Chromosomes & Sex Linkage

Y chromosome has SRY (sex-determining region Y) gene which encodes TDF (testis determining factor)
Haemophilia
1. Some blood clotting disorders are controlled by X-linked genes
2. Clot formation requires platelets and the protein fibrin
  1. Fibrin is produced in a series of steps mediated by many protein factors
3. Individuals with severe haemophilia have factor VIII levels that are less than 1% of normal
4. Moderate haemophiliacs (1-5% of normal factor VIII) have bleeding episodes after traumatic events
5. Factor VIII
  1. Can be purified from donor human plasma
  2. Can be produced by genetically modified hamster cells
6. Queen Victoria was a carrier of haemophilia
  1. Had an affected son & 2-3 carrier daughters
  2. Granddaughter was a carrier of haemophilia
    1. Only son (& heir to throne) had haemophilia
      1. Rasputin controlled bleeding episodes by hypnosis and herbs
Red-green Colourblindness
1. X-linked recessive trait
2. Red & green colourblindness are controlled by 2 genes on the X chromosome
3. Anomalously trichromatic people have all three photopigments, but one or more may be abnormal
Duchenne Muscular Dystrophy (DMD)
1. X-linked trait
2. Dystrophin links the cytoskeleton to proteins in the membrane
3. Rapid progression of muscle degeneration
  1. Leads to motor impairment and death
Inheritance
1. Carrier females can produce affected sons and normal sons
2. Affected females always have affected fathers
3. Affected males...
  1. ...give their X chromosomes to their daughters who will be affected
  2. ...give their Y chromosomes to their sons who will be normal
Barr Bodies
1. X0 females have no Barr Bodies
2. XXY males have 1 Barr Body
3. XXX females have 2 Barr Bodies
4. Inactivated X chromosomes
  1. Genes of this X chromosome are not transcribed
  2. Consist of highly compacted chromatin (heterochromatin)
Dosage Compensation
1. Inactivation of one of the X chromosomes in XX females equalizes X-linked gene expression between males and females
  1. X inactivation occurs in early embryonic development
Two cell possibilities in female mammals:
1. Maternal X is active. Paternal X is Barr Body
  1. All the descendants of this cell will inactivate the paternal X chromosome.
2. Paternal X is active. Maternal X is Barr Body
  1. All the descendants of this cell will inactivate the maternal X chromosome
Mammal females are mosaics
1. A mixture of two cell types:
  1. The proportions of these two cell types varies from female to female
Anhidrotic Ectodermal Dysplasia (AED)
1. X-linked disease
2. Teeth and nail abnormalities
3. Absence of sweat glands
4. XXe- females are heterozygous for the AED allele
  1. Mixture of 2 cell types
5. Mosaic phenotypes can be seen in 3 generations of females, all heterozygous for the X-linked AED gene
Genes, Proteins, and DNA
1. Genes are made of DNA
2. Our genome has ~20,000 protein-encoding genes
3. Our genes are packaged in chromosomes
4. Central Dogma of Molecular Biology and Genetics
  1. Genes packaged in chromosomes are located in the nucleus. The cellular machinery (ribosomes) to make proteins is located in the cytoplasm.

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