- Down Syndrome
- What are the different types of Down syndrome?
- Is it known why Down syndrome happens?
- How does Down syndrome affect the body?
- What risk factors are associated with Down syndrome?
- Screening and diagnosis for Down syndrome
- Treatment for Down syndrome and condition management
- Living with Down syndrome
- Down syndrome FAQs
What are the different types of Down syndrome?
With the advancements in understanding the fundamental cause of this genetic condition, medical professionals determined three types of Down syndrome based on the behaviour of chromosome 21 as fertilisation takes place.
The three types are:
- Trisomy 21: This type is the most common of all Down syndrome occurrences (90% to 95%) and is also known as non-disjunction. Although genetic, this condition is not a hereditary disorder (meaning not all children born to the same set of parents will have Down syndrome). This type of Down syndrome occurs when the two chromosomes 21 that originate from the mother fail to divide (or separate) and instead incorporate into either the mother’s egg or the father’s sperm (hence why this type is also known as non-disjunction). The fertilised egg then forms with 47 chromosomes that alter the formation of a foetus when the cells begin to divide and the additional chromosome replicates in every cell of the body.
- Mosaicism: This type (also not considered hereditary) is rarer than trisomy 21 (approximately 1% of cases) and occurs when the additional chromosome replicates in some cells of the body. The remainder of cells where replication has not occurred are normal. This means that those with this type of Down syndrome have few characteristics and symptoms or partial features of the condition. This type is also caused by non-disjunction shortly after embryo fertilisation and effectively forms a ‘mosaic’ pattern of normal and abnormal cells (some cells containing 47 chromosomes and others containing 46).
- Robertsonian translocation: When a part of chromosome 21 fuses with another chromosome (most often chromosome 14), this type of Down syndrome occurs. A foetus will develop a normal set of chromosomes, but has one with an extra set of genes (partial chromosomal material) from the chromosome 21 copy attached to it. Occurring in a very small percentage of cases (around 4%) as well, the partial fusing or segmental trisomy 21 can be inherited by a future child if a person with this type of Down syndrome reproduces.