Which choice below describes the expression of Tay-Sachs disease in humans at the biochemical level?

• A. Dominant
• B. Codominant
• C. Recessive
• D. Polygenic
• E. Incompletely dominant

Answer: (E)

Verified step by step guidance

1. Understand the genetic basis of Tay-Sachs disease: Tay-Sachs is caused by mutations in the HEXA gene, which encodes the enzyme beta-hexosaminidase A. This enzyme is crucial for breaking down GM2 gangliosides in nerve cells.
2. Recognize the inheritance pattern: Tay-Sachs disease is inherited in an autosomal recessive manner, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to express the disease.
3. Explore the biochemical expression: At the biochemical level, individuals with one normal allele and one mutated allele (carriers) produce less beta-hexosaminidase A enzyme than individuals with two normal alleles, but more than individuals with two mutated alleles.
4. Consider the concept of incomplete dominance: Incomplete dominance occurs when the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. In Tay-Sachs, carriers have intermediate enzyme levels, which is why the disease is described as incompletely dominant at the biochemical level.
5. Clarify the distinction: While Tay-Sachs disease is recessive in terms of disease expression (symptoms appear only in homozygous recessive individuals), the enzyme activity level in heterozygotes reflects incomplete dominance, as it is not fully dominant or recessive.