To clarify on A woman outside’s post- The genetics of late onset/adult onset Tay Sachs is the same as infantile Tay Sachs- the same gene- HEXA- is mutated, the affected individual has 2 mutated copies, one from each carrier parent. There are many different mutations that can occur in this gene. In the infant form, this results in the absence of the gene product hexosaminidase A, so symptoms are most severe and observed early. In the adult version, there is a deficiency in hexosaminidase A, the, but some enzyme activity is present, so that the disease progresses slowly, and at different rates in different individuals depending on the actual enzyme levels.
From your story, it seems like one of the dating partners (let’s assume the boy) has the disease, ie is homozygous for one of the mutations. Apparently, his parents were not tested, and since he was asymptomatic during childhood, he did not know he had Tay Sachs. The girl is negative, not even a carrier, which is why their numbers are compatible- if they were to marry, then their kids would all be carriers but not have the disease. So what it boils down to, is the girl willing to marry someone with what may very will be a debilitating degenerative disease. In this aspect, it is not any different from any other disease that could not have been predicted.
By the way, Dor Yesharin only works as far as known mutations. In early testing days, fewer mutations were identified and tested for. These have been expanded, also to include mutations common to non-Ashkenazik populations (not as common as the ones in Ashkenazik populations). If theoretically, someone was to develop a sporadic germ-line mutation in the gene that is not known and not tested for, they could be found compatible with a regular Tay Sachs carrier, the couple could marry and have a child with Tay Sachs at 25% probability.
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