Causes
The causes of trisomy 21 lie, as the name suggests, in a triple presence of chromosome 21 in the child’s genome. It is therefore a numerical, i.e. number-dependent, maldistribution of the chromosomes. Normally, every human being has two copies of all his autosomal chromosomes (this includes all 23 chromosomes, except for the sex chromosomes X and Y).
These copies are passed on to children. In Down syndrome, the mother passes not one but two copies of chromosome 21 to the child, so that the child has three copies. The risk of such a maldistribution increases with the age of the mother.
From the age of 35 onwards, the mother has a higher risk of a so-called nondisjunction. This means that the mother’s chromosome pairs do not separate in order to be evenly distributed among the daughter cells. There are different types of trisomy 21: In the case described above, the additional chromosome 21 is also free (free trisomy 21).
In 95% of the cases, it is therefore a new mutation. In other cases, which occur very rarely (about 5%), the 21 chromosome is located on another chromosome. This is called translocation trisomy 21.
Complications
Many, but not all, children with Down syndrome may have certain characteristics:
- Round Skull
- Outwardly rising (“mongoloid”) eyelid axis
- Skin fold at the inner corner of the eye (epicanthus)
- Large distance between the eyes
- Flat, broad nose root
- Large Tongue
- Deep set and roundish ears
- Short neck
- Narrow, high palate (speech disorders)
- Short, wide hands
- Four-finger furrow (furrow on the palm of the hand, which normally begins on the side of the little finger and extends between the middle and index finger (i.e. over 3 fingers), in Down syndrome it extends between the index finger and thumb (i.e. over 4 fingers))
- Sandal gap (wide gap between big toe and second toe)
- Mental disability, which varies greatly from person to person in terms of severity
- Slowed reflexes
- Heart failure
- Malformations of the hip
- Changes in the head and facial areaRound skullRising (“mongoloid”) eyelid axisSkin fold at the inner corner of the eye (epicanthus)Large distance between the eyesFlat, broad nose rootLarge tongueDeep and round earsShort neckSmall, high palate (speech disorders)
- Round Skull
- Outwardly rising (“mongoloid”) eyelid axis
- Skin fold at the inner corner of the eye (epicanthus)
- Large distance between the eyes
- Flat, broad nose root
- Large Tongue
- Deep set and roundish ears
- Short neck
- Narrow, high palate (speech disorders)
- Changes in arms and leg shortness, wide handsFour-finger furrow (furrow on palm of hand, which normally starts on the small finger side and extends between middle and index finger (i.e.
over 3 fingers), in Down syndrome to between index finger and thumb (i.e. over 4 fingers))Sandal gap (wide gap between big toe and second toe)
- Short, wide hands
- Four-finger furrow (furrow on the palm of the hand, which normally begins on the side of the little finger and extends between the middle and index finger (i.e. over 3 fingers), in Down syndrome it extends between the index finger and thumb (i.e. over 4 fingers))
- Sandal gap (wide gap between big toe and second toe)
- Other characteristics Mental disability, which varies greatly in severity from individual to individual Slowed reflexesHeart defectsFormities of the hip
- Mental disability, which varies greatly from person to person in terms of severity
- Slowed reflexes
- Heart failure
- Malformations of the hip