26. Down sysndrome is a chromosomal disorder that is caused by the presence of an extra chromosome at pair no 21. The incidence in the general population is 1 in 800 live births but the incidence increase as the age of the mother increases (1 to 50 for mothers over 45 years of age). Affected children have slanting eyes, flat noses and large tongues. Congenital heard defects and gastrointestinal malformations are common. Most of the children with Down Syndrome have mild to moderate mental retardation.