"At present there are three probable genetic mechanisms for the inheritance of AIS autosomal dominance (this produces a high proportion of affected members in a family), X linked transmission (X is the female sex chromosome) and what is called multifactorial. The last of these is particularly important and multiple genes (a gene is an active or functional segment of DNA which makes up a chromosome) are implicated. It is now known that environmental factors, using this term in a broad but not precisely defined way, play a key role in multifactorial inheritance. This is why research on the genetics of scoliosis in Australia needs to be done in this country. As yet it cannot be determined on clinical and X-ray evidence alone which form of inheritance has resulted in curve development in any one AIS patient.

An Australian study determined that AIS in this country is associated with advanced maternal age (an environmental factor) as it is in the United Kingdom. Further, the children of older mothers tend to have more severe curvatures. As there is now an established trend in our society to defer pregnancy until the late 20s and early 30s, it seems likely that AIS will become more common and curves larger."

http://www.scoliosis-australia.org/s...tic_basis.html

(Okay, I admit I was 35 when I had my daughter).