It is important to note that people with mosaic Down syndrome can have varying degrees of intellectual disability and physical abnormalities. Mosaic - in which only some of the cells have an extra chromosome 21. It is the most common cause of cognitive impairment. This extra genetic material causes the developmental changes and physical features of Down syndrome. Reality: While the individual odds of giving birth to a baby with Down syndrome increase with advanced maternal age, more babies with Down syndrome are born to younger mothers. Some have two copies of chromosome 21, and some have three. Also known as Down syndrome, trisomy 21 is a genetic condition caused by an extra chromosome. The primary features are infertility and small, poorly functioning testicles. This type affects about 2% of the people with Down syndrome. This disorder causes mental retardation and other problems. Only a percentage of cells have the extra chromosome associated with the various signs of DS, say 50% of cells. Sometimes, mosaic Down syndrome is so mild that it will go undetected. Some have the usual 46 chromosomes and some have 47. The presence of this extra part of chromosome 21 causes some Down syndrome characteristics. However, this blood test can only determine the level of mosaicism in the blood cell line. I would be thrilled if the Jolie-Pitt twins had Down syndrome. As recently as 1983, a person with Down syndrome lived to be only 25 years old on average. Turning heads - and possibly a few stomachs - US-born Angelina Jolie did what she does best, shocking onlookers by wearing a metal chin cuff to the premiere of her latest film. Individuals with down syndrome tend to have a. Humans have 46 chromosomes, which contain all of a person's genes and DNA. A karyotype was done on a lymphocyte culture and mosaic trisomy 21 was found in 3% of the 437 cells analysed.