What do chromosomes do?

  • Cells of the body have 22 pairs of chromosomes and 2 sex chromosomes; 46 chromosomes altogether. 
  • One chromosome from each pair comes from the mother and one from the father. 
  • The pairs are arranged by size. Chromosome 1 is the largest and chromosomes 21 and 22 are the smallest.  The sex chromosomes are called X and Y. 
  • Chromosomes can be seen with a microscope during a chromosome analysis test.  Test results are 46XX for females (this means there are 46 chromosomes including two X chromosomes) and 46XY for males (this means there are 46 chromosomes including one X and one Y chromosome.   

What about people with Down Syndrome?

People with Down syndrome have an extra chromosome 21.

There are three types of Down syndrome:

  1. Trisomy 21 occurs in about 95% of people with Down syndrome and means all cells will have three copies of chromosome 21.
  2. Translocation occurs in about 4% of people with Down syndrome.  There are still only two chromosome 21 in each cell but some extra chromosome 21 on another chromosome as well.
  3. Mosaic Down syndrome occurs in about 1% of people with Down syndrome.  Some cells will have the extra chromosome 21 and some will not.  People with Mosaic Down syndrome may have fewer physical features, medical problems and learning difficulties.