Final answer:
Each child of two carriers of the Tay-Sachs gene has a 25% probability of developing the condition, regardless of the genetic status of their siblings. The correct probability for the third child to develop Tay-Sachs is 25%.
Step-by-step explanation:
The question is asking about the probability that the third child will develop Tay-Sachs disease, assuming the first two did not but both parents are carriers of the recessive gene responsible for the condition. Tay-Sachs disease is an autosomal recessive disorder, meaning a child must inherit two copies of the defective gene, one from each parent, in order to manifest the disease. If both parents are carriers, there is a 25% chance that any given child will inherit two copies of the gene and thus have Tay-Sachs.
Therefore, given that each pregnancy is an independent event, the probability that the third child has Tay-Sachs is also 25%, assuming that the genetic makeup of the parents has not changed. The previous births do not affect the probability of the third child having the disorder.
The correct answer to the question is b) 25%.