Understanding the Genetics of Down Syndrome

Human cells generally enclose 23 pairs of chromosomes, for a total of 46 chromosomes in each cell, but an alteration in the number of chromosomes can cause problems with growth, development, and function of the body’s systems. These alterations can happen during the formation of multiplicative cells, in early fetal development, or in any cell after birth (National Institutes of Health, 2012).

Chromosome abnormalities usually occur when there is an error in cell division. There are two kinds of cell division, which are mitosis and meiosis. Mitosis is a result in two cells that are duplicates of the original cell and meiosis results in cells with half the number of chromosomes, 23 instead of the normal 46 which are the eggs and sperm. There are also other factors that can increase the risk of chromosome abnormalities which are maternal age and environment even though there is no conclusive evidence that specific environmental factors cause chromosome abnormalities, it is still a probability that the environment may play a role in the rate of genetic errors. (National Institutes, 2012). Some the most common genetic disorder includes Sickle cell disease, Huntington’s disease, Tay-Sachs disease, Klinefelter’s syndrome, Turner syndrome, Cystic fibrosis, and Down syndrome.

According to the article Trisomy 21 Causes Down Syndrome, Down syndrome is a genetic disorder caused by chromosomal abnormalities altering the course of development in a newborn. The risk of giving birth to a baby with Down syndrome increases in women who are over thirty years of age. Down syndrome is a condition that cannot be cured, currently; Down syndrome is one of the most common birth defects, affecting about one in every 750 live births (O’Connor, 2011). In addition, O’Connor explains that John Langdon Down was the first to describe this disorder in medical literature in 1866, by documenting the numerous symptoms related with the syndrome but failing to determine their cause persisted mysterious for nearly 100 years following Down's work. Then, in the 1950s, researchers determined the source of Down syndrome was the presence of an extra copy of chromosome 21, a condition often referred to as trisomy 21(O’Connor, 2011).

In Introduction to child and adolescent development, Mossler explains that trisomy 21 is when instead of two chromosomes on the 21st pair, people with Down syndrome have three; therefore this disorder is called Trisomy 21(Mossler, 2011). Further along Mosler describes that chromosomal abnormalities happen as a consequence of the environment interrelating with genetics in a way that is not entirely understood. Down syndrome is also characterized by moderate to severe deficits in physical and cognitive operational “usual physical features consist of short stature, flat facial features, and almond-shaped eyes” (Mossler, 2011).

The risk of infants being born with Down syndrome (Trisomy 21) increases significantly with the age of the mother. Furthermore, Mossler explains that among women younger than 25 years, the risk of Down syndrome is 1/1,600; the risk increases to 1/250 for women between the ages of 35 and 39 and 1/25 for women over 45,and perhaps even as high as 1 in 2 births for women over the age of 55 (Mossler, 2011).

Cells in the human body have 23 pairs of chromosomes, making a total of 46. Individual sperm and egg cells, however, have just 23 unpaired chromosomes. A person received half of their chromosomes from your mother's egg and the other half from their father's sperm cell. A male child receives an X chromosome from his mother and a Y chromosome from his father; females get an X chromosome from each parent. Genes are sections or segments of DNA that are carried on the chromosomes and determine specific human characteristics, such as height or hair color. Since each parent gives you one chromosome in each pair, you have two of every gene, except for some of the genes on the X and Y chromosomes in boys because boys have only one of each (Mossler, 2012).

Down syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and poor muscle tone (hypotonia) in infancy. The degree of intellectual disability varies, but it is usually mild to moderate. People with Down syndrome may be born with a variety of birth defects. About half of all affected children have a heart defect, and digestive abnormalities, such as a blockage of the intestine, are less common.

A person with Down syndrome has a greater risk of developing numerous medical conditions, which include gastroesophageal reflux, which is a backflow of acidic stomach contents into the esophagus, and celiac disease, which is an intolerance of a wheat protein called gluten. Approximately 15 percent of people with Down syndrome suffer from thyroid gland (hypothyroidism). The thyroid gland is a butterfly-shaped organ in the lower neck that produces hormones. Individuals with Down syndrome also have an increased risk of hearing and vision problems. Additionally, about 1 percent of children with Down syndrome develop cancer of blood- forming cells (leukemia). Grown-ups people with Down syndrome have an increased risk of developing Alzheimer disease, a brain disorder that results in a gradual loss of memory, judgment, and ability to function. Although Alzheimer disease is usually a disorder that occurs in older adults, about half of adults with Down syndrome develop this condition by age 50 (Nationa Institutes, 2012).


Mossler, R. (2011). Genetics, conception, and birth. In Child & adolescent development (pp. 1- 74). San Diego, CA: Bridgepoint Education.

National Institutes, 2012.
O'Connor, C. (2008) Trisomy 21 causes Down syndrome. Nature Education 1(1)

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