1. Chapter 9
The passage of life’s organization and information from one generation to the next
One way, but are there others?
How do organisms pass genetic information?
Are the contributions the same from males and females?
What kinds of mishaps occur and where do they originate?

2. General Life Strategies
Asexual reproduction
corms
fragmentation
bulbs
No exchange of genetic material
Offspring are genetically identical to parents
No time ‘wasted” finding a mate
No courtship

3. Figure 9.8 Bacterial Duplication Figure: 9.8 Caption:
Quaking aspen can reproduce asexually, leading to genetically identical clones. The photograph was taken in autumn, when quaking aspen leaves turn bright yellow.

4. Some Interesting Strategies
The life cycle of aphids can involve a mix of parthenogenetic (asexual) and sexual reproduction. Parthenogenetic reproduction provides the development of young from unfertilized eggs. The young are female and genetically identical to the parent. Eggs typically hatch in spring and develop into wingless females which then produce live young. After some generations of parthenogenesis, winged reproductive males and females are produced which mate and lay eggs.

5. Another Interesting Organism
In approximately 15 of the Cnemidophorus species there are no males. They reproduce by parthenogenesis.
Parthenogenesis is rare in vertebrates. The offspring of parthenogenic lizards are clones, identical to the mother.

6. Human Cloning
1998 Mice
United Nations (Nov. 20, 2001) - A key General Assembly committee backed a resolution calling for a treaty to ban the cloning of human beings, saying it was "contrary to human dignity.“ Under the draft resolution, a group would meet twice next year to define what should be negotiated in an international convention to ban reproductive cloning.
1997 Dolly
2000 Monkey Business

7. Box 9.3, Figure 1
But something else is happening: genetic recombination
BACTERIAL CONJUGATION AND RECOMBINATION
1. Hfr cells contain genes that allow them to transfer some or all of their chromosome to another cell.
Hfr cell
Normal cell
2. Conjugation tube connects Hfr cell to normal cell. Copy of Hfr chromosome begins to move to recipient cell.
Conjugation tube
3. Homologous sections of chromosome synapse.
Box 9.3, Figure: 1
Caption:
Recombination occurs in bacteria when a section of an Hfr chromosome enters a non-Hfr cell and integrates into its chromosome via crossing over.
4. Cells separate. Section of Hfr chromosome integrates into recipient chromosome by crossing over.

8. Some comparisons between asexual and sexual reproduction
Figure 9.9
Some comparisons between asexual and sexual reproduction
Asexual reproduction
Sexual reproduction
Generation 1
Generation 2
Figure: 9.9
Caption:
In this diagram, each female symbol and male symbol represents an individual. In the hypothetical example given here, every individual produces four offspring over the course of their lifetime, sexually reproducing individuals produce half males and half females, and all offspring survive to breed.  Question How many asexually produced offspring would be present in generation 4? How many sexually produced offspring?
Generation 3
So, what good are males???

9. Genetic Recombination: Sexual Reproduction
What are the benefits?
Two copies of each gene (provides instructions)
“Sharing” of beneficial genes
“Infinite” number of combinations (variation)

10. Genetic Recombination: Sexual Reproduction
What are the Costs?
Courtship expenses
Two parents investing resources
“Complicated” process to make gametes
Dangerous!

11. Genetic Recombination: Sexual Reproduction
What are the Costs?
Courtship expenses
Two parents investing resources
“Complicated” process to make gametes
Dangerous!

12. Genetic Recombination: Sexual Reproduction
What are the Costs?
Courtship expenses
Two parents investing resources
“Complicated” process to make gametes
Dangerous!

13. Genetic Recombination: Sexual Reproduction
What are the Costs?
Courtship expenses
Two parents investing resources
“Complicated” process to make gametes
Dangerous!

14. Life Cycle Strategies Involving Sexual Reproduction
Diploid Dominant (two copies of each chromosome)
Haploid Dominant (one copy of each chromosome)
Alteration of Generations

15. Figure 9.7a Diploid dominant 2n MEIOSIS: 2n >> n
Diploid adult
MITOSIS
FERTILIZATION
MEIOSIS:
2n >> n
Haploid gametes (n)
Diploid zygote
Diploid dominant
Figure: 9.7a
Caption:
(a) In animals, the gametes are the only haploid cells. Meiosis occurs in special reproductive tissues. 2n

16. Figure 9.7b Haploid dominant MEIOSIS MITOSIS Haploid cell Diploid cell
Caption:
(b) In many algae, the fertilized egg is the only diploid cell. When this cell undergoes meiosis, the haploid cells that are produced go on to form a multicellular adult.
Haploid adult
MITOSIS
FERTILIZATION
Haploid gametes

17. Alternation of generations
Figure 9.7c, upper
Alternation of generations
MEIOSIS
MITOSIS
Haploid cells
Diploid plant
Haploid gametes
Figure: 9.7c, upper
Caption:
(c) In land plants and some algae, there is a multicellular diploid stage and a multicellular haploid stage. Typically, one of these two stages is larger in size and longer in life span than the other. In ferns, the diploid stage is more prominent; in mosses the haploid stage is more prominent.
Haploid plant
Diploid cell
MITOSIS
MITOSIS
FERTILIZATIION

18. Figure 9.10a
Evidence for the benefits of sexual reproduction: resistance
Snails subject to parasitism by trematode worms (Lively)
Figure: 9.10a
Caption:
(a) Potamopyrgus antipodarum is a species of freshwater snail native to New Zealand. Some individuals in this species reproduce only sexually, while others reproduce only asexually.

19. Frequency of infection by parasites
Figure 9.10b
Are genetically diverse populations more resistant to parasites?
0.40
0.30
0.20
0.15
Male frequency
0.10
0.05
Figure: 9.10b
Caption:
(b) The x-axis (abscissa) on this graph plots the proportion of snails infected with trematode worms in a population while the y-axis (ordinate) plots the proportion of individuals in the same population that are male. (The proportion of males is an index of how many individuals reproduce sexually.)
0.01
0.00
0.00
0.05
0.15
0.30
0.50
Frequency of infection by parasites

20. Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis reduces the chromosome number by half, enabling sexual recombination to occur.
Meiosis of diploid cells produces haploid daughter cells, which may function as gametes. (Fig. 9.2a-c, 9.3)

21. Figure 9.2c
A full complement of chromosomes is restored during fertilization.
Female gamete
n = 23 in humans
Male gamete
n = 23 in humans
Fertilization
Diploid offspring contains homologous pair of chromosomes
Figure: 9.2c
Caption:
Meiosis reduces chromosome number by one-half. In diploid organisms, the products of meiosis are haploid.

22. Homologous pair of premeiotic chromosomes
Figure 9.2a
Each chromosome replicates prior to undergoing meiosis.
Maternal chromosome
Paternal chromosome
(n = 23 in humans)
(n = 23 in humans)
Duplication in
S phase
Sister chromatids
Figure: 9.2a
Caption:
Meiosis reduces chromosome number by one-half. In diploid organisms, the products of meiosis are haploid.
Centromere
Homologous pair of premeiotic chromosomes

23. Sister chromatids separate at meiosis II
Figure 9.2b
During meiosis, chromosome number in each cell is reduced.
Parent cell contains homologous pair of chromosomes
MEIOSIS I
Homologs separate
at meiosis I
Daughter cells contain just one homolog
MEIOSIS II
Sister chromatids separate at meiosis II
Figure: 9.2b
Caption:
Meiosis reduces chromosome number by one-half. In diploid organisms, the products of meiosis are haploid.
Four daughter cells contain one chromosome each. These cells become gametes.

24. Figure 9.3, left PRIOR TO MEIOSIS MEIOSIS I
Homologous chromosomes separate.
Chromosomes replicate, forming sister chromatids.
Tetrad (4 chromatids from homologous chromosomes)
Sister chromatids
Chiasma
Figure: 9.3, Left
Caption:
Exercise Label prophase, metaphase, anaphase, and telophase cells of meiosis I and meiosis II.
1. Chromosomes replicate in parent cell.
2. Synapsis of homologous chromosomes. Crossing over of non-sister chromatids.
3. Tetrads migrate to middle of cell.
4. Homologs separate.

25. Figure 9.3, right MEIOSIS II Sister chromatids separate
Caption:
Exercise Label prophase, metaphase, anaphase, and telophase cells of meiosis I and meiosis II.
5. Cell divides.
6. Chromosomes begin moving to middle of cell.
7. Chromosomes line up at middle of cell.
8. Sister chromatids separate.
9. Cell division results in four daughter cells.

26. Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis reduces the chromosome number by half, enabling sexual recombination to occur.
Gametes undergo fertilization, restoring the diploid number of chromosomes in the zygote.
23 pairs of chromosomes in humans
But what about the difference in size
between the egg and sperm?
Can be “extrachromosomal” factors in cytoplasm of egg:
Mitochondria, chloroplasts, infectious agents, chemicals

27. Box 9.1 Figure 1 Box 9.1 Figure 1 Caption:
Chromosomes undergoing mitosis are arranged randomly when first observed with the microscope. To determine a karyotype, a technician groups chromosomes by pairs and arranges them by number, as shown here. Question Is this human karyotype normal or does it reveal the presence of an extra chromosome?

28. Figure 9.1a,b 12 types of chromosomes in the lubber grasshopperk a d j X i h f c g
Each type of chromosome has two homologs.
Figure: 9.1a,b
Caption:
(a) Letters are placed next to each of the 12 distinct types of chromosomes found in lubber grasshopper cells. Note there are two of each type of chromosome. (b) The two members of a chromosome pair are called homologs. In this drawing, homologous chromosomes are indicated in blue and red. e k b d a j f X i h c g

29. Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis and fertilization introduce genetic variation in several ways:
Independent assortment of homologous pairs at metaphase I:
Each homologous pair can orient in either of two ways at the plane of cell division. (Fig. 9.5a,b)
The total number of possible outcomes = 2n (n = number of haploid chromosomes). (Fig. 9.6)
Crossing over between homologous chromosomes at prophase I.

30. Figure 9.5a Hypothetical example Eye color Hair color
Gene that contributes to brown eyes
Gene that contributes to blue eyes
Gene that contributes to black hair
Gene that contributes to red hair
Figure: 9.5a
Caption:
(a) In this hypothetical example, genes that influence hair color and eye color in humans are located on different chromosomes.
Maternal chromosome
Paternal chromosome
Maternal chromosome
Paternal chromosome

31. Figure 9.5b
During meiosis I, tetrads can line up two different ways before the homologs separate. OR
Figure: 9.5b
Caption:
(b) This diagram shows how gametes with different combinations of genes result from separation of homologous chromosomes during meiosis I.
Brown eyes Black hair
Blue eyes Red hair
Brown eyes Red hair
Blue eyes Black hair

32. Figure 9.6
Crossing over
EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING because of crossing over
Figure: 9.6
Caption:
This example shows some of the possible results of self-fertilization in an organism with four chromosomes (2n = 4).  
Exercise To the last line in this figure, add sketches showing the chromosome complements in additional offspring produced by selfing.
1. Parent cell with four chromosomes.
2. Crossing over during meiosis I.
3. Homologs separate. (Pairing of chromosomes depends on independent assortment.)
5. Offspring produced by selfing (only some of the possibilities shown.)
4. Gametes produced by meiosis II.

33. Box 9.2, Figure 1a,b: Crossing over involves breakage and reunion of chromatids
Shape of chromosome 9 varies in two maize strains
Knob
No knob
Long
Short
Strain 1
Strain 2
Genes on chromosome 9 also vary
Box 9.2, Figure 1a,b
Caption:
(a) In certain strains of maize, the size and shape of chromosome 9 differs. (b) The distinctive versions of chromosome 9 also contain distinctive genes that affect the color and texture of kernels in adult maize plants.
Colored kernels
Colorless kernels
Waxy kernels
Starchy kernels
Strain 1
Strain 2

34. Box 9.2, Figure 1c Predictions of crossing over hypothesis
If crossing over results in exchange of genetic material between two chromosomes, the products of meiosis will look like this:
Products of meiosis
Chromosome shape:
Box 9.2, Figure 1c
Caption:
(c) These diagrams show the types of gametes and offspring traits that should occur if crossing over results in the physical exchange of chromosome segments.
Long with knob
Short with knob
Long with no knob
Short with no knob
Traits contributed to offspring:
Colored, waxy kernels
Colored, starchy kernels
Colorless, waxy kernels
Colorless, starchy kernels
Experimental results support these predictions

35. Figure 9.4c Figure: 9.4c Caption:
Question and Exercise  (c) Is this meiosis I or II? What phase?

36. Figure 9.4b Figure: 9.4b Caption:
Question and Exercise  (b) What stage of meiosis I is illustrated here? Is this cell haploid or diploid?

37. Figure 9.4d Figure: 9.4d Caption:
Question and Exercise (d) What is happening in this photograph?

38. The Consequences of Meiotic Mistakes
Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II.
Fertilization can result in embryos that are 2n (a “trisomy”) or 2n - 1. (Fig. 9.11)
Abnormal copy numbers of one or more chromosomes is usually, but not always, fatal (Example: Down syndrome). (Fig. 9.12)
Human survivors: trisomics = 13, 18, 21

39. Figure 9.11
NONDISJUNCTION at Meiosis I: most common cause, weak meiosis I
alignment checkpoint in females???
n + 1
n + 1
n – 1
2n = 4   n = 2
n – 1
Figure: 9.11
Caption:
If homologous chromosomes fail to separate during meiosis I, the gametes that result will have an extra chromosome or lack a chromosome.  
Exercise Nondisjunction can also occur during meiosis II, after meiosis I has proceeded normally. Starting with a parent cell like the one shown here, make a diagram showing each step in meiosis when one set of sister chromatids fails to disjoin at meiosis II. How many of the resulting gametes are normal? How many have an extra chromosome or lack a chromosome?
1. Meiosis I starts normally. Tetrads line up in middle of cell.
2. Then one set of homologs does not separate (= nondisjunction).
3. Meiosis II occurs normally.
4. All gametes have an abnormal number of chromosomes--either one too many or one too few.

40. Incidence of Down syndrome per number of births
Figure 9.12
1 46
Incidence of Down syndrome per number of births
1 100
1 290
Figure: 9.12
Caption:
This graph plots mother’s age (on the x-axis) versus the incidence of Down syndrome, expressed as the number of affected infants per number of births.  Question Suppose that you are an obstetrician. Based on these data, at what age would you recommend that pregnant mothers undergo procedures to check the karyotype of the embryos they carry?
1 880
1 1200
1 2300
1 1600 28 32 37 42 20 24 47
Age of mother (years)

41. Other Consequences of Meiosis
Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II.
The resulting 2n gametes, if fertilized by normal sperm, create 3n zygotes (triploid).
Organisms with an odd number of chromosome sets cannot produce viable gametes (Example: seedless fruits).
3n = 2X1 chromosome separation
at meiosis I = unbalanced gametes,
undeveloped seeds

42. So where does this take us?
How do mitosis and meiosis figure into the passage of genetic information?
What are “patterns of inheritance”?
How do genes determine organismic characteristics