The Tay-Sachs allele in humans is expressed as incompletely dominant at the biochemical level , even though the disease itself shows recessive inheritance at the clinical level.

Key idea

  • People with two mutant alleles (homozygous recessive) have almost no functional Hexosaminidase A enzyme, so GM2 ganglioside accumulates massively in neurons.
  • Heterozygous carriers have reduced Hexosaminidase A activity compared with normal homozygotes, but still enough to prevent symptoms.
  • Because the enzyme level in heterozygotes is intermediate between normal and affected, the allele’s biochemical expression fits incomplete dominance, even though carriers are clinically normal and the disease appears recessive.

So, among multiple-choice options, the expression of the Tay-Sachs allele in humans at the biochemical level is best described as incompletely dominant.