The TAS2R38 gene encodes a receptor protein that influences the ability to taste bitterness. The gene has two alleles: a dominant, wild-type allele that enables an individual (taster) to taste bitterness and a recessive, mutant allele that interferes with the ability of an individual (nontaster) to taste bitterness. Three single nucleotide mutations in the coding region of the TAS2R38 gene are associated with the nontaster allele. The nucleotides present at the three positions are shown in the table below.

The table has a title NUCLEOTIDE VARIATION IN THE T A S 2 R 3 8 GENE. The table shows the position in the Nucleotide Sequence for different species. The data are as follows. Human nontaster; 145, G; 785, T; 886, A. Human taster; 145, C; 785, C; 886, G. Bonobo; 145, C; 785, C; 886, G. Chimpanzee; 145, C; 785, C; 886, G. Gorilla; 145, C; 785, C; 886, G.

A cladogram representing the evolutionary relatedness of selected primates is shown below.

The figure presents a cladogram with the root at the bottom left and three branches. From left to right, the branches are as follows: Orangutan, Gorilla, and Chimpanzee. Past the branches, the main line is labeled Human. Another branch, labeled Bonobo, is connected to the Chimpanzee branch.

The TAS2R38 receptor protein has been detected on the surface of cells from individuals who are homozygous for the nontaster allele of the TAS2R38 gene. Which of the following is the most likely effect of the mutations associated with the nontaster allele on TAS2R38 gene expression?

Question

The TAS2R38 gene encodes a receptor protein that influences the ability to taste bitterness. The gene has two alleles: a dominant, wild-type allele that enables an individual (taster) to taste bitterness and a recessive, mutant allele that interferes with the ability of an individual (nontaster) to taste bitterness. Three single nucleotide mutations in the coding region of the TAS2R38 gene are associated with the nontaster allele. The nucleotides present at the three positions are shown in the table below.

The table has a title NUCLEOTIDE VARIATION IN THE T A S 2 R 3 8 GENE. The table shows the position in the Nucleotide Sequence for different species. The data are as follows. Human nontaster; 145, G; 785, T; 886, A. Human taster; 145, C; 785, C; 886, G. Bonobo; 145, C; 785, C; 886, G. Chimpanzee; 145, C; 785, C; 886, G. Gorilla; 145, C; 785, C; 886, G.

A cladogram representing the evolutionary relatedness of selected primates is shown below.

The figure presents a cladogram with the root at the bottom left and three branches. From left to right, the branches are as follows: Orangutan, Gorilla, and Chimpanzee. Past the branches, the main line is labeled Human. Another branch, labeled Bonobo, is connected to the Chimpanzee branch.

The TAS2R38 receptor protein has been detected on the surface of cells from individuals who are homozygous for the nontaster allele of the TAS2R38 gene. Which of the following is the most likely effect of the mutations associated with the nontaster allele on TAS2R38 gene expression?

BlackTom AI · Accepted Answer

The question asks about how mutations associated with the TAS2R38 nontaster allele affect gene expression, given that these mutations are located in the coding region and three nucleotide changes distinguish nontasters from tasters.
Option 1: 'The mutations prevent transcription of the TAS2R38 gene.' Transcription is the process of making RNA from DNA, typically governed by promoter and enhancer regions, transcription factors, and chromatin structure rather than changes within the coding sequence itself. Coding-region mutations usually do not halt transcription entirely; they mainly alter the RNA sequence or the protein after translation. Thus this option is unlikely given the location of the mutations.
Option 2: 'The mutations change the primary structure of the encoded receptor protein.' The coding region directly encodes the amino acid sequence of the protein. Nucleotide changes here can lead to non-synonymous substitutions (missense), nonsense mutations, or frameshifts, all of which can alter the primary structure (the linear sequence of amino acids) of the TAS2R38 receptor. Since the nontaster alleles are defined by coding-region differences that affect the receptor’s function, this option is biologically plausible and aligns with the description of mutations in the coding region.
Option 3: 'The mutations prevent translation of the TAS2R38 mRNA.' Translation is the process of synthesizing protein from mRNA, and while coding-region mutations can cause misfolded proteins after translation, they do not typically prevent translation from occurring entirely. If translation were blocked, the protein would not be produced at all, which would be a more extreme effect than usually described for specific coding-region SNVs associated with receptor function. Therefore this option is less consistent with typical coding-region SNVs in this context.
Option 4: 'The mutations increase the stability of the TAS2R38 mRNA.' mRNA stability is generally influenced by untranslated regions (UTRs), regulatory elements, or specific sequence motifs that affect RNA decay. Coding-region mutations are less commonly described as increasing mRNA stability; moreover, stabilizing mRNA would more likely affect transcript levels rather than the receptor’s primary structure or function. In this scenario, the phenotype (nontasting) is tied to receptor function, not an appreciable change in mRNA stability, making this option less plausible.
In summary, the most coherent interpretation is that the three coding-region nucleotide changes alter the amino acid sequence of the TAS2R38 receptor, changing its primary structure and thus its ability to function as a taster receptor. The other options rely on effects on transcription, translation initiation, or mRNA stability, which are not as consistent with coding-region SNVs described for the nontaster allele.

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