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Active Lecture PowerPoint® Presentation for
Essentials of Genetics
Seventh Edition
Klug, Cummings, Spencer, Palladino
Chapter 5
Sex Determination and Sex
Chromosomes
Copyright © 2010 Pearson Education, Inc.
Outline
• Life Cycles & Sexual Differentiation
• Chlamydomonas
• Maize (Zea mays)
• Caenorhabditis elegans
• Humans
• Drosophila
• Reptiles
Asexual reproduction
vs.
Sexual reproduction
Life Cycles Depend on Sexual
Differentiation
• In multicellular organisms, it is important
to distinguish between:
1. primary sexual differentiation
- involves only gonads where gametes are
produced
and
2. secondary sexual differentiation
- involves overall appearance of organism
Sexual Differentiation
• Unisexual (dioecious or gonochoric):
- individuals contain only male or female reproductive
organs
• Bisexual (monoecious or hermaphroditic):
- individuals contain both male and female reproductive
organs
- can produce both male and female gametes
Life Cycle of Chlamydomonas
• spend most of life cycle in haploid phase
• asexually produce daughter cells by
mitotic divisions
• under unfavorable nutrient conditions,
certain daughter cells function as
gametes
Life Cycle of Chlamydomonas
• two gametes that fuse together during
mating are not morphologically
distinguishable
• such gametes are called isogametes
Figure 5-1
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Chlamydomonas Mating
•
Chlamydomonas haploid gametes are of
two mating types: mt– and mt+
• mt– cells can mate only with mt+ cells,
and vice versa
• there are chemical differences between
these mating types, though no visible
difference exits
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Maize (Zea mays) Life Cycle
• The diploid sporophyte
stage predominates
• Both male and female
structures are present on
adult plant (monoecious)
• Thus, sex determination
occurs differently in
different tissues of same
plant
Figure 5-2
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Prentice Hall, Inc.
Maize (Zea mays) Life Cycle
Figure 5-2
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Caenorhabditis elegans
•
The nematode worm C. elegans
has two sexual phenotypes:
1. Males - have only testes
2. Hermaphrodites - have both testes and ovaries
•
Males are X; hermaphrodites are XX
•
C. elegens lacks a Y chromosome. Maleness determined
by genes on both the X chromosome and autosomes
•
Ratio of X chromosomes:autosomes determines the
sex:Hermaphrodites have a ratio of 1.0 and males 0.5
Caenorhabditis elegans
• Self-fertilization occurs in
hermaphrodites and produces primarily
hermaphrodite offspring, with < 1% male
offspring
• As adults, males can mate with
hermaphrodites, producing ~½ male and
½ hermaphrodite offspring
Figure 5-3 Copyright © 2006 Pearson Prentice Hall, Inc.
X and Y Chromosomes in Sex
Determination
• X and Y chromosomes were first linked to
sex determination early in the twentieth
century
• XX/XO (Protenor )mode of sex determination
• XX/XY (Lygaeus) mode of sex determination
XX/XO (Protenor) Mode of Sex Determination
• Depends on random distribution of X
chromosome into ½ of male gametes
• The presence of two X chromosomes in
zygote results in female offspring
• The presence of only one
X chromosome in zygote
results in male offspring
http://natureproducts.net/Animals/Insects/Butterflies/Papilio
_protenor2.jpg
Figure 5-4a
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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) result in female
offspring
• Zygotes with one X and one Y
chromosome (heterogametous)
result in male offspring
http://perso.orange.fr/groisynature/images/punaiseimage/Lygaeus_saxatil
is_050815DSCN4482groisyweb.jpg
Figure 5-4b
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ZZ/ZW Sex Determination
• Females are heterogametic (ZW) sex and
males are homogametic (ZZ) sex
• Examples: some birds, fish, reptiles
Sex Determination in Humans
• The 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
• Y chromosome determines maleness in
humans
Sex Determination in Humans
Figure 5-5 Copyright © 2006 Pearson Prentice Hall, Inc.
Klinefelter Syndrome
• Persons with Klinefelter syndrome
have more than one X chromosome
(usually XXY or a 47,XXY karyotype)
• Occurs 1 in 1000 male births
• Have male genitalia; feminine sexual
development not entirely suppressed
Klinefelter Syndrome
Figure 5-6
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Turner Syndrome
• Persons with Turner syndrome usually
have a single X chromosome and no Y
chromosome (45, X karyotype)
• Have female genitalia; ovaries are
rudimentary
• They are short, have a low hairline,
relatively broad chest and folds of skin on
the neck.
• Occurs 1 in 3000 female births.
Turner Syndrome
Figure 5-6
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XXX Condition (Poly X females)
• The presence of three X chromosomes along with a
normal set of autosomes (47,XXX) results in female
differentiation
• Frequently, 47,XXX women have no distinctive
features other than a tendency to be tall and thin.
• 1 in 1000 female births
• Although a few are sterile, many menstruate regularly
and are fertile, and slight tendency towards mental
retardation may occur
XYY Condition
• Only consistently shared characteristic
found so far in the 47,XYY karyotype is
that such males are over 6 feet tall
Table 5-1
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Y Chromosome
• Y is a unique chromosome that determines sex in
mammals.
• Y chromosome contains far fewer genes than X (Y has
about 75 genes, X about 900-1400).
• Present on both ends of Y chromosome are the socalled pseudoautosomal regions (PARs)
• PARs share homology with regions on X chromosome
and synapse and recombine with it during meiosis
• The presence of such a pairing region is critical to
segregation of X and Y chromosomes during male
gametogenesis
Sexual differentiation in humans
• By the fifth week the human embryo is potentially
hermaproditic. There is no phenotypic difference in the
gonadal primordia, the tissues that will form the gonad.
• Gonadal ridge tissue developes to form either the
male or female gonads.
• Primodial germ cells migrate into these ridges.
• The cortex can develop into an ovary while the medula
can develop into a testis.
• In addition, two sets of undifferntiated ducts are
present. Wolffian ducts that will differentiate into male
reproductive tract and the Mullarian ducts that will
become structure of the female reproductive tract.
Y Chromosome
• Y chromosome
also contains
male-specific
region of the Y
(MSY)
• This includes Sexdetermining
region of the Y
(SRY)
Figure 5-7 Copyright © 2006 Pearson Prentice Hall, Inc.
Y Chromosome
• SRY or a closely related gene determines sex in all
mammals.
• In humans, the absence of Y chromosome almost
always leads to female development. Thus, SRY is
absent from X chromosome.
• SRY is the gene responsible for male sex
determination.
• It encodes a gene product that triggers the
undifferentiated embryos to produce testes.
• At 6-8 weeks the SRY gene becomes active and
causes the undifferentiated gonads of the embryo to
Y Chromosome
•
The testis-determining factor (TDF) is a protein
encoded by a gene in the SRY that triggers testes
formation
•
The MSY consists of three regions:
1. X-transposed region: originally derived from the X; nearly
identical to region in X called Xq21
2. X-degenerative region: originally derived from the X; distantly
related to parts in X
3. Ampliconic region: no counterpart in X, genes associated with
testes development
Q: In which region is the SRY located?
We’re Getting More Boys!!!
• Primary sex ratio vs. Secondary sex ratio
• Ratio of males to females in humans is
not 1.0!
• Why?
Dosage Compensation: Barr Bodies
• Darkly staining body in female nuclei that is
absent in male cells
• Barr body is an inactivated X chromosome
• Mechanism for dosage compensation
– If one X in females in inactivated, dosage of
genetic information that can be expressed in
males and females is equivalent
Barr Bodies
Female
Male
Figure 5-8
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Occurrence of Barr Bodies
None inactivated
One inactivated
Two inactivated
Three inactivated
The Lyon Hypothesis
• The inactivation of X chromosomes occurs
randomly in somatic cells at a point early in
embryonic development
• Once inactivation has occurred, all progeny
cells have same X chromosome
inactivated
• Makes females mosaics for X-linked traits
Calico cat
Figure 5-10
Tortoiseshell cat
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This figure not in the Sixth edition
Depiction of the absence of sweat
glands (shaded regions) in a female
heterozygous for the X-linked condition
anhidrotic ectodermal dysplasia.
The locations vary from female to
female, based on the random pattern
of X chromosome inactivation during
early development, resulting in unique
mosaic distributions of sweat glands in
heterozygotes.
Q: Are females homozygous for Xlinked traits mosaics also?
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The Mechanism of Inactivation:
Imprinting
• X-inactivation Center (Xic)
– Major control unit in humans
– Expression occurs only on X chromosome that is
inactivated
– Contains four genes
• X-inactive specific transcript (XIST)
– Believed to be critical gene in Xic
– Lacks an extended open reading frame (ORF)
– RNA product along with other RNA products of
Xic genes may coat X chromosome that
produced it
Sex Determination in Drosophila
• Females are XX & males are XY
• Unlike in humans, the Y chromosome is not involved
in sex determination in Drosophila
• Ratio of X chromosomes to sets of autosomes (X:A)
determines sex
• 1:1 X Chromosome: sets of autosome ratio = female
1:2 X Chromosome: sets of autosome ratio = male
• This is known as the Genic Balance theory of Calvin
Bridges
Drosophila
Chromosome
compositions and
sex
Figure 5-11
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Sex Determination in Reptiles
• In many reptiles sex is determined by sexchromosome composition
• However, in some reptiles, sex determination is
achieved according to incubation temperature
of eggs during a critical period of embryonic
development
• Temperature is thought to influence the
enzymes involved in production of estrogens
and androgens
• Examples: crocodiles, some turtles and lizards
Three Different Patterns of TemperatureDependent Sex Determination in Reptiles
Figure 5-12
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Genetics, Technology & Society
(p107)
A Question of Gender: Sex selection in
humans
Problems
Q 32-34 (p113)
The Problem of Cat Cloning
Cloned Cats are not Copy-Cats !