File:Trisomy21female.jpg: Difference between revisions

Trisomy 21 (Down Syndrome) Female Karyotype

Down syndrome or trisomy 21 is caused by nondisjunction of chromosome 21 in a parent who is chromosomally normal and is one of the most common chromosomal abnormalities in liveborn children. The frequency of trisomy 21 in the population is approximately 1 in 650 to 1,000 live births, in Australia between 1991-97 there were 2,358 Trisomy 21 (Down) infants.

The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. The karyotype is the characteristic chromosome complement as identified by staining and can only be identified during cell division when chromosomes are folded. The chromosomes when organised as an image in sequence are called a karyogram or idiogram.

Chromosome 21

• the smallest human chromosome
• about 47 million base pair
• about 1.5 % of the total DNA in cells
• contains between 300 and 400 genes (of human estimated 20,000 to 25,000 total genes)
• trisomy 21 or Downs Syndrome - additional copy of this chromosome
• partial monosomy 21 - missing segment of the chromosome in each cell
• ring chromosome 21 - circular structure, occurs when there are two breaks in a chromosome and the ends of the chromosome then fuse together

Image Source: UNSW Embryology http://embryology.med.unsw.edu.au/Defect/page21.htm