|
|
|
Down syndrome in the family |
|
What information is important for you to know if you are pregnant? |
Regarding a family member with Down syndrome, it is important to ascertain the following details:
- Is this investigation specifically for Down syndrome or for
mental retardation due to an unknown cause?
- Have the patient's chromosomes been examined? If so, where, when, and what was the result?
- What was the age of the patients mother at the time of his/her birth?
- What is the patient's relationship to you?
|
|
Why is this important? |
It is important to ascertain the answers to these questions since there are a number of causes of
Down syndrome, some of which are hereditary (listed below).
|
|
|
|
Every healthy person has 23 pairs of chromosomes (a total of 46 chromosomes)
arranged and numbered by shape and size. When the chromosomes of people with
Down syndrome are examined, it can be seen that there are three types of pathological karyotypes:
- Trisomy: in approximately 95% of cases, the patients karyotype shows a trisomy of
chromosome number 21. There are three number 21 chromosomes instead of the normal two,
so that there are in total 47 chromosomes instead of 46, a condition known as trisomy.
In most cases, this occurs in women who are pregnant at an advanced age.
The risk of recurrence is only relevant to the mother of the affected child, and there are no
risks to other family members.
- Translocation: in about 3-4% of cases there is a
Robertsonian translocation between chromosome 21
and another chromosome. In Down syndrome, such a translocation involves a fusion between the extra
chromosome 21 and chromosomes 13, 14, 15, 21 or 22. These patients have two normal number 21 chromosomes
and a translocated chromosome 21. The translocated chromosome is the one that is connected to the
other chromosome, and the commonest example of this is a translocation between
chromosome 21 and chromosome 14. In such a case it is important to know whether the patient's parents
have been examined and whether one of them carries an identical, balanced translocation (one normal chromosome
and 1 translocated chromosome 21 that is connected to another chromosome).
Such individuals have a normal total number of copies of the genetic material in chromosome 21.
If this is the case, a number of close relatives may also carry the same translocation, and such people
therefore are also at risk. It is therefore important to refer these relatives for chromosome testing.
If neither of the patients parents carries an identical translocation, the other family members are not at high risk.
- Mosaicism: in approximately 2-3% of cases, there is a mosaic of normal cells and trisomy 21 cells.
In these cases, there is no increased risk for other family members.
|
It must be emphasized that in all cases of Down syndrome it is important to obtain appropriate
genetic counseling. Such counseling will evaluate the risk of having an infant with Down syndrome
based on data that the parents give the counselor about their family history, the relationship of
the affected family member, and which tests the individual underwent.
If the parents had a previous pregnancy involving a fetus with Down syndrome, it is recommended
that the mother undergo prenatal diagnosis in each pregnancy (chorionic villus sampling or amniocentesis).
Such a woman is at high risk of having a child with a structural chromosomal disorder.
When Down syndrome is caused by an additional chromosome 21 (trisomy 21), there is a 1% increment in
the risk of chromosomal problems in each additional pregnancy.
In the case of translocation, the risk for recurrence in future pregnancies depends on whether the
translocation carried by the carrier parent is balanced or not. If it is, the risk of recurrence is about 15%.
Mosaicism presents no significant added risk for recurrence.
|
|
|