AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

DOWN'S SYNDROME

• Down syndrome is set of mental and physical symptoms that result from having an extra copy of Chromosome 21.
• It is named after John Langdon Down, the British doctor who described the syndrome in 1866.
• The disorder was identified as a chromosome 21 trisomy by Jerome Lejeune in 1959.
• The chromosome nomenclature is 47, +21 for Down Syndrome
• Other names people use for Down syndrome
  • Down's Syndrome
  • Trisomy 21
  • 47,XX,+21
  • 47,XY,+21
• How do people get Down syndrome?
  • If a pair of number 21 chromosomes fails to separate during the formation of an egg (or sperm), that egg (or sperm) unites with a normal sperm (or egg) to form an embryo, that embryo ends up with three copies of chromosome 21 instead of the normal two.
  • In rare cases Down syndrome is caused by a Robertsonian translocation, which occurs when the long arm of chromosome 21 breaks off and attaches to another chromosome at the centromere.
  • The carrier of such a translocation will not have Down syndrome, but can produce children with Down syndrome.
• What are the symptoms of Down syndrome?
  • People with Down syndrome have very distinct facial features: a flat face, a small broad nose, abnormally shaped ears, a large tongue, and upward slanting eyes with small folds of skin in the corners.
  • Health concerns for individuals with Down syndrome include a higher risk for : respiratory infections, gastrointestinal tract obstruction (blocked digestive tract), leukemia, heart defects, hearing loss, hypothyroidism, and various eye abnormalities.
  • Individuals with Downs’ syndrome are mentally retarded and are also called Mongoloid idiots because their facial features resemble those of Mongoloid race.
  • They also exhibit moderate to severe mental retardation; children with Down syndrome usually develop more slowly than their peers, and have trouble learning to walk, talk, and take care of themselves.
  • Because of these medical problems most people with Down syndrome have a decreased life expectancy. About half live to be 50 years of age.