Final answer:
The 30-year-old man has a 2/3 chance of being a carrier for Tay-Sachs disease because his parents, who had two affected children, are both likely carriers (heterozygous). We exclude the possibility of him being affected as he is phenotypically normal.
Step-by-step explanation:
Tay-Sachs disease is an autosomal recessive condition where individuals must receive two copies of the defective gene to exhibit symptoms. Since the 30-year-old man is phenotypically normal and had siblings who died from this disease, we can infer that his parents were both carriers (heterozygous) for Tay-Sachs. The possible genotypes for the siblings would be:
- Healthy and not a carrier (AA)
- Carrier but healthy (Aa)
- Affected by Tay-Sachs (aa)
Because Tay-Sachs is a lethal condition that affects individuals who are homozygous recessive (aa), we know that both deceased siblings had the genotype 'aa'. The probability that the parents would have a child who is a carrier (Aa) is 2/3, considering that we are excluding the possibility of them being affected (since the man is phenotypically normal).
Therefore, the risk that the man is a heterozygous carrier of Tay-Sachs is 2/3. This calculation assumes that since he is unaffected, he could either be non-carrier (AA) or a carrier (Aa), and given his family history, the likelihood of being a carrier is higher.