First, let's assume that there are two genes: A and A.

From the meaning of the question, it can be seen that the disease-causing gene is not necessarily manifested, so it can be determined as a recessive gene disease.

According to "a couple with normal behavior has a normal boy and a girl with a certain genetic disease", their genes must be Aa type, otherwise they will not behave normally but have a sick child.

Then the next question is the genetic determination of two married people. Look at the woman first. Her mother is ill, so she must have genes. She is normal, so the genotype must be Aa. Boys are more complicated, either Aa or AA.

Let's look at the situation that the boy is aa first. The probability that a boy is AA is 1/3 (since he is known to be normal and the possibility of AA is excluded, it is 1/3 instead of 1/4). The probability that a boy and a girl with AA will have a normal son with pathogenic genes is 1/2, so

Looking at the case that the boy is Aa, the probability that the boy is Aa is 2/3, and the probability that the boy with Aa and the girl with Aa have normal sons carrying pathogenic genes is 2/3, so in this case, the probability that the son carries pathogenic genes is 2/3 times 2/3, which is 4/9.

The probability of adding up the above two situations is1/6+4/9 =11/8, so the correct answer is a.