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Wheeler High School
The Center for Advanced Studies in Science, Math &
Technology
Sex Determination and Sex Chromosomes
Post-AP DNA/Genetics – Ms. Hager
Sexual Differentiation and Life Cycles
•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.
Isogametes
Post-AP DNA/Genetics – Ms. Hager
Sexual Differentiation
and Life Cycles
•In maize (Zea mays), the
diploid sporophyte stage
predominates and both
male and female
structures are present on
the adult plant, indicating
that sex determination
must occur differently in
different tissues of the
same plant.
Post-AP DNA/Genetics – Ms. Hager
Sexual Differentiation and Life Cycles
The nematode worm
Caenorhabditis elegans
has two sexual
phenotypes: males, which
have only testes, and
hermaphrodites, which
have both testes and
ovaries. Self-fertilization
occurs in the
hermaphrodites and
produces primarily
hermaphrodite offspring,
with less than 1% male
offspring.
Male or Hermaphrodite? How can
you tell?
Sex determination in C. elegans results from the
presence of only one X chromosome in the
males and two in the hermaphrodites.
Figure 7- 4a
Copyright © 2006 Pearson Prentice Hall, Inc.
The Y Chromosome Determines
Maleness in Humans
• The human karyotype shows that one pair of
chromosomes differs in males and females:
females have two X chromosomes; males
have one X and one Y chromosome.
Post-AP DNA/Genetics – Ms. Hager
The traditional human karyotypes derived from
a normal female and a normal male. Each
contains 22 pairs of autosomes and two sex
chromosomes. The female (a) contains two
X chromosomes, while the male (b) contains
one X and one Y chromosome (see arrows).
Figure 7-6
Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-6a
Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-6b
Copyright © 2006 Pearson Prentice Hall, Inc.
Klinefelter and Turner Syndrome
• Persons with Klinefelter syndrome have
male genitalia but have more than one X
chromosome (usually XXY, or a 47,XXY
karyotype).
• Persons with Turner syndrome usually have
a single X chromosome and no Y
chromosome (45,X karyotype) and have
female genitalia. Such syndromes provide
evidence that the Y chromosome determines
maleness.
Post-AP DNA/Genetics – Ms. Hager
Klinefelter and Turner Syndrome
The karyotypes and phenotypic depictions of individuals with (a) Klinefelter
syndrome (47,XXY) and (b) Turner syndrome (45,X).
Figure 7-7
Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-7a
Copyright © 2006 Pearson Prentice Hall, Inc.
Klinefelter Syndrome
Figure 7-7b
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Turner syndrome
Frequency of XXY Individuals
Table 7-1
Copyright © 2006 Pearson Prentice Hall, Inc.
Why does ‘Y’ make the Guy?
Let’s ask SRY……………..
•Sex-determining region
(SRY) codes for testis
determining factor (TDF).
–Around 6-8 weeks TDF
becomes present which
causes the
undifferentiated gonadal
tissue of the embryo to
form testes.
–Y chromosome contains
the male-specific region
of the Y (MSY) and a sexdetermining region of the
Y (SRY)
Post-AP DNA/Genetics – Ms. Hager
Any Questions?
Wheeler High School
The Center for Advanced Studies in Science, Math &
Technology
X Inactivation and Dosage Compensation
Post-AP DNA/Genetics – Ms. Hager
Dosage Compensation
• The term ‘dosage compensation’ describes
the genetic mechanisms that balance the
dose of X chromosome gene expression in
females and males.
Post-AP DNA/Genetics – Ms. Hager
Barr Bodies
•The Barr body is an
inactive X chromosome
•No Barr bodies in males
•If one of the 2 X
chromosomes is inactive
in the female, then the
dosage of genetic
information expressed in
males is the same as
females.
Figure 7-9
Copyright © 2006 Pearson Prentice Hall, Inc.
Dosage Compensation Prevents
Excessive Expression of X-Linked
Genes in Humans and Other
Mammals
Occurrence of Barr
bodies in various
human
karyotpyes
where all X
chromosomes
except 1 are
inactivated.
Post-AP DNA/Genetics – Ms. Hager
The Lyon Hypothesis
•Mary Lyon hypothesized
that X chromosome
inactivation occurs
randomly in somatic cells
at some point during early
embryonic development.
•Possibly during the
blastocyst stage
•Once inactivation occurs,
all decedent cells have the
same X inactivated as the
parent cell
(a) A calico cat, where the random distribution of
orange and black patches illustrates the Lyon
hypothesis. The white patches are due to another
gene; (b) A tortoiseshell cat, which lacks the white
patches characterizing calicos.
Figure 7-11
Copyright © 2006 Pearson Prentice Hall, Inc.
Depiction of the absence
of sweat glands (shaded
regions) in a female
heterozygous for the Xlinked 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.
Figure 7-12
Copyright © 2006 Pearson Prentice Hall, Inc.
Mechanism of X Inactivation
Critical
gene for Xinactivation
• Xist lacks open reading frames (ORF) which are
necessary for translation. Since there are no
ORF’s in this region you only have transcription
and NOT translation.
– Scientists speculate that RNA products of Xist
produce a ‘cage’ that entraps and inactivates the
chromosome
Sex Determination in Drosophila
• The Ratio of X Chromosomes to Sets of
Autosomes Determines Sex in Drosophila
The Y chromosome is
not involved in D.
melanogaster sex
determination
Post-AP DNA/Genetics – Ms. Hager
X Chromosomes and Autosomes
• In 1916, Calvin Bridges realized that the
critical factor in determining the sex is the
ratio of X chromosomes to the number of
haploid sets of autosomes.
– In this case it is not the ‘Y that makes the
guy’…in fact, it’s the expression of genes on the
X chromosome that cause a fly to be male
– Females = XX
– Males = X (may have the Y but don’t need to)
Post-AP DNA/Genetics – Ms. Hager
Figure 7-13
Copyright © 2006 Pearson Prentice Hall, Inc.
Bilateral gynandromorph
formed following the loss
of one X chromosome in
one of the 2 cells during
the 1st mitotic division.
The left side of the fly is
male and the right is
female.
Figure 7-14
Copyright © 2006 Pearson Prentice Hall, Inc.
Temperature Variation Determines
the Sex of Reptiles
Three different patterns of temperature-dependent sex
determination (TSD) in reptiles. The relative pivotal
temperature is crucial to sex determination during a critical
point during embryonic development (FTfemaledetermining temperature; MTmale-determining
temperature).
Any Questions?
Figure 7-11a
Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-11b
Copyright © 2006 Pearson Prentice Hall, Inc.
7.5 Dosage Compensation
Prevents Excessive
Expression of X-Linked
Genes in Humans and Other
Mammals
7.5.3 The Mechanism of Inactivation
7.6 The Ratio of X Chromosomes
to Sets of Autosomes
Determines Sex in
Drosophila
Figure 7-15
Copyright © 2006 Pearson Prentice Hall, Inc.