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TRISOMY 18

Edward syndrome (trisomy 18, 47,XX+18 or 47,XY+18): numerical chromosomal abnormality,
autosomal trisomy.
Features are
 Infants usually die in the first few weeks after birth
 Delayed developmental milestones
 Mental retardation
 Prominent occiput
 Short palpebral fissures
 Narrow palatal arch
 Micrognathia
 Clenched hands with over-riding of fingers.
 Short sternum
 Umbilical or inguinal hernia
 Congenital heart defects

 Chromosome
 Each chromosome is made up of condensed DNA which is tightly coiled many times around
proteins called histones and visible under metaphase stage.
 Each chromosome has a constriction point called the centromere, which divides the chromosome
into two “arms.” The short arm of the chromosome is labeled the “p arm” and the long arm “q
arm.”
 Depending on the position of centromere the chromosomes are classified morphologically into
o Metacentric- centromere is in the center of chromosome, where length of two arms is
equal. Chromosomes 1, 3, 16, 19
o Submetacentric- centromere is away from the center of chromosome, where one arm is
shorter than the other arm. Chromosomes 2, 4, 5, 6 to 12, 17, 18, X
o Acrocentric – centromere is towards one end of the chromosome, where one of the arms is
too short and the other arm is too long. Chromosomes 13, 14, 15, 20, 21, Y

  Depending upon the length, position of centromere, bands, they are classified into 7 groups.
Group A- 1, 2, 3; group B- 4, 5; group C- 6-12, X; group D- 13, 14, 15; group E- 16, 17, 18; group F-
19, 20; group G- 21, 22, Y
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