 and a sperm with 46 would make a child with 46 + 46 = 92 chromosomes.
The next generation. That child's gametes would carry 92 each, so the grandchild would have 92 + 92 = 184.
The pattern. The count doubles every generation and never settles.
The real rule, halving. Meiosis cuts the count in half: 46 ÷ 2 = 23 in each gamete.
The real rule, restoring. At fertilization an egg with 23 meets a sperm with 23, and 23 + 23 = 46.

In words: without halving, the count would go from 46 to 92 to 184, doubling each time; with meiosis and fertilization it stays at 46, generation after generation.

Now the two terms that do the job.

Meiosis is the two-part cell division that makes gametes, turning one diploid cell into four genetically different haploid cells after a single round of DNA copying.
Fertilization is the joining of an egg and a sperm, which combines two haploid sets of chromosomes and restores the diploid number.

Together they make sexual reproduction: reproduction in which two parents each contribute a haploid gamete, so the offspring carries a new combination of both parents' chromosomes. Week 9's asexual reproduction made a copy of one parent. Sexual reproduction makes something new from two.

Misconception check. Students sometimes think a gamete carries 46 chromosomes "so the baby has enough". The idea is tempting because 46 is the number you memorized for a human cell. But the baby's 46 is the sum of two gametes, 23 from each parent. If each gamete carried 46, the count would double every generation. Meiosis halves; fertilization restores.

Memory hook: half from each parent; whole in the child.

3. Run meiosis I: the homologs pair and separate

Picture a square dance. The dancers start in pairs, partners side by side. At a call, the couples line up across the middle of the room. At the next call, every couple splits up, and the two partners walk to opposite walls. Meiosis I is that dance, and the dancers are homologous chromosomes.

Before the dance starts, one thing happens that you already know from Week 9. In the S phase the cell copies its DNA, so each chromosome enters meiosis as two sister chromatids joined at the centromere. That copying happens once, before meiosis begins, and never again during meiosis.

Meiosis I is the first division of meiosis, called the reductional division, in which homologous chromosomes pair up and then separate, leaving two haploid cells whose chromosomes are still doubled.

Here is meiosis I, in order.

Prophase I: the pairs find each other. The chromosomes condense, and each chromosome lines up beside its homolog along its whole length. Synapsis is the pairing of homologous chromosomes side by side in prophase I of meiosis. Mitosis has no such step; in mitosis, homologs never pair.
Metaphase I: the pairs line up as pairs. The paired homologs line up at the middle of the cell. In mitosis the chromosomes lined up single file; here they line up two by two.
The pairs split. The homologs separate and move to opposite poles. Each chromosome's two sister chromatids stay joined and travel together.
Two cells form. The cell divides into two. Each new cell holds one chromosome from every pair, so it is haploid, but each of those chromosomes is still two chromatids.

It is called the reductional division because it reduces the number of chromosome sets from two to one.

Worked example — the model cell through meiosis I. You follow the three-pair model cell from this week's lab. It has a long, a medium, and a short pair, so n = 3. B stands for the copy from one parent and R the copy from the other.

Count the chromosomes. 2n = 2 × 3 = 6 chromosomes.
Copy the DNA once. After the S phase each chromosome is two chromatids, so the cell holds 6 × 2 = 12 chromatids.
Pair and separate. The long B pairs with the long R, the medium B with the medium R, and the short B with the short R. Then each pair splits.
Count the result. The cell becomes 2 cells, each with 3 chromosomes, one from each pair. Each chromosome is still doubled, so each cell holds 3 × 2 = 6 chromatids.

In words: meiosis I takes the model cell from 6 chromosomes in pairs to 2 haploid cells with 3 doubled chromosomes each.

Misconception check. Students often say meiosis I separates sister chromatids, because separating sisters is what they learned in mitosis. In meiosis I the homologs separate and the sisters stay together. The cure is the dance: in meiosis I the partners split, and each partner is still holding hands with its own copy.

Memory hook: meiosis I splits the pairs.

4. Run meiosis II: the sister chromatids separate

Now each of the two cells from meiosis I has three doubled chromosomes. Think of each one as a pair of socks still clipped together from the laundry. Meiosis II unclips them.

Meiosis II is the second division of meiosis, called the equational division, in which sister chromatids separate, leaving four haploid cells with single chromosomes.

Meiosis II looks a lot like mitosis. In each of the two cells, the doubled chromosomes line up, the sister chromatids separate and move to opposite poles, and the cell divides. Two cells each divide once, so you get 2 × 2 = 4 cells.

Notice what does not happen between meiosis I and meiosis II: there is no S phase and no second round of DNA copying. The chromosomes were copied once, and the cell divides twice. That is how the number of chromosomes per cell ends up halved.

The order of the two divisions is a fact you can be graded on:

meiosis I (homologs separate) → meiosis II (sister chromatids separate)

Worked example — the model cell through meiosis II. You finish the three-pair model cell.

Start. 2 cells, each with 3 doubled chromosomes.
Separate the sisters. In each cell every chromosome splits into its two chromatids, which move apart.
Count the cells. Each of the 2 cells divides once, so 2 × 2 = 4 haploid cells form.
Count the chromosomes. Each of the 4 cells holds 3 single chromosomes, one from each pair.

In words: one diploid model cell with 6 chromosomes became 4 haploid cells with 3 single chromosomes each, after one round of DNA copying and two divisions.

Misconception check. Students often think the DNA is copied again between meiosis I and meiosis II, since it seems the cell would need fresh copies before dividing a second time. It is copied once, before meiosis I. The second division only pulls apart the sister chromatids that the single copying already made. Students also say meiosis makes identical cells, because mitosis does. The point of meiosis is the reverse: its four cells differ, for the two reasons in the next section.

Memory hook: copy once, divide twice.

5. Find the two sources of genetic variation

Go back to the sibling puzzle. Both children got 23 chromosomes from each parent. So why are they different? Because the gametes that made them were different, and meiosis makes them different in two ways. Genetic variation is the differences in DNA among the members of a family or a species, the raw material that natural selection acts on.

Source one: crossing over, in prophase I

While the homologs are paired in prophase I, they trade pieces.

Crossing over is the swap of matching segments between nonsister chromatids of paired homologous chromosomes in prophase I, which makes new combinations within a chromosome.
Nonsister chromatids are chromatids that belong to different chromosomes of a homologous pair, one from each parent's copy, rather than to the same chromosome.

Back to the recipe decks. Suppose the soup card from one grandparent swaps its bottom half with the bottom half of the soup card from the other grandparent. Now each card carries part of each recipe. After crossing over, a chromosome leaves meiosis carrying a new mix of the two parents' versions, a combination that neither parent's chromosome had.

Source two: independent assortment, in metaphase I

When the pairs line up in metaphase I, each pair can face either way. The mother's copy may face the left pole and the father's copy the right, or the reverse. Each pair settles its direction by chance, and no pair's direction affects any other's.

Independent assortment is the random, independent way each homologous pair lines up in metaphase I, which sends either parent's copy of each pair into a gamete and makes new combinations between chromosomes.

Think of getting dressed from a closet in the dark. In an illustrative closet with 5 shirts and 3 jackets, you have 5 × 3 = 15 possible outfits, because every shirt can go with every jacket. The choices multiply. Independent assortment works the same way, except that every chromosome pair offers exactly 2 choices.

The multiplier: random fertilization

Finally, any one egg can meet any one sperm.

Random fertilization is the chance meeting of any one egg with any one sperm, which multiplies the variety that crossing over and independent assortment create.

Two kittens from the same litter are a good everyday case. The same two parent cats made both, but each kitten came from a different egg and a different sperm, each one shuffled by crossing over and independent assortment, so the kittens can look quite unlike each other.

Worked example — which source is it? You name the source of variation from a description.

Two paired homologs trade matching segments. Crossing over, in prophase I. It makes a new combination within one chromosome.
The long pair's B copy goes to one pole while the short pair's R copy goes to that same pole, by chance. Independent assortment, in metaphase I. It makes a new combination between chromosomes.
Of all the sperm a father makes, one in particular reaches the egg. Random fertilization. It multiplies the other two.

In words: crossing over shuffles within a chromosome, independent assortment shuffles between chromosomes, and fertilization multiplies both.

Misconception check. Students often think crossing over happens in mitosis too. It is tempting because the chromosomes and chromatids are the same ones. But crossing over needs homologs lying side by side, and only prophase I pairs them. Mitosis never pairs homologs, so it never crosses them over, and its two daughter cells stay identical.

Memory hook: crossing over mixes within; assortment mixes between.

Pause and reflect. Choose one of these two prompts and think it through, or jot a few sentences in your notes.

(a) Think of two siblings you know, in your family or a friend's. Which differences between them could come from meiosis, and which might come from their surroundings? How would you tell the two apart?
(b) Explain crossing over and independent assortment to someone who has never taken biology, using an everyday comparison of your own that is not the recipe cards or the closet.

6. Count the gametes: the doubling rule

Now put a number on independent assortment. In the model cell, each of the 3 pairs sends either its B copy or its R copy into a gamete. That is 2 choices for the long pair, 2 for the medium pair, and 2 for the short pair.

The rule for any number of pairs is short. From independent assortment alone, a cell with n chromosome pairs makes 2ⁿ genetically different gametes. 2ⁿ means 2 multiplied by itself n times.

Worked example — the three-pair model cell. You count and then list every gamete the model cell can make.

Count the choices. 2 × 2 × 2 = 2³ = 8 gametes.
List them. Written long, medium, short, the eight are BBB, BBR, BRB, BRR, RBB, RBR, RRB, RRR.
Check the list. Count them: 8 different gametes and no repeats, matching 2³ = 8.

The eight gametes are an illustrative paper model built for this course; the table sorts them by how many B copies each one carries.

The eight gametes of the three-pair model cell, written long, medium, short, sorted by the number of copies from parent B (illustrative paper model)
B copies in the gamete	Gametes	How many
3	BBB	1
2	BBR, BRB, RBB	3
1	BRR, RBR, RRB	3
0	RRR	1

The four counts add up: 1 + 3 + 3 + 1 = 8. Two of the eight gametes carry copies from only one parent, BBB and RRR; the other six are mixes.

Worked example — climbing the ladder to a human. You watch what each added pair does.

Two pairs. 2² = 4.
Three pairs. 2³ = 8.
Four pairs. 2⁴ = 16.
Five pairs. 2⁵ = 32.
The pattern. Each added pair doubles the count, because every gamete from the smaller cell can now take either copy of the new pair.
A human. A human has n = 23 pairs, so independent assortment alone gives 2²³ = 8,388,608 genetically different gametes.

In words: a human can make 8,388,608 kinds of gametes from independent assortment alone, and that number is only a floor, because crossing over raises it without limit.

Fertilization multiplies again. Two model parents, each able to make 8 kinds of gametes, can pair them 8 × 8 = 64 ways from independent assortment alone. That is 64 pairings versus 8 kinds of gametes per parent, 8 times as many, and it is why two siblings almost never draw the same combination.

In practice (your tools). In the lab you check the doubling rule in a spreadsheet such as Google Sheets. Type the number of pairs, 2 through 5, down column A, then type =2^A2 in cell B2 and fill it down. For the human count, =2^23 or =POWER(2,23) returns 8,388,608. A spreadsheet is fine for powers of 2; your job is to say what the number means.

Caveat: real chromosomes do not come in two neat colors, and real meiosis does not hand you a list. The paper model shows the counting rule at a size you can hold, and the rule, not the model, is what scales to 23 pairs.

Misconception check. Students often give the human gamete count as 23 or 46. Those are the numbers they know best: 23 pairs and 46 chromosomes. But neither one counts combinations. Each pair doubles the count, so the count is 2 multiplied by itself 23 times, 2²³ = 8,388,608. Another slip is 2 × 23 = 46, which doubles the number of pairs once instead of doubling the count once for every pair.

Memory hook: each pair doubles the count.

7. Compare mitosis and meiosis, and see why variation matters

You now know both kinds of nuclear division. They share the same parts, chromosomes, sister chromatids, and phases with the same names. That shared machinery is exactly why they are easy to blur. The table sets them side by side on six features.

Mitosis and meiosis compared on six features, from number of divisions to purpose
Feature	Mitosis	Meiosis
Divisions	1	2
Cells made	2	4
Chromosome sets in each new cell	diploid, like the parent	haploid, half the parent's number
How the new cells compare	identical to each other and to the parent	genetically different from each other and from the parent
Do homologs pair?	never	yes, in prophase I, where they can cross over
Purpose	growth, repair, replacement, asexual reproduction	making gametes for sexual reproduction

Both divisions follow one round of DNA copying in the S phase. The difference is what happens next: mitosis divides once and keeps the pairs together in every cell; meiosis divides twice and splits the pairs between cells.

Worked example — mitosis, meiosis, or both? You sort each statement.

Makes four haploid cells. Meiosis.
Repairs a scraped knee. Mitosis.
Is preceded by one round of DNA copying in the S phase. Both.
Pairs homologous chromosomes so they can cross over. Meiosis, in prophase I.
Produces two cells identical to the parent. Mitosis.
Separates sister chromatids at some point. Both: mitosis in anaphase, meiosis in meiosis II.

In words: four of the six statements belong to one division only, and two belong to both, because the two divisions share their machinery but not their results.

The evolutionary lens

Week 1 named evolution as change in the heritable traits of a population over generations, with natural selection favoring individuals whose traits suit their surroundings. Natural selection can only work with differences that already exist. If every offspring were a copy of its parent, there would be nothing to choose between. Meiosis and fertilization supply new combinations in every generation, which is why the two siblings at that family dinner each got a different hand. Variation is the raw material natural selection acts on, and that is why sexual reproduction is worth what it costs a living thing.

Honesty note: this course gives you the evolutionary lens in one paragraph. The full story of how populations change belongs to the second semester of biology.

Misconception check. Students often say mitosis and meiosis are basically the same thing, because both divide a nucleus and both use phases with the same names. They differ on all six rows of the table. The fastest cure is two numbers: mitosis makes 2 identical cells, and meiosis makes 4 unique ones.

Memory hook: mitosis copies; meiosis shuffles.

Key terms

Diploid — Describes a cell that carries two full sets of chromosomes, one set from each parent, written 2n; a human body cell is diploid, with 46 chromosomes. (first used: 1. Count the sets: diploid, haploid, and homologous pairs)
Haploid — Describes a cell that carries one complete set of chromosomes, written n; a human egg or sperm is haploid, with 23 chromosomes. (first used: 1. Count the sets: diploid, haploid, and homologous pairs)
Gamete — A sex cell, an egg or a sperm, that carries one set of chromosomes and joins with another gamete at fertilization. (first used: 1. Count the sets: diploid, haploid, and homologous pairs)
Homologous chromosomes — The two chromosomes of a matched pair, one from each parent, that carry the same genes in the same order, possibly as different versions of those genes. (first used: 1. Count the sets: diploid, haploid, and homologous pairs)
Meiosis — The two-part cell division that makes gametes, turning one diploid cell into four genetically different haploid cells after a single round of DNA copying. (first used: 2. Solve the doubling problem: meiosis halves, fertilization restores)
Fertilization — The joining of an egg and a sperm, which combines two haploid sets of chromosomes and restores the diploid number. (first used: 2. Solve the doubling problem: meiosis halves, fertilization restores)
Sexual reproduction — Reproduction in which two parents each contribute a haploid gamete, so the offspring carries a new combination of both parents' chromosomes. (first used: 2. Solve the doubling problem: meiosis halves, fertilization restores)
Meiosis I — The first division of meiosis, called the reductional division, in which homologous chromosomes pair up and then separate, leaving two haploid cells whose chromosomes are still doubled. (first used: 3. Run meiosis I: the homologs pair and separate)
Synapsis — The pairing of homologous chromosomes side by side in prophase I of meiosis. (first used: 3. Run meiosis I: the homologs pair and separate)
Meiosis II — The second division of meiosis, called the equational division, in which sister chromatids separate, leaving four haploid cells with single chromosomes. (first used: 4. Run meiosis II: the sister chromatids separate)
Genetic variation — The differences in DNA among the members of a family or a species, the raw material that natural selection acts on. (first used: 5. Find the two sources of genetic variation)
Crossing over — The swap of matching segments between nonsister chromatids of paired homologous chromosomes in prophase I, which makes new combinations within a chromosome. (first used: 5. Find the two sources of genetic variation)
Nonsister chromatids — Chromatids that belong to different chromosomes of a homologous pair, one from each parent's copy, rather than to the same chromosome. (first used: 5. Find the two sources of genetic variation)
Independent assortment — The random, independent way each homologous pair lines up in metaphase I, which sends either parent's copy of each pair into a gamete and makes new combinations between chromosomes. (first used: 5. Find the two sources of genetic variation)
Random fertilization — The chance meeting of any one egg with any one sperm, which multiplies the variety that crossing over and independent assortment create. (first used: 5. Find the two sources of genetic variation)

Summary

A human body cell is diploid, 2n = 46; a human gamete is haploid, n = 23.
Homologous chromosomes are a matched pair from two parents; sister chromatids are identical copies of one chromosome.
Meiosis halves the chromosome number; fertilization restores it.
The DNA is copied once, before meiosis I; the cell then divides twice.
Meiosis I separates homologs; meiosis II separates sister chromatids; the result is four haploid cells.
Crossing over in prophase I mixes within a chromosome; independent assortment in metaphase I mixes between chromosomes.
Random fertilization multiplies both sources of variation.
From independent assortment alone, n pairs give 2ⁿ kinds of gametes; a human's 23 pairs give 8,388,608.
Mitosis makes 2 identical diploid cells; meiosis makes 4 unique haploid cells.
Variation is the raw material natural selection acts on.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

An illustrative flowering plant has 14 chromosomes in each body cell. What are its 2n and n, how many chromosome pairs does it have, and how many genetically different gametes can it make from independent assortment alone?
An illustrative animal has 10 chromosomes in each body cell. Count its chromosomes and chromatids after the S phase, after meiosis I (per cell), and after meiosis II (per cell), and say how many cells there are at the end.
Sort each statement as mitosis, meiosis, or both, and explain one of your choices: (a) makes the cells that repair a cut on your hand; (b) includes synapsis; (c) produces cells with half the parent's chromosome number; (d) starts from a diploid cell.
Someone claims, "Two puppies from the same parents should be nearly identical, since they got all their chromosomes from the same two dogs." Explain, naming three sources of variation, what is wrong with that claim.

Answers (worked reasoning — read after attempting)

The body cell has two full sets, so 2n = 14. One set is n = 14 ÷ 2 = 7, so the plant has 7 pairs. From independent assortment alone it can make 2⁷ = 2 × 2 × 2 × 2 × 2 × 2 × 2 = 128 kinds of gametes. The count is not 7 and not 14; each of the 7 pairs doubles it.
After the S phase: still 10 chromosomes, because each X counts as one, and 10 × 2 = 20 chromatids. After meiosis I: the homologs have separated, so each of the 2 cells has 10 ÷ 2 = 5 chromosomes, still doubled, so 5 × 2 = 10 chromatids per cell. After meiosis II: the sisters have separated, so there are 2 × 2 = 4 cells, each with 5 single chromosomes. Each final cell is haploid, n = 5.
(a) Mitosis: repair needs identical diploid cells. (b) Meiosis: synapsis is the pairing of homologs in prophase I, and mitosis never pairs homologs. (c) Meiosis: it is the halving division that makes haploid gametes. (d) Both: mitosis divides diploid body cells, and meiosis begins with a diploid cell and ends with haploid ones.
The puppies share parents, but not gametes. Crossing over in prophase I gave each chromosome in each egg and sperm a new mix of the parents' versions. Independent assortment in metaphase I sent a different set of copies into each gamete. Random fertilization meant a different egg met a different sperm for each puppy. So each puppy carries its own combination, and littermates can differ a great deal.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz; next week, Mendel's segregation rule follows one gene through the gametes meiosis makes.

Built with The Course Maker · samples.thecoursemaker.com