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.

Trisomy 21 Links: Trisomy 21 | Image T21 female karyotype | Image T21 male karyotype | chromosome 21 | Category:Trisomy 21 | Genetic Abnormalities | Prenatal Diagnosis

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