Down syndrome is caused by an extra (third) chromosome 21. Individuals with Down syndrome have therefore not 46, but 47 chromosomes. Whereas everyone has two chromosomes 21, they have 3. Therefore, Down syndrome is also called trisomy 21. The trisomy is usually due to an abnormality of the egg of the mother: normally all reproductive cells (eggs and sperms) carry only 23 chromosomes (one of each). At fertilisation the 23 chromosomes from the female egg than join the 23 chromosomes from the male sperm to form a fertilised egg or zygote with the normal 46 chromosomes. When one of the reproductive cells has an extra chromosome 21, the fertilised egg has 47 chromosomes with 3 chromosomes 21, resulting in Down syndrome.
Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) have a similar origin.
