What is a fatal genetic disorder inherited in an autosomal recessive pattern in which harmful quantities of a fatty substance called ganglioside accumulate in the nerve cells in the brain?

The disorder you are referring to is Tay-Sachs disease. It is an autosomal recessive genetic disorder caused by a deficiency of the enzyme hexosaminidase A, leading to the accumulation of gangliOSide GM2 in nerve cells. This buildup results in progressive neurological damage, typically manifesting in infancy and leading to severe disabilities and early death. There is currently no cure, and affected individuals usually do not survive past early childhood.