Down's syndrome - inheritance?
I read the section on Down’s syndrome and would like to ask a question. My boyfriend’s aunt and uncle have the syndrome i.e. his mother’s sister and brother. I was told that either he or is sister could have a child with Down’s. Is Down’s syndrome hereditary and can it be passed down, skipping a generation?
The significance of a family history of Down’s syndrome depends on the type of chromosomal abnormality in the affected person. There are three main types of chromosome abnormality in Down’s syndrome. The commonest type, which occurs in over 95% of cases, is known as trisomy 21, which means that the affected person has three chromosomes 21 instead of the usual complement of two. Trisomy 21 occurs sporadically and there is a very marginal risk of recurrence for other family members. The additional risk is so slight that it is not worth being concerned about. Also given the fact that your boyfriend is a nephew to the affected people that would reduce the risk even further. The second type is called translocation, which means that the extra chromosome is translocated or attached to another chromosome. This type of inheritance is seen in 3% of Down’s sufferers. In that situation one of the parents is a carrier for the translocation defect. In that situation there is an increased risk of Down’s recurring. The third type is referred to as mosaicism, which means that some cells have 47 chromosomes and the others have 46. This occurs in less than 1% of affected people. The only way you can find the specific information you are looking for would be to establish the nature of the defect in your boyfriend’s relatives. If they have trisomy 21 there is no significant increased risk of you and your boyfriend having a child with Down’s syndrome. If your boyfriend’s relatives have inherited Down’s syndrome through translocation of chromosome 21 then the question arises if he could be a carrier but the risk of that happening would be negligible. The other question that arises is the possible genetic testing of your boyfriend. The question you raise is quite complex and at the end of the day it is important that we keep a sense of proportion about the risks of you and your boyfriend producing a child with Down’s syndrome. The risk is very small.