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BIOLOGY: RESOURCES: Genetics - Down Syndrome
OF DOWN SYNDROME AND NORMAL KARYOTYPES
These are the chromosomes of a normal karyotype. Try pairing the chromosomes yourself (as has been done for the Down Syndrome karyotype below).
Down Syndrome results when three, rather than the normal two, copies of chromosome 21 are present in each cell. An abnormal number of chromosomes results when a mistake called nondisjunction occurs in the production of eggs or sperm.
In nondisjunction, the paired copies of a particular chromosome do not separate at cell division and therefore, two copies of a chromosome are placed in a gamete, rather than one. If that gamete is used in a fertilization event, its two chromosomal copies are united with one chromosomal copy from the other gamete, resulting in an embryo with three copies of that particular chromosome (called trisomy).
Whereas in most cases three copies of a chromosome are lethal to the embryo, in certain cases an extra or missing chromosome is tolerated. However, such individuals usually have serious medical problems due to the imbalance in gene products expressed from that chromosome. In the case of Down Syndrome, trisomy 21 results in mental retardation, characteristic facial features, and decreased muscle tone.
In the Down Syndrome karyotype below, the chromosomes have been arranged in their pairs. Note the extra copy of chromosome 21.