Down Syndrome

Down syndrome is a set of mental and physical symptoms that result from having all or part of an extra Chromosome 21. It is named after John Langdon Down, a British doctor who first described the syndrome in 1866. Currently, the incidence of Down syndrome is estimated to be one out of 800 live births.

Symptoms

Even though people with Down syndrome may have some physical and mental features in common, symptoms of Down syndrome can range from mild to severe. Usually, mental development and physical development are slower in people with Down syndrome than in those without the condition.

Mental retardation is a disability that causes limits on intellectual abilities and adaptive behaviors (conceptual, social, and practical skills people use to function in everyday lives). Most people with Down syndrome have IQs that fall in the mild to moderate range of mental retardation. They may have delayed language development and slow motor development.

Some common physical signs of Down syndrome include:

Flat face with an upward slant to the eye, short neck, and abnormally shaped ears
Deep crease in the palm of the hand
White spots on the iris of the eye
Poor muscle tone, loose ligaments
Small hands and feet

There are a variety of other health conditions that are often seen in people who have Down syndrome, including:

Congenital heart disease
Hearing problems
Intestinal problems, such as blocked small bowel or esophagus
Celiac disease
Eye problems, such as cataracts
Thyroid dysfunctions
Skeletal problems
Dementia—similar to Alzheimer’s

Causes

To understand why Down syndrome occurs, the structure and function of the human chromosome must be understood. The human body is made of cells; all cells contain chromosomes, structures that transmit genetic information. Most cells of the human body contain 23 pairs of chromosomes, half of which are inherited from each parent. Only the human reproductive cells, the sperm in males and the egg in females, have 23 individual chromosomes, not pairs. Scientists number them 1 through 22. The 23rd chromosome is a sex-determining chromosome. It is named X in an egg and either X or Y in a sperm.

When the reproductive cells, the sperm and egg, combine at fertilization, the fertilized egg that results contains 23 chromosome pairs. A fertilized egg that will develop into a female contains chromosome pairs 1 through 22, and the XX pair. A fertilized egg that will develop into a male contains chromosome pairs 1 through 22, and the XY pair. When the fertilized egg contains extra material from chromosome number 21, the Down syndrome appears. This may happen in three different ways: trisomy 21, mosaic trisomy 21, and translocation trisomy 21.

Trisomy 21

In most cases, approximately 92% of the time, Down syndrome is caused by the presence of an extra chromosome 21 in all cells of the individual. In such cases, the extra chromosome originates in the development of either the egg or the sperm (through a mechanism called nondisjunction). Consequently, when the egg and sperm unite to form the fertilized egg, three--rather than two--chromosomes 21 are present. As the embryo develops, the extra chromosome is repeated in every cell. This condition, in which three copies of chromosome 21 are present in all cells of the individual, is called trisomy 21.

Mosaic Trisomy 21

In approximately 2-4% of cases, the extra chromosome 21 is present in some, but not all, cells of the individual. For example, the fertilized egg may have the right number of chromosomes, but, due to an error (nondisjunction) in cell division early in embryonic development, some cells acquire an extra chromosome 21. Thus, an individual with Down syndrome due to mosaic trisomy 21 will typically have 46 chromosomes in some cells, but will have 47 chromosomes (including an extra chromosome 21) in others. In this situation, the range of the physical problems may vary, depending on the proportion of cells that carry the additional chromosome 21.

Translocation Trisomy 21

Approximately 3-4% of individuals with Down syndrome do not contain a whole extra Chromosome 21, but only extra material from part of Chromosome 21. The extra material is caused by a mechanism called Robertsonian translocation. In such situations, cells from individuals with Down syndrome have two normal chromosomes 21, but also have additional chromosome 21 material on the translocated chromosome. Thus, there is still too much material from chromosome 21, resulting in the features associated with Down syndrome.

Reference:

National Institute of Child Health and Human Development, USA.