Offspring develop the disorder if both its parents have that recessive trait in their genes. If both parents are carriers of it then there is a 25% chance that the offspring can get the disorder. If both parents have that disorder there is a 100% that the offspring will get it. If one parent has it and one parent doesn't, and isn't a carrier as well, there is a 0% chance of the offspring getting it. If one parent is a carrier and one parent doesn't have it, it is also a 0% chance.
