Definition:
Chromosome - packages of genetic information contained in every cell of the human body. Most people have 46 chromosomes. People with Down syndrome have 47 chromosomes.
People with Down syndrome have an extra chromosome 21. Most people with Down syndrome have an entire extra #21 chromosome in every cell of their body - a condition called trisomy 21. People can also have Down syndrome as a result of an extra #21 in only some of their cells (called Mosaic Down syndrome) or because two of their chromosomes are stuck together, a condition called a translocation.
Pronunciation: "chrome" "o""sew""m"
Examples:
Down syndrome is caused by an extra chromosome number 21.
