Galactosemia is a recessive human disease that is treatable by restricting lactose and glucose in the diet. Susan Smithers and her husband are both...

Galactosemia is a recessive human disease that is treatable by restricting lactose and glucose in the diet. Susan Smithers and her husband are both heterozygous for the galactosemia gene.

If the couple has four children, what is the probability that the first two will have galactosemia and the second two will not?

ANSWER: (1/4 x 1/4 x 3/4 x 3/4)6 = 54/256

Where did the 6 come from? I know order matters but what does the 6 mean?