DOWN SYNDROME

Fertilization and ensuing mitosis with typical chromosome counts:

Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21

One in every 691 babies in the U.S. is born with Down syndrome.

There are three types of Down syndrome:

TRISOMY 21 - accounts for 95% of cases of Down syndrome

- caused by error in cell division called nondisjunction

- a pair of 21st chromosomes in either the sperm or the egg fails to separate; can occur during any point of cell division: mitosis, meiosis I, meiosis II

- extra chromosome from nondisjunction is replicated in every cell of the body as the embryo develops, resulting in an embryo with 2 copies of chromosome 21 instead of 2

TRANSLOCATION - accounts for 4% of cases of Down syndrome

- part of chromosome 21 breaks off during cell division and attaches to another chromosomes (usually chromosome 14)

- presence of an extra part of chromosome 21 causes characteristic of Down syndrome, even though there is a total of 46 chromosomes in the cells

MOSAICISM - accounts for 1% of cases of Down syndrome

- nondisjunction of chromosome 21 takes place in one of the initial cell divisions after fertilization

- creates a mixture of cells containing the usual 46 chromosomes and cell containing 47 chromosomes

The cause of nondisjunction is unknown, but research has shown that it occurs more frequently as a woman ages.

The additional genetic material alters the course of development and causes the physical characteristics associated with the syndrome.

Individuals with Down Syndrome commonly have low muscle tone, small stature, upward slant to the eyes, single deep crease across the center of the palm.

Sources:
NDSS, Genetics Home Reference, Kids Health