Fiona Challoner Malone

Background

About Fiona Challoner Malone

What is Down syndrome? Down syndrome occurs in approximately one in every 450 births in Ireland. In every cell in the human body there is a nucleus, where genetic material is stored in genes. Genes carry the codes responsible for all of our inherited traits and are grouped along rod-like structures called chromosomes. In traditionally-developed individuals, each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21. Down syndrome can affect a child’s learning ability, their physical development and can come with a range of health and developmental challenges. There are three types of Down syndrome: Trisomy 21 (Nondisjunction), Translocation and Mosaicism. Trisomy 21 This is the most common cause of Down syndrome. Trisomy 21 results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down syndrome accounts for 95% of cases. Translocation In translocation – which accounts for about 4% of cases of Down syndrome – the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome. Mosaicism Mosaicism (or Mosaic Down syndrome) is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. Mosaicism is the least common form of Down syndrome and accounts for only 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 of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess. Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. Down syndrome is not a sickness or a disease, it is a genetic disorder or chromosomal anomaly that occurs at the time of conception.