Sex Linked Genes

• The fact that sexual reproduction allows new genotypes to occur and thus permits evolution is assumed to be why most eukaryotes have sexes
• apomictic plants: unfertilized eggs double to produce viable diploids
• unusual methods for creating sexes:

  • temperature
  • crowd sensing
• Most plants have perfect flowers, with pistils and anthers on each flower
• monoecious: male and female flowers separated on same plant
• dioecious: male and female flowers on separate plants
• Males are heterogametic (sperm carry X or Y)
• XX females are homogametic
• Only genes on the X chromosome are called sex-linked genes
• Genes on the Y chromosome are "holandric" and pass only from father to son
• Show sex-linked genes as you would "powers" on the X
• In pedigrees where a sex-linked dominant is present, more females than males are typically affected
• Lyon's Law:

  Early in embryology, every extra X in all somatic cells becomes condense and inactive

  Which X is inactivated in an XX cell is pure chance

  Due to slower replication, the same X is inactive in every subsequent daughter cell after mitosis
• Barr bodies are inactivated X's
• Specific regions of specific chromosomes are inactivated during gametogenesis so are not expressed in the fertilized egg or resulting cells until the inactivation marks (5mC in promoter usually) are removed
• Phenotypes can expose a defect in the one allele that should function
• Prader-Willi syndrome arises if the male-active region of Ch 15 is defective
• Angelman syndrome reflects a missing or inactive adjacent region of Ch 15 that should be active in female gametes
• Phenotypes can reflect a failure to properly imprint
• Beckwith-Weidemann disease causes abnormal over-growth if regions of Ch 11 are not properly imprinted
• sex limited traits: traits such as lactation that occur only in one sex although genes and alleles are present in both
• sex influenced traits: traits that are dominant in one sex and recessive in the other
• In bacteria, males contain a fertility (F+) plasmid, a circle of DNA that codes for proteins that enable a copy of the plasmid to be transferred into cells that do not already contain a copy (F-)
• In fungi, many require different alleles at one or two gene loci in order to cross; generally, either "mating type" can be male or female in transfer of genetic information
• In turtles, the temperature of eggs may trigger male or female development
• Females have 2 X's so all eggs get one X
• examples of sex-linked traits:

  • colorblindness
  • Duchene Muscular Dystrophy
  • Hemophilia A
• Dominant genes on the X chromosome tend to show up more often in females than males
• T/F: A sex linked dominant gene would be passed from an affected man to all his sons

  False
• T/f: A sex linked dominant gene would be passed from an affected man to all his daughters

  True
• T/F: A sex linked dominant gene would be passed from a normal female to half her sons

  False
• T/F: A sex linked dominant gene would be passed from a normal female to half her daughters

  False