The second reason that might be involved is called "penetrance-. If one allele causes a condition to be present in some people but not others, that is called "variable penetrance,"" and that appears to be what happens with trisomy 21: the alleles don not do the same thing to every person who has it. Both reasons may be why there is such variation in children and adults with Down syndrome.
RELATIONSHIP OF DOWN SYNDROME INCIDENCE TO MOTHERS' AGE.
Mothers Age Incidence of Down Syndrome.
Under 30 Less than 1 in 1,000.
30 1 in 900.
35 1 in 400.
36 1 in 300.
37 1 in 230.
38 1 in 180.
39 1 in 135.
40 1 in 105.
42 1 in 60.
44 1 in 35.
46 1 in 20.
48 1 in 16.
49 1 in 12.
Source: Hook, E.G., Lindsjo, A. Down Syndrome in Live Births by Single Year Maternal Age.
Although many theories have been developed, it is not known what actually causes Down syndrome. Some professionals believe that hormonal abnormalities, X-rays, viral infections, immunologic problems, or genetic predisposition may be the cause of the improper cell division resulting in Down syndrome. It has been know for some time that the risk of having a child with Down syndrome increases with advancing age of the mother. As the older the mother gets when she has children, the greater the possibility that she may have a child with Down syndrome. However, most babies with Down syndrome (more that 85 per cent) are born to mothers younger than 35 years. Some investigators reported that older fathers might also be at an increased risk of having a child with Down syndrome. It is well known that the extra chromosome in trisomy 21 could either originate in the mother of the father. Most often, however, the extra chromosome is coming from the mother. Parents who have a child with Down syndrome have an increased risk of having another child with Down syndrome in future pregnancies. It is estimated that the risk of having another child with Down syndrome is about 1 in 100 in trisomy 21 and mosaicism.
