1. Sexual Reproduction and Meiosis
Chapter 9
Sexual Reproduction and Meiosis
Ultrasound: © Chad Ehlers/Glow Images
Copyright © McGraw-Hill Education.  All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

2. Why Sex? Reproduction occurs in two main forms: asexual and sexual.
Section 9.1
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Figure 9.1

3. Why Sex?
In asexual reproduction, an organism replicates its DNA and splits the contents of one cell into two. This amoeba is reproducing asexually.
Asexual reproduction generates identical offspring.
Section 9.1
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Figure 9.1

4. Why Sex?
In sexual reproduction, the DNA of the offspring comes from two parents. Humans, cats, and many other familiar organisms reproduce sexually.
Section 9.1
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Figure 9.1

5. Why Sex?
A female parent contributes an egg, and a male parent produces sperm. These cells fuse, yielding the first cell of the next generation.
Section 9.1
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Figure 9.1

6. Why Sex?
Sexual reproduction produces genetic diversity among offspring, which increases the chance that some offspring survive in changing environments.
Section 9.1
Figure 9.2

7. Clicker Question #1
Which two organisms are genetically the most similar?
Pair A
Pair B
Flower: © Doug Sherman/Geofile/RF; Amoeba: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images

8. Clicker Question #1
Which two organisms are genetically the most similar?
Pair A
Pair B
Flower: © Doug Sherman/Geofile/RF; Amoeba: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images

9. Diploid Cells Contain Two Homologous Sets of Chromosomes
Most cells in a sexually reproducing organism have two sets of chromosomes: one from the father and one from the mother.
Cells with two sets of chromosomes are called diploid cells. The image to the right shows all of the chromosomes from a diploid human cell.
Section 9.2
Figure 9.3
All karyotype photos: © CNRI/Science Source

10. Diploid Cells Contain Two Homologous Sets of Chromosomes
The numbered chromosomes are called autosomes. These chromosomes are the same for both sexes.
The chromosomes denoted by a letter are the sex chromosomes. In humans, an individual with two
X chromosomes is female, and an individual with one X and one Y chromosome is male.
Section 9.2
Figure 9.3
All karyotype photos: © CNRI/Science Source

11. Diploid Cells Contain Two Homologous Sets of Chromosomes
Notice that each chromosome is paired with an equally-sized chromosome (except for when the X chromosome pairs with the smaller Y chromosome).
Section 9.2
Figure 9.3
All karyotype photos: © CNRI/Science Source

12. Diploid Cells Contain Two Homologous Sets of Chromosomes
The chromosomes of a homologous pair look alike and carry the same sequence of genes for the same traits.
However, the members of a homologous pair may carry different alleles, which are alternative versions of the same gene.
Section 9.2
Figure 9.4

13. Clicker Question #2
The photo below shows two chromosomes from one cell. Which statement is true?
Both came from the person’s mother.
The 1-2 pair is identical to the 3-4 pair.
1 is identical to 2.
The chromosomes have not been replicated yet. 2 3 4 1
Flower: © Doug Sherman/Geofile/RF

14. Clicker Question #2
The photo below shows two chromosomes from one cell. Which statement is true?
Both came from the person’s mother.
The 1-2 pair is identical to the 3-4 pair.
1 is identical to 2.
The chromosomes have not been replicated yet. 2 3 4 1
Flower: © Doug Sherman/Geofile/RF

15. Meiosis Is Essential in Sexual Reproduction
Gametes, like the sperm and egg cells in this image, are haploid sex cells. A haploid cell has only one set of chromosomes.
When two haploid cells fuse at fertilization, one diploid zygote is formed. A zygote is the first cell of a new organism.
Section 9.3
Figure 9.15
Sperm contacting egg: © Francis Leroy, Biocosmos/Science Source

16. Meiosis Is Essential in Sexual Reproduction
The organism grows by mitotic cell division.
In adults, specialized diploid cells called germ cells divide to form haploid gametes.
Section 9.3
Figure 9.5
Mating frogs: © Nature Picture Library/Britain On View/Getty Images

17. Meiosis Overview
Meiosis halves the chromosome number and scrambles alleles.
n=2
n=2
2n=4
Section 9.3
Figure 9.6
Human female karyotype: © James Cavallini/Science Source

18. In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
The cell on the left has two pairs of homologous chromosomes, whereas the cells on the right have two unpaired,
unreplicated chromosomes.
Section 9.4
Figure 9.11

19. In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
Section 9.4
Figure 9.7
All photos © Ed Reschke

20. In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
Section 9.4
Figure 9.7
All photos © Ed Reschke

21. Clicker Question #3
A zygote of a ferret contains 40 chromosomes. How many chromosomes are in a ferret’s sperm cell? 10 20 40 80
Flower: © Doug Sherman/Geofile/RF

22. Clicker Question #3
A zygote of a ferret contains 40 chromosomes. How many chromosomes are in a ferret’s sperm cell? 10 20 40 80
Flower: © Doug Sherman/Geofile/RF

23. Meiosis Generates Variability
The gametes produced in meiosis are different from one another because of:
crossing over (prophase I)
independent assortment (metaphase I)
Section 9.5
Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images

24. Meiosis Generates Variability
During crossing over, two homologous chromosomes exchange genetic material during prophase I.
Section 9.5
Figure 9.8

25. Meiosis Generates Variability
In independent assortment, chromosome pairs
align randomly during metaphase I.
A germ cell with three pairs of chromosomes has four possible arrangements during metaphase I,
yielding eight possible gametes (23 = 8).
Section 9.5
Figure 9.9

26. Meiosis Generates Variability
If meiosis generates such great variability among gametes, how can siblings be identical?
Sometimes, early in development, an embryo splits in two. Each embryo then develops independently.
Section 9.5
Figure 9.10
Identical twins: © Barbara Penoyar/Getty Images RF

27. Meiosis Generates Variability
Fraternal twins are a testament to the variation among gametes; two sperm cells fertilize two separate egg cells, and the offspring might look very different.
Section 9.5
Figure 9.10
Fraternal twins: © Image Source Black/ Getty Images RF

28. Clicker Question #4
Many sets of triplets consist of a “pair and a spare;” that is, two identical babies and one non-identical baby. Producing this arrangement requires ___ sperm and ___ eggs.
2 … 2
1 … 3
3 … 1
2 … 3
Flower: © Doug Sherman/Geofile/RF

29. Clicker Question #4
Many sets of triplets consist of a “pair and a spare;” that is, two identical babies and one non-identical baby. Producing this arrangement requires ___ sperm and ___ eggs.
2 … 2
1 … 3
3 … 1
2 … 3
Flower: © Doug Sherman/Geofile/RF

30. Mitosis and Meiosis Compared
Meiosis occurs in germ cells; mitosis occurs in body cells.
2n=4
2n=4
n=2
2n=4
Section 9.6
Figure 9.11

31. Mitosis and Meiosis Compared
Cytokinesis occurs once in mitosis, but twice in meiosis.
2n=4
2n=4
n=2
2n=4
Section 9.6
Figure 9.11

32. Mitosis and Meiosis Compared
In mitosis, daughter cells have the same chromosome number as the parent cell. In meiosis, chromosome number is halved.
2n=4
2n=4
n=2
2n=4
Section 9.6
Figure 9.11

33. Mitosis and Meiosis Compared
In meiosis, homologous chromosome pairing allows for crossing over. This process does not occur in mitosis.
2n=4
2n=4
n=2
2n=4
Section 9.6
Figure 9.11

34. Errors Sometimes Occur in Meiosis
If chromosomes fail to separate during anaphase I or anaphase II, then abnormal gametes form.
This image shows how nondisjunction in meiosis I produces gametes with either an extra or a missing chromosome.
Section 9.7
Figure 9.12

35. Errors Sometimes Occur in Meiosis
This image shows how nondisjunction in meiosis II produces two gametes with either an extra or a missing chromosome, and two normal gametes.
Section 9.7
Figure 9.12

36. Errors Sometimes Occur in Meiosis
Individuals with missing chromosomes often cease developing before birth. An extra chromosome often causes fewer problems than a missing chromosome.
Section 9.7
Figure 9.12

37. Errors Sometimes Occur in Meiosis
An extra copy of a chromosome—a trisomy—causes genetic disorders such as Down syndrome.
Section 9.7
Figure 9.13
Karyotype: © CNRI/Science Source; Child with Down syndrome © George Doyle/Stockbyte/Getty Images RF

38. Errors Sometimes Occur in Meiosis
Extra or missing sex chromosomes cause sex disorders.
Section 9.7
Table 9.1

39. Errors Also Occur Within Chromosomes
Mutations in chromosome structure can have major effects on an organism.
Section 9.7
Figure 9.14
Chromosomes: © Addenbrookes Hospital/Science Source

40. Clicker Question #5
In Turner syndrome, a female is born with only one X chromosome. This disorder could arise as a result of nondisjunction in ___.
sperm formation
egg formation
either sperm or egg formation
Flower: © Doug Sherman/Geofile/RF

41. Clicker Question #5
In Turner syndrome, a female is born with only one X chromosome. This disorder could arise as a result of nondisjunction in ___.
sperm formation
egg formation
either sperm or egg formation
Flower: © Doug Sherman/Geofile/RF

42. 9.7 Mastering Concepts
How can nondisjunction during meiosis lead to gametes with extra or missing chromosomes?
Ultrasound: © Chad Ehlers/Glow Images

43. Meiosis Produces Human Gametes
Sperm cells form in the testes. Egg cells form in the ovaries.
Section 9.8
Figure 9.15
Sperm contacting egg: © Francis Leroy, Biocosmos/Science Source

44. Meiosis Produces Human Gametes
Section 9.8
Figure 9.16

45. Meiosis Produces Human Gametes
Section 9.8
Figure 9.17

46. Meiosis Produces Spores in Plants
Section 9.8
Figure 9.18