of Down Syndrome
Down syndrome is a genetic
condition that can be caused by one of three chromosomal abnormalities.
- The majority of persons
with Down syndrome (93-96%) receive three copies of chromosome 21 (trisomy 21) as a result of nondisjunction which occurs during Meiosis I or Meiosis II.
Consider visiting the YouTube Biologybyme's channel video for a thorough explanation.
Nondisjunction is not a heritable defect, rather it is a random event that occurs during the formation of one batch of reproductive cells. The result is a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21. If a child results from the fertilization of such an atypical reproductive cells, the child will have an extra chromosome 21 in every somatic cells.
- Three to four percent
of Down syndrome results from translocation, that occurs when some chromosome 21 genes are added to another chromosome. The most common chromosome to accept additional genes from 21 is 14, although it is also known to attach to 13, 15, 21, and 22. Translocations can be inherited from an apparently unaffected parent. The parent may have a balanced translocation in which segments are swtiched between chromosomes. Unbalance translocations result in Down syndrome when extra genetic material from chromosome 21 is present in the genome.
- Finally, 1-2% of all
persons with Down syndrome have trisomy 21 mosaicism, a condition in which
only some of the bodys cells have three copies of the 21st chromosome.
Adults with Down syndrome
almost always have some degree of mental retardation, but it is usually in the
mild to moderate range. There is a wide variation in the communicative and cognitive
status of adults with Down syndrome. Some speak clearly and understandably,
others have significant issues with grammar and articulation, and a small number
depend on augmentative communication devices. Many adults with Down syndrome
attain functional levels for reading, writing, and math; others have been less
successful in these areas. While there is usually some degree of impairment
in both fine and gross motor skills, many adults with Down syndrome are good
athletes and hold responsible jobs.
Adults with Down syndrome
tend to have certain physical characteristics, including:
- hypotonia, or low muscle
- a small, brachycephalic
head (flat in back at the occiput)
- flattened mid-face
- small, cup-shaped ears
- small mouth with a large
tongue that may protrude
- epicanthal folds in the
- excess skin folds on
the back of the neck
- wide space between the
first and second toes
- short limbs, hands, and
- a single crease across
one or both palms
To learn more, consider visiting the National Down Syndrome Society web page
What is Down Syndrome?
Adults with Down syndrome often
have certain physical characteristics, including low muscle tone and distinct