NAU Biology BIO 372
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BIO372 : Biological & Ecological Implications of Sex : Sex Determination

Sex Determination: Lesson

Use the outline below to guide your study of the material in this lesson. The outline indicates those topics the instructor feels are most important for you to learn in the course. You should read all the pages assigned, open and study the links, and read the terms in the glossary.

I. Chromosomal

  1. Sex chromosomes
  2. A different kind of male: a third sex or a special strategy?

II. Temperature-dependent sex determination

  1. Leopard gecko, crocodiles, and snapping turtles
    • Low and high temperatures result in females
    • Intermediate temperatures result in males
    • Estrogen overrides temperature
  2. Turtles
    • Cool temperatures result in males
    • Warm temperatures result in females
  3. Alligators
    • Cool temperatures result in females
    • Warm temperatures result in males
Black caiman: Melanosuchus niger

III. Behavior-dependent sex determination

  1. Sequential hermaphrodites: Sequence depends on ecological conditions
  2. Simultaneous hermaphrodites
  3. Potential mates

IV. Parthenogenesis

  1. Whiptail lizards
  2. Aphids
  3. Daphnia

V. Sex ratios
  1. Birds and mammals
  2. Hymenoptera: ants, wasps, bees
  3. Aphids: parthenogenesis plus sexual reproduction with males
  4. Rotifers: no males?
  5. Humans
  6. Theory
ant: Formica yessensis:

VI. Alternation of generations
  1. Cycle of asexual and sexual forms
  2. Coelenterates
  3. Moss
  4. Fern
  5. Flowering plants
  6. Chlamydomonas
Cnidaria: carybdeid from Darwin, Australia:

VII. Imprinting: dosage compensation in mammals

  1. Mammals need both parents, but frogs don't
  2. Methylation
  3. DNA competition

When you have completed this lesson, go on to Review Questions

E-mail Professor Frederiksen at

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