Preimplantation genetic diagnosis (PGD) was originally developed for couples 
whose potential offspring were at risk of severe Mendelian disorders, but has 
since been extended to other indications. One possible use of PGD is to perform 
gender selection for couples whose offspring are at increased risk of disorders 
that do not follow Mendelian inheritance, but which are substantially more 
common in one sex than another (unequal sex incidence). Here, we examine the 
clinical and ethical issues to be considered prior to offering PGD gender 
selection to reduce the risk of a child being affected by a non-Mendelian 
condition with unequal sex incidence. Factors to be considered include: the risk 
that a child of either sex will be affected by the condition; the overall 
reduction in risk provided by gender selection and the potential harms of the 
procedure. Consideration should also be given to the interests of the family and 
the child to be born, the seriousness of the condition and the couple's 
procreative autonomy. To illustrate these issues we use the example of autism, a 
non-Mendelian disorder that is considerably more common in males than in 
females.