Reproductive Biology BI399

1
Reproductive BiologyBI-399

• Lecture 4
• Genetic basis of sex

2
Vertebrate sex chromosomes

• Homogametic
• gender with sex chromosomes that are same in
  size/shape
• sex chromosome homomorphism
• Heterogametic
• gender with sex chromosomes that are different in
  size/shape
• sex chromosome heteromorphism

3
Genetic basis of sex

• Diploid (2n)
• Mammalian chromosome compliment
• Autosomes
• humans with 22 pair 44
• Sex chromosomes
• humans with 1 pair 2
• total of 23 pair of chromosomes 46
• Male (XY)
• heterogametic for sex chromosomes
• Female (XX)
• homogametic for sex chromosomes

4
Mammalian sex chromosomes

• Female - XX
• Male - XY
• Y chromosome is necessary for creating male
  through directing development
• Activates other genes
• SRY gene
• Sex-determining region of Y chromosome
• 35 kb region on short arm
• known from humans and mice (Sry)

5
Vertebrate sex chromosomes

• Cyclostomes (hagfish and lampreys)
• sex chromosomes unknown
• Elasmobranchii
• Carchariniformes
• sex chromosomes unknown
• Rajiformes
• XY system
• Teleosts
• XY and ZW systems known
• depends on species

6
Vertebrate sex chromosomes

• Amphibians and reptiles
• XY
• some frogs, salamanders, and lizards
• ZW
• some frogs, salamanders, and snakes
• Birds
• Males
• ZZ
• Females
• ZW

7
Sex chromosomes in snakes

• Phylogenetic transition in sex chromosomes
• Females as heterogametic sex
• Differentiation in sex chromosomes
• More recent groups with greater differentiation

8
Sex chromosome peculiarities

• Some vertebrates with multiple sex chromosomes in
  normal diploid condition
• results from nondisjunction of sex chromosomes
• Anolis, Sceloporus, Tachyglossus, some marsupials
  and rodents
• male - X1X2Y
• female - X1X1X2X2

9
Lack of sex chromosomes

• Temperature-dependent sex determination (TSD)
• seen in Crocodylia, most Chelonia, and some
  lizards (leopard gecko - Eublepharus macularis)
• incubation temperatures direct the sexual
  development of developing embryos
• temperature sensitive genes that induce formation
  of gonads

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TSD
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TSD
12
Human sex chromosomes

• Autosomes Sex chromosomes Gonad Syndrome
• 44 XO ovary Turners
• 44 XX ovary normal ?
• 44 XXX ovary Super ?
• 44 XY testis normal ?
• 44 XXY testis Klinefelters
• 44 XYY testis Super ?
• 66 XXX ovary Triploids
• 66 XXY testis (nonviable)
• 44 XXsxr testis Sex reversed

13
Turners Syndrome (XO)

• Manifestations
• short stature
• webbed neck
• low hairline in back
• abnormal eye features (drooping of eyelids)
• abnormal bone development, for example a
  'shield-shaped', broad, flat chest
• absent or retarded development of secondary
  sexual characteristics that normally appear at
  puberty including sparse pubic hair, small breasts

14
Turners Syndrome (XO)

• Manifestations (cont.)
• infertility
• tearing, decreased
• menstruation, absent
• simian crease (a single crease in the palm)
• absence of normal moisture in vagina, painful
  intercourse

15
Klinefelters Syndrome (XXY)
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Klinefelters Syndrome (XXY)

• Manifestations
• small penis
• small firm testicles
• diminished pubic, axillary, and facial hair
• sexual dysfunction
• enlarged breast tissue (called gynecomastia)
• tall stature
• abnormal body proportions (long legs, short
  trunk)
• learning disabilities
• personality impairment
• simian crease