Ptc Genetics Lab Student Worksheet printable pdf download

PTC

PTC Genetics Lab Student Worksheet

Experiment Objective

To formulate a hypothesis and an experimental method to test it. To develop an understanding of electrophoresis

and genetic principles. To analyze results and to deduce genotypes from given phenotype and pedigree information.

Background

In 1931, a chemist named Arthur Fox started to measure some powdered phenylthiocarbamide (PTC). Pouring hastily,

Fox accidentally caused some of the chemical to blow into the surrounding air. Fox’s lab mates nearby complained of

the bitter taste in the air due to the chemical. Yet, Fox was perplexed- he tasted nothing. Since that day, PTC has been

used to show genetic variation in tasting abilities. When people sample PTC, some people taste a strong bitterness,

others taste a slightly bitter taste, and others taste nothing at all. Using genetics, we can try to understand why some

people can taste this chemical and others can’t (refer to Appendix A for more information on genetic inheritance).

Taste and Genetics

The sensation of taste can be categorized into five basic types: sweet, sour, salty, bitter, and umami (the taste of

monosodium glutamate). These five tastes serve to classify compounds into potentially nutritive and beneficial

(sweet, salty, umami) or potentially harmful or toxic (bitter, sour). The ability to taste is due to the presence of

chemically sensitive, specialized taste receptor cells on the surface of the tongue and throat. When we eat something

sweet, the soluble molecules in the food dissolve in saliva and binds to specific receptor proteins on the surface of the

receptor cells that detect sweet taste. The stimulated receptor cells send nerve impulses to gustatory region of the

brain where the sense of taste is interpreted. Different types of taste receptors are activated by different chemicals,

and the nerve impulses they send to the brain are interpreted as different tastes.

Scientists have shown that the TAS2R taste receptor proteins are responsible for the human ability to taste bitter

substances. These taste receptor proteins are encoded by about 30 different genes. One of the best-studied genes,

the TAS2R38, codes for a G protein-coupled receptor (GPCR) which contributes to the tasting of the chemical

Phenylthiocarbamide (PTC). When molecules of PTC bind to the TAS2R38 receptor protein, some people can taste the

bitterness while others taste nothing at all – we call them “Tasters” and “Non-Tasters”, respectively. PTC is a man-

made chemical that resembles toxic alkaloids found in some poisonous plants. Although PTC is not found in nature,

the ability to taste it correlates strongly with the ability to taste other bitter substances that do occur naturally, many

of which are toxins.

This genetic variation in the ability to taste PTC has been of great interest to those who study genetics. The variation

in PTC sensitivity is determined by two common alleles of the TAS2R38 gene: the functional allele and the mutated

allele. The DNA sequence between these two alleles is only different at a single base pair. This type of polymorphism

within a DNA sequence is termed “Single Nucleotide Polymorphism (SNP)”. Just a single point mutation changes the

DNA coding sequence, the protein produced, and the protein’s function. Any person with a single functional allele

can make the PTC receptor protein and therefore can taste the bitterness of PTC. Some studies have shown that

when homozygous tasters (who carry two functional alleles) attempt to taste PTC, they experience a more intense

bitterness than heterozygous tasters do (who only carry one functional allele). On the other hand, homozygous non-

tasters (with two mutant alleles) cannot taste the bitterness of PTC at all.

In this kit, the provided DNA samples include only a short region of the TAS2R38 gene. This region of the TAS2R38

gene is 221 base pairs (bp) in length. As mentioned above, the functional allele and the mutant allele only differ by

a single base pair. To distinguish between these two alleles, we could either 1) sequence the DNA fragments to find

the exact nucleotide sequence, or 2) combine the techniques of restriction enzyme digestion and gel electrophoresis

to reveal the difference. In our PTC kit, we will focus on the second method which is an easier and more affordable

method for college and high school students. This method does not require the use of expensive equipment to

analyze the DNA sequence.

Ptc Genetics Lab Student Worksheet

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