3

• Lecture 3: Chimera and sex chromosome


  March-18-2024
• Chimera


  An ancient Greek name used to define a mythical creature that has been formed from a combination of different animals
• Can plants be naturally chimeric?
• Plants


  • Oranges can be chimeric
• Chimeras happen in nature all the time, especially in plants
• Can animals be chimeric?
• Scientists have artificially created chimeric animals
• Natural chimerism


  The fusion of more than one fertilized zygote in the early stages of prenatal development

  
• Hyper ovulation


  1. Two eggs release instead of one

  2. Two different sperm cells may fuse with the eggs, forming two separate zygotes instead of one

  3. If development proceeds, this can give rise to fraternal twins

  
• Human chimera cases


  • Lydia
  • Karen Keegan
• Over 100 cases of chimeric humans have been reported worldwide
• Chromosomal sex


  • Determined at fertilization
  • Sexual differences begin in the 7th week

  
• Sex


  Influenced by genetic and environmental factors
• Females (XX) do not have a Y chromosome

  
• Males (XY) have a Y chromosome

  
• Development of ducts


  Shown in diagram
• Hermaphrodites


  Have both male and female gonads
• Androgen insensitivity


  XY males become phenotypic females
• Pseudo hermaphrodites


  XY males at birth are phenotypically female; at puberty develop a male phenotype
• 1. chromosome inactivation (XCI)


  Mammalian females have evolved a unique mechanism of dosage compensation to correct the imbalance of having two copies of the X chromosome

  
• Dosage compensation


  The process called X-chromosome inactivation (XCI) where female mammals transcriptionally silence one of their two X's in a complex and highly coordinated manner
• Barr body


  The inactivated X chromosome condenses into a compact structure and is stably maintained in a silent state

  
• Sex-linked traits


  Traits found on either the X or Y chromosomes

  
• Sex-linked traits


  • Hemophilia, red-green color blindness, congenital night blindness, some high blood pressure genes, Duchenne muscular dystrophy, Fragile X syndrome
• Males are more often afflicted with sex-linked disorders because they only have one X chromosome
• Color blindness (Color vision deficiency)


  Decreased ability to see color or differences in color
• People with total color blindness (achromatopsia) may also be uncomfortable in bright environments and have decreased visual acuity
• Duchenne Muscular Dystrophy (DMD)


  A genetic disorder characterized by progressive muscle degeneration and weakness due to alterations of the dystrophin protein
• DMD is one of four conditions known as dystrophinopathies
• DMD symptom onset is in early childhood, usually between ages 2 and 3
• In Europe and North America, the prevalence of DMD is approximately 6 per 100,000 individuals
• DMD carriers


  Females who have a normal dystrophin gene on one X chromosome and an abnormal dystrophin gene on the other X chromosome

  
• Most carriers of DMD do not themselves have signs and symptoms of the disease, but a minority do
• Until relatively recently, boys with DMD usually did not survive much beyond their teen years
• Thanks to advances in cardiac and respiratory care, life expectancy is increasing and many young adults with DMD attend college, have careers, get married, and have children
• Survival into the early 30s is becoming more common than before for individuals with DMD