Lec12 Sex Determination
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Lec12 Sex Determination

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Lec12 Sex Determination Lec12 Sex Determination Presentation Transcript

  • SEXRELATEDINHERITANCE
    Biology 3 Kent Kawashima
  • HOW ISSEX (gendeR)DETERMINED?
  • How is Sex Determined?
    XY
    ZW
    X0
  • XY System
    • Most mammals and some insects use the “X” and “Y” chromosomes determine sex
    • In humans, males are XY (heterogametic) while females are XX (homogametic)
    • In humans, a single gene (SRY) on the Y chromosome acts as a signal to set the developmental pathway towards maleness
    XY
  • ZW System
    • Birds, most fishes and some insects use the “Z” and “W” chromosomes determine sex
    • The ovum determines the sex of the offspring
    • In birds, males are ZZ (homogametic) while females are ZW(heterogametic)
    • The W chromosome is thought to be essentail for female sex determination or at least contain female-determining genes.
    ZW
  • X0 System
    • Crickets, grasshoppers and some insects use the number of X chromosomes to determine sex
    • The X0 system only has one sex chromosome – X
    • Males have only one X chromosome, (X0), females have two (XX)
    • The maternal gamete always contains an X chromosome. The sperm determines sex – containing either one X chromosome or no sex chromosomes at all.
    X0
  • SEXLIMITEDTraits
  • Sex-Limited Traits
    Autosomal, not found in the sex chromosomes
    Genes are carried by both males and females, but only one sex would ever express them
    Example: genes that influence how much milk a lactating mother produces when she’s nursing a baby
    Another example: cryptorchidism (undescended testicles)
  • SEXINFLUENCEDTraits
  • Sex-Influenced Traits
    Autosomal, not found in the sex chromosomes
    Genes are carried by both males and females, but there is a difference in the way the two sexes express them
    Example: pattern baldness in humans. The baldness allele behaves like a dominant allele in males, while in females it behaves like a recessive allele
  • SEXLINKEDTraits
  • Sex-Linked Traits
    Genes are found on the sex chromosomes
    Sex-linked traits are behave differently from autosomal traits when sex chromosomes are non-homologous
    May be dominant or recessive in nature
    Phenotype is dependent also on the presence of a complete gene pair or just one allele
    Example: Hemophilia (X-linked)