C4.5 - The Nature of Sex

Term Differentiation

• sex: biological term for male / female
• gender: societal term for male / female
• sperm: small gamete, male
• pollen: small gamete, male, plants
• egg: large gamete, female

Sex Determination

• human sex distinguished at birth
• external sexual structures

6 Week Old Fetus

• no anatomic sexual difference
• bipotential gland
• in presence of hormones, gonad develops into
  • testes (male)
  • ovaries (female)

Early Views of Sex Determination

• Aristotle summarized that sex is determined from
  • father’s temperature; and
  • level of excitement
  • during sexual intercourse
• Apparently, higher temp. and excitement produced boys

Advances?

• Sex and blending
• 1890s: sex determination during pregnancy and other environ. factors
  • what about fraternal twins?
• fraternal twins: twins where one is boy, other is girl

20^th Century: Study of Chromosomes

Chromosomes with XX sex pair

• hereditary roles of chromosomes understood
• 1916: fruit flies
  • females: XX
  • males: XO or XY (O is placeholder)
• 1923: human sex chromosomes discovered

Further Study

• 1959: Turner syndrome (XO)
  • very skinny
  • masculinized females
• Klinefelter (XXY)
  • feminized males
• sex determined by presence of Y chromosome

Exceptions

X chromosome (left) and Y chromosome (right)

• XX Male
  • male sex structures
  • no sperm / eggs
• XY Female
  • female sex structures
  • no sperm / eggs
• 1 / 20,000 individuals

Sex-Determining Gene

• XX male: carries part of Y on X
• XY female: missing SRY from Y
• Gene: Sex-determining region Y (SRY)

Further Test

• mouse experiment
• XX egg injected w/ SRY gene → transgenic egg
  • implanted in uterus
• XX mouse born
  • appears male

Y Chromosome

• broadly divided into 2 functional regions
• reg. 1: recombinant regions
  • at extreme tips of chromosome
• reg. 2: non-recombinant region
  • does not exchange genetic information with X

Y Clones

• Implications for inheritance of Y along male lines
• Y reproduced clonally
• Allows genetic analysis of male line
• Y-chromosomal Adam, 60 000 years b.p.

Genes on the Y

• contains other genes besides SRY
• cell housekeeping (16 genes)
• have counterparts on X
• sperm production
  • i.e. DAZ
  • 9 families
• total: 26 genes and gene families
• 1% of total genome

Evolution of Y Chromosome

• sex determined by temp. in mammalian ancestor
• autosomes only
• X and Y nearly identical
• Y chromosomes 300 my in the making

Rise of SRY

• SRY arises as mutation in X
• SRY commandeers X autosome
• Y stops swapping genes w/ X
• proto X and proto Y

Development of Y

• Y undergoes series of inversions (internal recombination)
• results in removal of genes
• Y shortens
• cannot recombine w/ X (except at tips)
• Y periodically renewed by other autosome genes
• DAZL (spermatogenesis) gene donated from autosome
  • now called DAZ
  • 4 copies

Sex Determination in Other Animals

Reptiles

• ectothermic: cold-blooded
• temperature-dependent sex determination
• crocodiles
  • warm temps: males ♂
  • cool temps: females ♀
• turtles
  • warm temps: females ♀
  • cool temps: males ♂

Birds

• endothermic: warm-blooded
• ZZ: males ♂
• ZY: females ♀
• actual mode of sex determ. unclear
• may be analogous to mammals

Other Mammals

• endothermic
• monotremes: duck-billed platypus
  • 10 sex chromosomes
  • female: XXXXXXXXXX ♀
  • male: YYYYYXXXXX ♂
• complex segregation during meiosis

Sex Determination in Mammals Evolutionarily

• Sex chromosomes necessary for mammals and other endothermic animals
• Mammals develop internally
  • exceptions: platypus
• Confers advantage onto offspring
• Temperature-related sex determination impossible