Lec12 Sex Determination

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Lec12 Sex Determination

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

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