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Bellringer
In humans, pointed eyebrows (B) are
dominant over smooth eyebrows (b). Mary’s
father has pointed eyebrows, but she and her
mother have smooth.
• What is the genotype of her father?
• What are the chances that her future sibling
will have smooth eyebrows as well?
Which of these would be a
normal gamete?
•
•
•
•
•
GgRr
GRr
Gr
GgR
None of these are normal gametes.
Chapter 10
Meiosis and Sexual
Reproduction
Why Mitosis?
Mitosis maintains the same chromosome
number in the daughter cells as were found
in the original cell.
Why Meiosis?
Meiosis (or reduction division) reduces the
chromosome number from 2N to N during
the production of gametes.
Asexual Reproduction
• Only one parent
• Advantages:
– Fast
– Many offspring
– All identical—good when well adapted to the
environment
– No need to find a mate
Disadvantage: no variations
Figure 13.1 The asexual reproduction of a hydra
Sexual Reproduction
• Usually involves two parents
• Involves two events: meiosis and
fertilization
• Advantage: recombination of the inherited
traits of two parents; therefore, offspring
may be able to survive environmental
changes or other stress
Figure 13.2 Two families
Vocabulary
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•
•
•
•
Diploid (2N)
Haploid (1N)
Autosomes
Sex chromosomes
Homologous chromosomes
Figure 13.4 The human life cycle
Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization
(syngamy)
Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number
Figure 13.7 The stages of meiotic cell division: Meiosis I
Figure 13.10 The results of crossing over during meiosis
Figure 13.9 The results of alternative arrangements of two homologous chromosome
pairs on the metaphase plate in meiosis I
Figure 13.7 The stages of meiotic cell division: Meiosis II
Meiosis Square Dance
• https://www.youtube.com/watch?v=iCL6d0
OwKt8
Three Sources of Variation in
Sexual Reproduction
1. Crossing-over
2. Independent assortment (223 possible
combinations  8 million possibilties!)
3. Random fertilization
Compare Mitosis and Meiosis
• DNA replication
• Number of divisions
• Type of cell you start with
• Where it occurs
• When it occurs
• Role
• Product
• Events
Figure 13.8 A comparison of mitosis and meiosis
Spermatogenesis
Oogenesis
Genetic Abnormalities related to
meiosis: sex chromosomes
• Turner Syndrome
(XO)
• Poly-X syndrome
(XXX)
• Klinefelter
Syndrome (XXY)
• Jacob Syndrome
(XYY)
Why no YO?
Genetic Abnormalities related to
meiosis: autosomal
• Down Syndrome
(trisomy 21)
• Trisomy 18
• Trisomy 13
• Few other autosomal
abnormalities are
known or occur in
high frequencies –
WHY?
Do not survive
past the first year
of life
Remember…
• These are not gene mutations – they are
IMBALANCES IN NORMAL
CHROMOSOME NUMBER
• How are these diagnosed?
Figure 13.3 Preparation of a human karyotype (Layer 1)
Figure 13.3 Preparation of a human karyotype (Layer 2)
Figure 13.3 Preparation of a human karyotype (Layer 3)
Fetal material can be obtained three ways:
1. Amniocentesis
2. Chorionic villi sampling
3. IVF
Controversial…why?
Figure 13.3 Preparation of a human karyotype (Layer 4)
Figure 13.x2 Human female chromosomes shown by bright field G-banding
Figure 13.x3 Human female karyotype shown by bright field G-banding of
chromosomes
Figure 13.x5 Human male karyotype shown by bright field G-banding of
chromosomes
What causes chromosomal issues
like these: NONDISJUNCTION?