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Concepts of Genetics
Ninth Edition
Klug, Cummings, Spencer, Palladino
Chapter 7
Sex Determination
and Sex Chromosomes
Lectures by David Kass with contributions from
John C. Osterman.
Copyright
© 2009©Pearson
Education,
Inc.
Copyright
2009 Pearson
Education,
Inc.
Section 7.1
• 7.1 Life Cycles Depend on Sexual
Differentiation
• In multicellular organisms, it is important to
distinguish between:
• primary sexual differentiation
• involves only the gonads where gametes are
produced
• secondary sexual differentiation
• involves the overall appearance of the organism
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Section 7.1
• Some organisms (e.g. Chlamydomonas)
spend most of their life cycle in the haploid
phase, asexually producing daughter cells
by mitotic division.
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Copyright © 2009 Pearson Education, Inc.
Figure 7.1
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Figure 7.2
Section 7.1
• In maize (Zea mays), the
diploid sporophyte stage
predominates and both
male and female structures
are present on the adult
plant.
• This indicates that sex
determination must occur
differently in different
tissues of the same plant
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Copyright © 2009 Pearson Education, Inc.
Figure 7.3.2
Section 7.1
• The nematode worm Caenorhabditis
elegans has 2 sexual phenotypes:
• Males, which have only testes
• Hermaphrodites, which have both testes and
ovaries
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Copyright © 2009 Pearson Education, Inc.
Figure 7.4
Section 7.2
• XX/XO (Protenor) mode of sex
determination
http://www.natureproducts.net/Ani
mals/Insects/Butterflies/Papilio_prot
enor2.jpg
• depends on random distribution of the X
chromosome into half of the male gametes.
• Presence of two X chromosomes in the
zygote = female offspring
• Presence of only one X chromosome =
male offspring
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Copyright © 2009 Pearson Education, Inc.
Figure 7.5a
Section 7.2
• XX/XY (Lygaeus) mode of sex
determination:
• female gametes all have an X
chromosome
• male gametes have either an X or a Y
chromosome
• Zygotes with two X chromosomes
(homogametous) = female
offspring
• Zygotes with one X and one Y
chromosome (heterogametous) =
male offspring
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http://bugguide.net/node/view/193531/bgimage
Copyright © 2009 Pearson Education, Inc.
Figure 7.5b
Section 7.2
• ZZ/ZW sex determination:
• females are the heterogametic (ZW) sex
• males are the homogametic (ZZ) sex
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Section 7.3
• 7.3The Y Chromosome Determines
Maleness in Humans
• Human karyotype revealed that one pair of
chromosomes differs in males and
females:
• females have two X chromosomes
• males have one X and one Y chromosome
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Normal Human Karyotypes
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Figure 7.6
Section 7.3
• Persons with
Klinefelter
syndrome have:
• male genitalia
• more than one X
chromosome
(usually XXY, or a
47,XXY karyotype)
http://all4freehere.com/2009/07/what-is-klinefelters-syndrome/
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Figure 7.7a
Section 7.3
• Persons with
Turner syndrome
usually have:
• a single X
chromosome
• no Y chromosome
(45,X karyotype)
• female genitalia
http://www.lucinafoundation.org/assets/turner-syndrome.jpg
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Figure 7.7b
Section 7.3
• 47, XXX – Trisomy X
• 3 X chromosomes along with a normal set of
autosomes results in female differentiation.
• Usually normal
• In other cases, underdeveloped secondary
sex characteristics, sterility, and mental
retardation may occur.
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Section 7.3
• 47, XYY – Jacobs Syndrome
• Males are usually over 6 feet tall.
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Copyright © 2009 Pearson Education, Inc.
Figure 7.8
Section 7.3
• The Y chromosome contains far fewer
genes than the X chromosome.
http://blogs.discovermagazine.com/notrocketscience/files/2010/07/XY.jpg
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Section 7.3
• Pseudoautosomal Regions (PARs)
• regions on Y chromosome that share homology with
regions on the X chromosome
• synapse and recombine with it during meiosis
• Presence of such a pairing region is critical to
segregation of the X and Y chromosomes during male
gametogenesis
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Section 7.3
• Y chromosome
contains:
• the malespecific region
of the Y (MSY)
• a sexdetermining
region of the Y
(SRY)
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Section 7.3
• Testis-determining factor (TDF)
• a protein encoded by a gene in the SRY that
triggers testes formation.
• The MSY consists of three regions:
• X-transposed region
• X-degenerative region
• ampliconic region
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Section 7.4
• 7.4The Ratio of Males to Females in
Humans is not 1.0
• Primary sex ratio reflects the proportion
of males to females conceived in a
population.
• Secondary sex ratio reflects the
proportion of each sex that is born.
http://partywarehouse.co.nz/zen/images
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Section 7.5
• 7.5Dosage Compensation Prevents
Excessive Expression of X-Linked Genes
in Humans and Other Mammals
• Dosage compensation balances the
dose of X chromosome gene expression in
females and males.
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Section 7.5
• The inactive X is highly condensed, can be
observed in stained interphase cells, and
are referred to as Barr bodies (Figure
7.10).
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Copyright © 2009 Pearson Education, Inc.
Figure 7.11
Section 7.5
• The Lyon hypothesis states that
X-inactivation occurs randomly in somatic
cells.
• This is evident in the calico cat (Figure 7.12).
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Lyonization in
Humans:
- Red-green color
blindness
- Anhidrotic
ectodermal
dysplasia
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Figure 7.13
Section 7.5
• The X-inactivation center (Xic) is active
on the inactive X.
• It consists of the X-inactive specific
transcript (XIST) gene.
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7.6 The Ratio of X Chromosomes to Sets of
Autosomes Determines Sex in
Drosophila
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Copyright © 2009 Pearson Education, Inc.
Figure 7.15
7.7 Temperature Variation Controls Sex
Determination in Reptiles
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Section 7.7
• For all crocodiles, most turtles, and some
lizards, sex determination is achieved
according to the incubation temperature of
eggs during a critical period of embryonic
development.
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Section 7.7
• There are three different patterns of
temperature sex determination in reptiles
(Figure 7.18).
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Copyright © 2009 Pearson Education, Inc.
Figure 7.18