Down syndrome is a chromosomal condition that can result in a variety of birth defects. It exists all across the globe and commonly results in variable effects on learning styles, physical characteristics or overall general health.
Chromosomes are thread-like molecules of DNA that carry genetic information and it is this DNA which determines how the person will develop. Each cell in the human body contains 46 chromosomes - 23 pairs with half inherited from each parent.
People with Down syndrome have 47 chromosomes in all or some of their cells instead of 46. This because of an extra chromosome 21 in the cells.
Having extra or abnormal genetic material changes the way the brain and body develop. People with Down syndrome are at an increased risk of developing several medical conditions. Delayed development and behaviour problems are often reported in children with Down syndrome.
Down syndrome has been prevalent for ages. It was only in the year 1866 that an English physician, John Langdon Down, published an accurate description of a person affected by the condition. Although attempts had previously been made to recognise the characteristics of the syndrome, it was Down who described the condition as a distinct and separate entity.
Down syndrome occurs at conception and experts still do not known why it happens. It is a life-long condition with no cure, and can happen across all ethnic and social groups and to parents of all ages.
Some of the common physical traits of Down syndrome are low muscle tone (degree of muscle tension or resistance during rest or in response to stretching), small stature, an upward slant to the eyes and a single deep crease across the centre of the palm. It is important to note that each person with Down syndrome is a unique individual and may possess these characteristics to different degrees or not at all.
Children with Down syndrome also tend to have a flat face and a short neck. They also have some degree of intellectual disability. For many people with Down syndrome, speaking clearly can be a difficulty. Very often, people with Down syndrome can understand a lot more than they can express with words. This often means that their abilities are underestimated, which can make them feel frustrated.
Many children with Down syndrome are also born with heart, intestine, ear or breathing problems. These conditions can often lead to other problems such as respiratory infections or hearing loss, but most these problems can be treated.
Down syndrome is more likely to occur if women are older when they get pregnant. A 35-year-old woman has about a one in 350 chance of conceiving a child with Down syndrome, and this increases gradually to one in 100 by the age of 40. At the age of 45, the chance is approximately one in 30.
Having a brother or sister who has Down syndrome also increases the risk of conceiving a child with Down syndrome. A previous instance of having a child with Down syndrome also increases the risk.
Screening tests such as an ultrasound or a blood test during the first or second trimester of pregnancy can help show if the developing baby is at risk of developing Down syndrome. These screening tests, however, can sometimes give misleading results. Diagnostic tests such as chorionic villus sampling or aminocentesis can show if a baby has Down syndrome.
There are three types of Down syndrome:
Trisomy 21 (Nondisjunction)
Down syndrome is usually caused by an error in cell division called "nondisjunction", which 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, which accounts for 95 per cent of cases, is called trisomy 21.
Mosaicism, also known as mosaic Down syndrome, is characterised by a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those with 47 chromosomes contain an extra chromosome 21.
Mosaicism is the least common form of Down syndrome and accounts for only about 1 per cent of all cases. Research has also indicated that those with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome.
In translocation, which accounts for about 4 per cent 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.
Though Down syndrome is a genetic condition, only 1 per cent of all cases have a hereditary component. Heredity is not a factor in trisomy 21 and mosaicism. However, there is a hereditary component in one-third of cases of Down syndrome resulting from translocation.
- The author is Information Assistant, PIB Srinagar.