Download Trisomy 21: The Story of Down Syndrome What is Down syndrome?

Trisomy 21: The Story of Down Syndrome
What is Down syndrome?
Down syndrome is a set of physical and mental traits caused by a gene problem that happens
before birth. Children with Down syndrome tend to have certain features, such as a flat face and
a short neck. They also have some degree of mental retardation. This varies from person to
person, but in most cases it is mild to moderate. Down syndrome is a lifelong condition. But with
care and support, most children with Down syndrome can grow up to have healthy, happy,
productive lives.
The Chromosomes
Chromosomes are thread-like structures composed of DNA and other proteins. They are present
in every cell of the body and carry the genetic information needed for that cell to develop. Genes,
which are units of information, are "encoded" in the DNA. Human cells normally have 46
chromosomes which can be arranged in 23 pairs. Of these 23, 22 are alike in males and females;
these are called the "autosomes." The 23rd pair are the sex chromosomes ('X' and 'Y'). Each
member of a pair of chromosomes carries the same information, in that the same genes are in the
same spots on the chromosome. However, variations of that gene ("alleles") may be present.
(Example: the genetic information for eye color is a "gene;" the variations for blue, green, etc.
are the "alleles.")
Human cells divide in two ways. The first is ordinary cell division ("mitosis"), by which the
body grows. In this method, one cell becomes two cells which have the exact same number and
type of chromosomes as the parent cell. The second method of cell division occurs in the
ovaries and testicles ("meiosis") and consists of one cell splitting into two, with the resulting
cells having half the number of chromosomes of the
parent cell. So, normal eggs and sperm cells only
have 23 chromosomes instead of 46.This is what a
normal set of chromosomes looks like. Note the 22
evenly paired chromosomes plus the sex
chromosomes. The XX means that this person is a
female. The test in which blood or skin samples are
checked for the number and type of
chromosomes is called a karyotype, and the results look like the picture above.
Many errors can occur during cell division. In meiosis, the pairs of chromosomes are supposed to
split up and go to different spots in the dividing cell; this event is called "disjunction." However,
occasionally one pair doesn't divide, and the whole pair goes to one spot. This means that in the
resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes. This
accident is called "nondisjunction." If a sperm or egg with an abnormal number of
chromosomes merges with a normal mate, the resulting fertilized egg will have an abnormal
number of chromosomes. In Down syndrome, 95% of all cases are caused by this event: one cell
has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st
chromosomes. Hence the scientific name, trisomy 21. Recent research has shown that in these
cases, approximately 90% of the
abnormal cells are the eggs. The
cause of the nondisjunction error
isn't known, but there is definitely
connection with maternal age.
Research is currently aimed at trying
to determine the cause and timing of
the nondisjunction event.
Analysis Questions
1. Draw a nondisjunction in anaphase 1 or 2 during meiosis. Assume the cell starts with two
pairs of chromosomes before replication.
Frayer Model
2. Create a Frayer Model for the word
chromosome.
3. Explain how trisomy 21 (Down Syndrome) is
identified with a karyotype.
4. Analyze the two karyotypes above to
determine the sex of each individual.
Extension
5. Turner syndrome is a condition where females
have one x chromosome instead of two. Explain
the mechanism or cause of Turner Syndrome.
6. Edwards syndrome (trisomy 18), Patau
Syndrome (trisomy 13), Turner Syndrome and
Down Syndrome (trisomy 21) are the only
disease caused by a nondisjunction. Predict the impact of a nondisjunction of other chromosomes
in a sperm or egg that is fertilized.