Escola Secundária de Loulé
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.
Many errors can occur during cell division. Sometimes, an error occurs when the 46 chromosomes are being divided in half and an egg or sperm cell keeps both copies of the #21 chromosome instead of just one copy. If this egg or sperm is fertilized, the baby ends up with three copies of the #21 chromosome and this is called "trisomy 21" or Down syndrome. The features of Down syndrome result from having an extra copy of chromosome 21 in every cell in the body. The name "Down syndrome" comes from the physician, Dr. Langdon Down, who first described the collection of findings in 1866.
Ninety-five percent of Down syndrome results from trisomy 21.
Occasionally, the extra chromosome 21 or a portion of it is attached to another chromosome in the egg or sperm; this may result in what is called "translocation" Down syndrome (3 to 4 percent of cases). This is the only form of Down syndrome that can sometimes be inherited from a parent. Some parents have a rearrangement called a balanced translocation, where the #21 chromosome is attached to another chromosome, but it does not affect his/her health.
A very rare case is a form of Down syndrome called "mosaic" Down syndrome that may occur when an error in cell division occurs after fertilization (1 percent to 2 percent of cases). These persons have some cells with an extra chromosome 21 and others with the normal number.
Karyotype of an individual with trisomy 21. Credit: Wikipedia
A child with Down syndrome may have eyes that slant upward and small ears that may fold over slightly at the top. Their mouth may be small, making the tongue appear large. Their nose also may be small, with a flattened nasal bridge. Some babies with Down syndrome have short necks and small hands with short fingers. Rather than having three "creases" in the palm of the hand, a child with Down syndrome usually has one single crease that goes straight across the palm, and a second crease that curves down by the thumb. The child or adult with Down syndrome is often short and has unusual looseness of the joints. Most children with Down syndrome will have some, but not all, of these features.
Human Karyotype. 22 pairs of autossomes and one pair of sexual chromosomes. Credit: Wikipedia
Many errors can occur during cell division. Sometimes, an error occurs when the 46 chromosomes are being divided in half and an egg or sperm cell keeps both copies of the #21 chromosome instead of just one copy. If this egg or sperm is fertilized, the baby ends up with three copies of the #21 chromosome and this is called "trisomy 21" or Down syndrome. The features of Down syndrome result from having an extra copy of chromosome 21 in every cell in the body. The name "Down syndrome" comes from the physician, Dr. Langdon Down, who first described the collection of findings in 1866.
Ninety-five percent of Down syndrome results from trisomy 21.
Occasionally, the extra chromosome 21 or a portion of it is attached to another chromosome in the egg or sperm; this may result in what is called "translocation" Down syndrome (3 to 4 percent of cases). This is the only form of Down syndrome that can sometimes be inherited from a parent. Some parents have a rearrangement called a balanced translocation, where the #21 chromosome is attached to another chromosome, but it does not affect his/her health.
A very rare case is a form of Down syndrome called "mosaic" Down syndrome that may occur when an error in cell division occurs after fertilization (1 percent to 2 percent of cases). These persons have some cells with an extra chromosome 21 and others with the normal number.
Karyotype of an individual with trisomy 21. Credit: Wikipedia
A child with Down syndrome may have eyes that slant upward and small ears that may fold over slightly at the top. Their mouth may be small, making the tongue appear large. Their nose also may be small, with a flattened nasal bridge. Some babies with Down syndrome have short necks and small hands with short fingers. Rather than having three "creases" in the palm of the hand, a child with Down syndrome usually has one single crease that goes straight across the palm, and a second crease that curves down by the thumb. The child or adult with Down syndrome is often short and has unusual looseness of the joints. Most children with Down syndrome will have some, but not all, of these features.
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