Probability of getting a down syndrome baby?

I'm seriously wondering, if 2 down syndrome people have sex, what is the likelihood of conceiving a baby that also has down syndrome?

>>3
but in the case of down syndrome, down is a negative trait

multiplying two negatives yields a positive outcome
-1x-1=1
1x1=1
-1x1=-1
1x-1=1

so as long as BOTH parents have down syndrome, you should be fine