Chapter
7
Sex Determination and
Sex Chromosomes
Lecture Presentation by
Dr. Cindy Malone,
California State University Northridge
© 2015 Pearson Education, Inc.
Chapter 7: Introduction
 In animals, including humans, differentiation of
sexes is evident via phenotypic dimorphism
 Heteromorphic chromosomes
– Dissimilar
– Example: Sex chromosomes X and Y
© 2015 Pearson Education, Inc.
Section 7.1: Sexual Differentiation
 Life cycles depend on sexual differentiation
 Primary sexual differentiation
– Involves only gonads where gametes are
produced
 Secondary sexual differentiation
– Involves overall appearance of organism
© 2015 Pearson Education, Inc.
Section 7.1: Sexual Differentiation
 Unisexual, dioecious, gonochoric
– Have only male or female reproductive organs
 Bisexual, monoecious, hermaphroditic
– Have both male and female reproductive organs
– Common in plants and animals
– Can produce egg and sperm
© 2015 Pearson Education, Inc.
Section 7.1: Chlamydomonas – Green Algae
 Chlamydomonas – green algae
– Asexual reproduction
– Some organisms spend their life cycle in haploid
phase
– Asexually producing daughter cells by mitotic
division
© 2015 Pearson Education, Inc.
Section 7.1: Isogametes
 Isogametes
– Under unfavorable nutrient conditions,
Chlamydomonas daughter cells function as
gametes
– Two gametes fuse together during mating
– Gametes not usually morphologically
distinguishable (isogametes)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-1
Section 7.1: Zea mays (Corn)
 Plant life cycles alternative between haploid
gametophyte and diploid sporophyte stages
 Maize (Zea mays)
– Diploid sporophyte stage predominates
– Both male and female structures are present on
adult plant
– Indicates sex determination occurs differently in
different tissues of same plant (Figure 7-2)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-2
Section 7.1: Caenorhabditis elegans
 C. elegans (major model organism)
– Nematode worm Caenorhabditis elegans has two
sexual phenotypes
– Males have only testes
– Hermaphrodites have both testes and ovaries
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-3
Section 7.1: The X Chromosome
 Determination of maleness
– Ratio of X chromosomes to autosomes
determines male or hermaphrodite in C. elegans
– No Y chromosome in C. elegans
– Sex determination results from presence of only
one X chromosome in males and two in
hermaphrodites
© 2015 Pearson Education, Inc.
Section 7.2: Homogametous and
Heterogametous
 Homogametous
– Producing like chromosomes
– Zygotes with two X chromosomes
– Results in female offspring
 Heterogametous
– Producing unlike chromosomes
– Zygotes with one X and one Y chromosome
– Result ins male offspring (Figure 7-4b)
© 2015 Pearson Education, Inc.
Section 7.2: Heterogametic Females
 Females as heterogametic sex
– ZZ/ZW sex determination
– Females are the heterogametic (ZW) sex
– Males are the homogametic (ZZ) sex
– Example: chickens
© 2015 Pearson Education, Inc.
Section 7.3: Y Chromosome—Maleness
 Y chromosome determines maleness
– Human karyotype
 22 pairs of autosomal chromosomes
 2 sex chromosomes
 Reveals one pair of chromosomes differs in males and
females
– Females: XX
– Males: XY
 Figure 7-5
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-5
Section 7.3: Klinefelter and Turner
Syndromes
 Klinefelter and Turner Syndrome
– Two human abnormalities
– Characterized by aberrant sexual development
– Both syndromes result from nondisjunction
 Failure of X chromosomes to segregate during meiosis
© 2015 Pearson Education, Inc.
Section 7.3: Klinefelter Syndrome
 Klinefelter syndrome (47,XXY)
– Tall, long arms and legs
– Large hands and feet
– Internal ducts are male, rudimentary testes fail to
produce sperm
– Feminine development not suppressed
 Enlarged breasts common, rounded hips
(Figure 7-6a)
© 2015 Pearson Education, Inc.
Section 7.3: Tuner Syndrome
 Turner syndrome (45,X)
– Phenotypically female
 Female external genitalia and internal ducts
 Ovaries are rudimentary
 Underdeveloped breasts
– Short stature
– Cognitive impairment
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-6
Section 7.3: 47,XXX Syndrome
 47,XXX syndrome: Triplo-X
– Three X chromosomes
– Normal set of autosomes
– Results in female differentiation
– Sometimes women are perfectly normal
– Sometimes underdeveloped secondary sex
characteristics (sterility and mental retardation)
occur
© 2015 Pearson Education, Inc.
Section 7.3: 47,XYY Condition
 47,XYY Condition
– Only consistently shared characteristic – males
are over 6 feet tall
– Subnormal intelligence
– Personality disorders
© 2015 Pearson Education, Inc.
Section 7.3: Y Chromosome
 Y chromosome and male development
– Y chromosome has at least 50 genes
– Fewer genes than X chromosome (100 genes)
 PARs: Pseudoautosomal regions
– Present on both ends of Y chromosome
– Share homology with regions on X chromosome
– Synapse and recombine with X during meiosis
© 2015 Pearson Education, Inc.
Section 7.3: MSY and SRY regions
 Pairing region critical to segregation of X and Y
chromosomes during male gametogenesis
 MSY: Male-specific region of the Y
– Nonrecombining region of Y chromosome
 SRY: Sex-determining region Y
– Located adjacent to PAR of the short arm of Y
chromosome
– Controls male development
 Figure 7-7
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-7
Section 7.3: TDF
 TDF: Testis-determining factor
– At 6–8 weeks of development SRY gene
becomes active in XY embryos
– Encodes protein that triggers testes formation
© 2015 Pearson Education, Inc.
Section 7.3: MSY
 MSY: Male-specific region Y
 23 million base pairs
 Divided into three regions
– X-transposed region (15 percent of MSY)
– X-degenerative region (20 percent)
– Ampliconic region (30 percent)
 Encodes proteins specific to development and function
of testis
© 2015 Pearson Education, Inc.
Section 7.5: Barr Bodies
 Barr bodies (sex chromatin bodies)
– Genetic mechanism compensates for X dosage
disparities
– Inactive X chromosome, highly condensed
– Darkly stained body in interphase nerve cells
observed: Barr bodies (Figure 7-8)
– Random inactivation
– Occurs early in embryonic development
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-8
Section 7.5: X-inactivation
 X-inactivation
– Explains dosage compensation
– Follows N  1 rule (N  total number of X
chromosomes)
– Then why do we have Turner and Kleinfelter
syndromes?
© 2015 Pearson Education, Inc.
Section 7.5: Inactivation and Syndromes
 Why does X-inactivation not affect
syndromes such as Turner or Kleinfelter?
– Chromosome inactivation not in early stages of
development for cells destined for gonadal tissue
– Not all X chromosomes forming Barr bodies are
inactivated
 15% escape inactivation
© 2015 Pearson Education, Inc.
Section 7.5: Lyon Hypothesis
 Lyon hypothesis
– Inactivation of X chromosome is random
– Occurs in somatic cells at early stage of
embryonic development
– All descendant cells have same X-inactivation
– Example: Calico cats and fur color/patterns
 Figure 7-10
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-10
Section 7.6: Dosage Compensation in
Drosophila
 Dosage Compensation in Drosophila
– Drosophila females have two copies of X-linked
genes
– X-inactivation not observed in Drosophila
– Male X-linked genes transcribed at twice rate of
females
© 2015 Pearson Education, Inc.
Section 7.7: Temperature Variations
 TSD: Temperature-dependent sex
determination
– Controls sex determination in reptiles
– Three different patterns of temperature sex
determination in reptiles (Figure 7-15)
– Crocodiles, most turtles, and some lizards:
incubation temperature of eggs during embryonic
development determines sex
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7-15