Types of Down Syndrome

Trisomy 21 (nondisjunction)

This is the most commonly occurring type of Down syndrome, accounting for 95% of cases. Normally, babies are born with 23 chromosome pairs (or 46 chromosomes in total) in each cell – one inherited from the mother and one from the father. In Trisomy 21, there is an error in cell division called “nondisjunction”. This results in a baby being born with an extra copy of chromosome 21, meaning there are three copies of chromosome 21 instead of the usual two. As the embryo develops, the extra chromosome is replicated in every cell of the body.

Translocation

Translocation accounts for about 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.

Mosaicism

Mosaicism occurs when nondisjunction of chromosome 21 takes place in one - but not all – of the initial cell divisions after fertilization. In these cases, there is a mixture of two types of cells - some containing the usual 46 chromosomes and others containing 47. Those cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism accounts for about 1% of all cases of Down syndrome. Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.

Incidence

One in every 700 babies is born with Down syndrome, making it the most common genetic condition on a worldwide basis.

Down syndrome can occur in people of all races and economic levels. We do know that older women have an increased chance of having a child with Down syndrome. Since many couples are postponing parenting until later in life, the incidence of Down syndrome conceptions is expected to increase. Therefore, genetic counselling for parents is becoming increasingly important.