1. 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. 2n
Cell division (mitosis)
And development
fertilization +
Sperm egg
(1n) (1n)
One-celled
Zygote--> 2n
gametes
meiosis

2. The Events of Meiosis Meiosis I
Replicated, homologous chromosomes undergo synapsis; crossing over occurs.
Homologous pairs align at the cell center; are pulled into two different daughter cells.
Each chromosome of the haploid daughter cells consists of two sister chromatids.

3. The Events of Meiosis Meiosis II:
Sister chromatids are pulled apart at the centromere and move to opposite poles, forming a total of four haploid daughter cells.

4. A full complement of chromosomes is restored during fertilization.
Female gamete (1n)
Male gamete (1n)
Fertilization
Diploid offspring contains homologous pairs of chromosomes
Figure: 9.2c
Caption:
Meiosis reduces chromosome number by one-half. In diploid organisms, the products of meiosis are haploid.

5. The problem with 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

6. The Role of Sexual Reproduction in Evolution
Natural Selection favors genotypes that pass on the greatest number of genes to the next generation
Sexual reproduction in a population should decline in frequency relative to asexual reproduction.
Asexual reproduction: no males are needed, all individuals can produce offspring.
Sexual reproduction: only females can produce offspring, therefore fewer are produced.

7. So why aren’t all species asexual?
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.

8. The Role of Sexual Reproduction in Evolution
Sexual reproduction produces genetically variable offspring (crossing over, independent assortment, random union of gametes)
Sexual reproduction may exist because it provides genetic variability that reduces susceptibility of a population to pathogen attack.

9. Testing the hypothesis
Snails subject to parasitism by trematode worms
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.
Potamopyrgus can reproduce sexually (males & females)
or asexually (females only)

10. Frequency of infection by parasites
Is sexual reproduction more common in populations with lots of 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

11. Mistakes during meiosis can lead to offspring with abnormal numbers of chromosomes

12. Humans have 23 pairs of homologous chromosomes
22 pairs of autosomes
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?
1 pair of sex chromosomes
(XX in females, XY in males)
23 pairs total = 46 chromosomes

13. Human gametes have one of each type of chromosome
Female:
22 autosomes
1 sex chromosome (X)
Male:
1 sex chromosome (X or Y)
22 + 1X
egg
22 + 1X
22 + 1Y or
sperm
sperm

14. The Consequences of Meiotic Mistakes
Nondisjunction occurs 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 (monosomy).

15. 2. Then one set of homologs does not separate (nondisjunction).
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?
2. Then one set of homologs does not separate (nondisjunction).
1. Meiosis I starts normally. Tetrads line up in middle of cell.
4. All gametes have an abnormal number of chromosomes--either one too many or one too few.
3. Meiosis II occurs normally.

16. The Consequences of Meiotic Mistakes
Nondisjunction occurs when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II.
Abnormal copy numbers of one or more chromosomes in humans is usually, but not always, fatal (Example: Down syndrome).

17. Karyotype of a person with Down syndrome

18. Incidence of Down syndrome per number of births
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 1600
1 1200
1 2300 28 32 37 42 20 24 47
Age of mother (years)

19. Karyotype of a person with Turner syndrome

20. Karyotype of a person with Klinefelter syndrome

21. The Consequences of Meiotic Mistakes
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).

22. The Consequences of Meiotic Mistakes
Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II.
Organisms with an odd number of chromosome sets cannot produce viable gametes (Example: seedless fruits).
Bananas
Seedless watermelon
Triploid grass carp are
used to control the growth
of aquatic plants