Genetics Ch. 4

• Sex Determination and Sex-Linked Characteristics

  Mechanisms and characteristics related to biological sex
• Sex Determination

  • There are several different mechanisms
  • Alternates between haploid and diploid states
  • Most organisms have two sexual phenotypes, male and female
  • Male and female gametes differ in size
• The X and Y chromosomes pair during meiosis, even though they are not homologous (the genes located on each are different)
• Sex Determination Mechanisms

  • Hermaphroditism (both sexes in the same organism)
  • Monoecious (both male and female reproductive structures in the same organism)
  • Dioecious (either male or female reproductive structures in one organism)
• Chromosomal Sex-Determination Systems

  • XX-XO system (XX female, XO male) - grasshopper
  • XX-XY system (XX female, XY male) - mammals
  • ZZ-ZW system (ZZ male, ZW female) - birds, snakes, butterflies, amphibians and fishes
  • Haplodiploidy system (haploid male, diploid female) - bees, wasps, and ants
• Heterogametic sex

  The sex that produces gametes with different sex chromosomes
• Homogametic sex

  The sex that produces gametes with the same sex chromosome
• Genic sex-determining system has no distinct sex chromosomes, sex is determined by genes on undifferentiated chromosomes
• Environmental sex determination

  Sex is determined fully or in part by environmental effects
• In Drosophila melanogaster, sex is determined by the genic balance system based on the X:A ratio (X, number of X chromosomes; A, number of haploid sets of autosomes)
• Chromosome complements and sexual phenotypes in Drosophila

  • XX AA (1.0) - Female
  • XY AA (0.5) - Male
  • XO AA (0.5) - Male
  • XXY AA (1.0) - Female
  • XXX AA (1.5) - Metafemale
  • XXXY AA (1.5) - Metafemale
  • XX AA (0.67) - Intersex
  • XO AA (0.33) - Metamale
  • XXXX AA (1.3) - Metamale
• SRY gene


  Gene on Y chromosome that determines maleness
• Role of sex chromosomes

  • X chromosome contains genetic information essential for both sexes, at least one copy required
  • Male-determining gene (SRY) is located on Y chromosome
  • Absence of Y results in female phenotype
• Androgen-insensitivity syndrome

  Caused by defective androgen receptor
• Klinefelter syndrome is the phenotype of a person with XXXY sex chromosomes
• Sex-linked characteristics

  Determined by genes on the sex chromosomes
• Dosage compensation

  The amount of protein produced by X-linked genes is randomly inactivated in the two sexes; meaning females don't have an advantage with two X chromosomes
• Number of Barr bodies in human cells with different complements of sex chromosomes

  • XX - 1 Barr body
  • XY - 0 Barr bodies
  • XO (Turner) - 0 Barr bodies
  • XXY (Klinefelter) - 1 Barr body
  • XXYY (Klinefelter) - 1 Barr body
  • XXXY (Klinefelter) - 2 Barr bodies
  • XXXXY (Klinefelter) - 3 Barr bodies
  • XXX - 2 Barr bodies
  • XXXX (Poly-X female) - 3 Barr bodies
  • XXXXX (Poly-X female) - 4 Barr bodies
• A male with XXXYY chromosomes has two Barr bodies in each of his cells
• Limpets

  • Their sex is determined by their position on the stack
  • A larva that settles on an unoccupied substrate develops into a female - and produces a chemical that attracts other larvae
  • The attracted larva settles on top and becomes male - the two become mates with the original female
  • Time passes and the makes on top becomes female
  • The female larvae attract more larvae which becomes male
• Turner Syndrome (XO)

  short stature, webbed in the neck, underdeveloped, and infertility issues
• Klinefelter Syndrome (XXY, XXXY, XXYY)

  presenting phenotypically male; issue with sexual functioning, underdeveloped, and cognitive delays
• The Role of Sex Chromosomes

  • X chromosome contains genetic information essential for both sexes, at least one X is required
  • A single Y, even in the presence of several Xs, still produces a male phenotype (no Y results in a female pheotype)
• Androgen-Insensitivity Syndrome


  The receptors aren't sensitive to androgen male hormones, so the receptor doesn't receive them. This causes female developments - phenotypical female, genetically male
• Sex-Linked Characteristics Determination

  • X linked: rare to find in females as one X chromosome can be affected and they would be considered a carrier
  • Y-Linked Characteristics: only present in males, the Y chromosome is lost over time as it becomes shortened (causing lack of crossing over)
• Barr Bodies

  inactivated X chromosome
• Random X Inactivation

  Requires Two Steps:

  1. the cells count/assesses, how many X chromosomes are present
  2. One X chromosome is selected to go active while the others are inactive