The Evolution of Sex Determination Mechanisms, Summaries of Nursing

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Genetic

Sex Determination

Sex is determined by the

heat of male partner during

intercourse…..

Aristotle (384-322 B.C.) Courtesy of Humphrey Yao

The Jost Paradigm Genetic Sex Gonadal Sex Phenotypic Sex Switches Genetic Sex Determination Sex Chromosomes Environmental Sex Determination Temperature, social cues, etc Courtesy of Humphrey Yao

Mammals Birds Crocodiles Turtles Snakes Amphibians Fish

Both

Both

GSD

Both

ESD

GSD

GSD

Evolution of Sex Determination Mechanisms Courtesy of Humphrey Yao

Sex Determination

• Earthly Venus - offspring of different

races

– Pierre Louis Moreau de Maupertuis

– Venus physique / The Earthly Venus

– La Haye, 1745

• Speculation on organismal adaptation to environment 100 yrs before Darwin
• Examined debate on source of humans
  • Sperm
  • Egg
• How did he test this?
  • Hint: della Porta

Genetics of Sex Determination

• rediscovery of Mendel /others suggested

genetic factor

1902 Clarence McClung: the “Accessory chromosome” 1959 Welshons & Russell: the role of the Y chromosome 1991 Lovell-Badge et al: discovery of the SRY gene

Human Primary Sex Determination

• 1. gonadal determination
• 2. chromosomal
  • a. female = XX
  • b. male = XY
• 3. number of X chromosomes not important
• 4. presence of the Y is critical

Y Chromosome

• Represents 2% of haploid complement
  • differ between species in size and gene content
  • Contains over 200 genes
  • Contains over 50 million base pairs, of which

approximately 50% have been determined

• Genes for
  • Sex determination
  • Histocompatibility
  • Spermatogenesis
  • Growth
  • Cancer

X Inactivation

• Forms the Barr Body

• Condensation of some of

the genes on one of the

X chromosome

• Why?

• Double dose of genes

thus,

• Double dose of proteins

Barr body

• Mexico City Olympics - 1968

– introduced genetic testing in the form

of a sex chromatin (Barr body)

• Barcelona games

– PCR for Y chromosome gene, SRY

Chronology

1. 1959: Y chromosome shown to determine males

2. 1966: Testis determining gene localized

-short arm of Y chromosome

3. 1986-1990: XX males and XY females identified

and examined

• isolated a 35 kilobase in region 1 of Y chromosome

-the SRY - S ex-determining R egion of the Y

SRY

• A. codes for a 223 AA protein,
• B. a transcription factor
  • ligand unknown
• C. has an HMG box region, found in other transcription factors - 'box' binds/folds the DNA - essential for sex determination - 10-14kb genomic fragment in transgenic mice = sex reversal - mutation = sex reversal
• D. found in normal males and XX males (full male genotype)
• E. lacking in normal females and XY females

Mouse SRY

• homologous region

• found in developing gonad just before testis

formation (2 days prior to testis formation)

• Also seen in the brain

• suppressed late in development

Day 14 Day 20 Day 26

Mouse testis formation - in situ whole mount for SRY

Koopman et al J Exp Zool 290 (2001)