Genes and human disease Genes and chromosomal diseases Down
Syndrome is a type of mental retardation caused by extra genetic
material in chromosome 21. This can be due to a process called
nondisjunction, in which genetic materials fail to separate
during a crucial part of the formation of gametes, resulting in
an extra chromosome (called trisomy 21). The cause of
nondisjunction is not known, although it correlates with a
woman’s age reference. The additional material present
influences development and results in the state known as Down
Syndrome. Some physical characteristics of Down Syndrome in
infants are decreased muscle tone, a flat face, eyes slanting
up, irregular shaped ears, ability to extend joints beyond the
usual, large space between the big toe and its neighbouring toe,
large tongue relative to the mouth, etc. Down Syndrome patients
can also be afflicted with other conditions, including heart
disease, Alzheimer’s disease, and leukaemia. The estimated
incidence of Down Syndrome is between 1 in 1,000