How Gene Mutations Change Your Ability to Taste

The genetic material within our cells, our genome, plays a large role in determining our traits, such as how we look and how we function. Every person—with the exception of identical twins, triplets, etc.—has a unique set of genes inherited from their parents. These genes consist of DNA (deoxyribonucleid acid). DNA is a genetic code that is made up of four different nucleotides that each include a different base molecule: adenine (A), thymine (T), guanine (G), and cytosine (C). The nucleotides are organized in a double helix structure, made up of two complimentary DNA strands: the sense, or coding, strand, and the antisense strand. The antisense DNA strand is turned into RNA (ribonucleic acid) in our bodies in a process called transcription.

The resulting RNA has the same sequence as the sense strand of the DNA; the only difference is that in RNA, a nucleotide called uracil (U) substitutes for each thymine molecule. The RNA then goes through a process called translation to turn into amino acids (Figure 1). During translation, every three RNA nucleotides encodes a single amino acid. This set of three nucleotides is called a codon, and different codons may code for the same amino acid, as shown in Figure 2. In the end, a sequence of DNA is turned into a sequence of amino acids joined together in a long chain, which is called a protein. Proteins are responsible for most our cell function.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 1. A DNA sequence is first transcribed into RNA, and then translated into a protein.

Image Credit: Pixabay / Public Domain
Figure 2. In the genetic code, every nucleotide triplet (codon) encodes for one amino acid. Some codons are "stop" codons, which signal the end of a protein.

Many things can happen during this process that can result in an altered structure of the protein or prevent a gene from turning into a functional protein. For example, if a single DNA nucleotide is mutated in a gene from an adenine (A) to a guanine (G), this may cause the wrong amino acid to be made. This is called a missense mutation. If the wrong amino acid is made and assembled into a long chain of amino acids, the resulting protein might have different properties than intended, or might not work at all. This is because different amino acids have different characteristics, which affects how they interact with each other as well as the molecules that surround them. Other single DNA nucleotide mutations might not lead to a change in the amino acid sequence, in which case they are called silent mutations and do not have any effects on the protein function.

Gene mutations may result in the same gene occurring in slightly different versions. These variants are called alleles. For example, a gene that encodes for a flower's color can have a purple-flower allele or a white-flower allele. The set of alleles that a person inherits make up the individual's genotype. As each person inherits two alleles—one from each parent —these two alleles can be the same, in which case the person is called a homozygote, or different, which is called a heterozygote. The traits, or observable characteristics such as color, shape, behavior, or other properties of an organism that result from the expression of its genotype, is called the phenotype. In the flower color example, this would mean that depending on what alleles are present in the plant, its phenotype (flower color) would be purple or white. This is simple if a plant has two purple alleles or two white alleles, but what if it has one white and one purple allele? In this case, it depends on which allele is dominant and which is recessive. The dominant allele always determines the phenotype, which explains why different genotypes can produce the same phenotype.

Over time, numerous mutations have created a great variety of alleles in the human genome, which is the reason for the genetic variety we see. In this lesson plan, students will explore how mutations in a taste receptor gene have resulted in two phenotypes, a "taster" and a "non-taster," with respect to a certain chemical called phenylthiocarbamide, or PTC. Depending on alleles present in their genome, students will either be able to taste bitterness in the PTC sample, or will not detect a taste in the PTC at all. Researchers have found that this difference is based on three single nucleotide mutations in the PTC receptor gene. Students will find out if they are a PTC "taster" or "non-taster" and will explore the reason for the two different phenotypes.