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.
fertilization
Cell division (mitosis)
And development
+
Sperm
(1n)
egg
(1n)
One-celled
Zygote--> 2n
gametes
meiosis
2n
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.
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.
A full complement of chromosomes is restored during
fertilization.
Female
gamete
(1n)
Fertilization
Male
gamete
(1n)
Diploid offspring
contains homologous
pairs of chromosomes
The problem with sexual reproduction
Asexual reproduction
Generation 1
Generation 2
Generation 3
Sexual reproduction
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.
So why aren’t all species asexual?
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.
Testing the hypothesis
Snails subject to parasitism by trematode worms
Potamopyrgus can reproduce sexually (males & females)
or asexually (females only)
Is sexual reproduction more common in populations
0.40
with lots of parasites?
0.30
Male frequency
0.20
0.15
0.10
0.05
0.01
0.00
0.00
0.05
0.15
0.30
Frequency of infection by parasites
0.50
Mistakes during meiosis can lead to offspring with
abnormal numbers of chromosomes
Humans have 23 pairs of homologous chromosomes
2n = 46
22 pairs of autosomes
1 pair of sex chromosomes
(XX in females, XY in males)
23 pairs total = 46 chromosomes
Human gametes have one of each type of chromosome

Female:
 22 autosomes
22 +
1X
 1 sex chromosome (X)

egg
Male:
 22 autosomes
 1 sex chromosome (X or Y)
22 +
1X
sperm
or
22 +
1Y
sperm
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 + 1
(a “trisomy”) or 2n - 1 (monosomy).
NONDISJUNCTION
n+1
n+1
n–1
2n = 4
n=2
1. Meiosis I starts
normally. Tetrads
line up in middle
of cell.
n–1
2. Then one set of
homologs does
not separate
(nondisjunction).
4. All gametes have
3. Meiosis II
occurs normally. an abnormal
number of
chromosomes-either one too many
or one too few.
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).
Karyotype of a person with Down syndrome
Incidence of Down syndrome
per number of births
1
46
1
100
1
290
1
2300
1
1600
1
1200
1
880
20
24
28
32
Age of mother (years)
37
42
47
Karyotype of a person with Turner syndrome
Karyotype of a person with Klinefelter syndrome
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).
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