Chromosome

• Euchromatin, which consists of DNA that is active, e.g., expressed as protein.
• Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
  • Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.
  • Facultative heterochromatin, which is sometimes expressed.

• Chromosomal aberration or Partial chromosomal dysplasia (malformation), which are usually the result of a defective crossover. Examples are:
  • Cri du chat syndrome, which is caused by the deletion of part of the short arm of chromosome 5. Victims make high-pitched cries that sounds like a cat. They have wide-set eyes, a small head and jaw and are mentally retarded.
  • Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4. It is characterized by severe growth retardation and mental defect.
• Missing or additional chromosomes, called aneuploidy, which are the result of an incomplete chromosomal segregation, many of which are associated with cancer. Examples are:
  • Down syndrome (extra chromosome 21). This is also know as mongolism or trisomy 21. Symptoms are decreased muscle tone, asymmetrical skull, slanting eyes and mental retardation.
  • Klinefelters Syndrome (XXY). Men with Klinefelter syndrome are usually sterile. They tend to have longer arms and legs and tend to be taller than their peers. Other common symptoms are fatigue, apathy, lack of emotion, and an increased tendency to develop psychiatric disorders.
  • Turner syndrome (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. People with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development and a "caved-in" appearance to the chest.

• sex-determination system
• XYY
• Genetic deletion