When one person in a family is diagnosed with Down syndrome, this does not mean that other family members need to be tested for Down syndrome as well. This is true for several reasons. First, Down syndrome is a condition which is almost always apparent at birth. Therefore, it is extremely unlikely that other family members would have Down syndrome but would not have been suspected as having it until a new baby is born in the family and diagnosed with the condition. Second, most cases of Down syndrome occur for the first time in a family in the individual who is diagnosed. If an individual is diagnosed with the standard trisomy 21 or mosaic trisomy 21 forms of Down syndrome, genetic specialists do not recommend performing chromosome testing on any other family members. This is because these forms of Down syndrome are not inherited; therefore, relatives of individuals with these forms of Down syndrome are not at increased risk to have Down syndrome as well nor are they at risk to carry chromosome rearrangements.
If an individual has been diagnosed with Down syndrome due to a Robertsonian translocation of chromosome 21, genetic specialists do recommend a chromosome karyotype (an organized image of the chromosomes within a cell obtained by microscopic evaluation) for each parent be obtained to determine if the chromosome abnormality was inherited from a parent, or if it occurred for the first time in the diagnosed individual. If neither parent of an individual with translocation Down syndrome has a translocation themselves, other family members do not need to have chromosome analysis. If one of the parents is found to carry a translocation, their family members should also consider being tested for the translocation.
