Download Conclusion Introduction Background The PTC Sensitivity Gene

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Introduction
Individuals vary greatly in their sensitivity to the bitter compound
Phenylthiocarbamide (PTC). This is one of the best known genetic traits
in the human population and historically has been the most popular
teaching subject in inheritance. However, the classic PTC paper test falls
short of differentiating between homozygous vs heterozygous in the taster
alleles. Here we introduce a PCR method to identify an individual’s DNA
haplotypes for the PTC receptor gene.Upon gel electrophoresis, the
homozygous or heterozygous state of the taster/nontaster alleles can be
correctly determined, allowing accurate prediction of one’s PTC taste
phenotype. This kit provides an excellent hands-on opportunity for
students to correlate their genotypic variations to differences in
phenotypes.
Individuals’ DNA that are homozygous for the taster or nontaster alleles
will only be amplified either with taster or nontaster specific primers,
respectively. DNA of people who are heterozygous for the taster/nontaster
alleles will have amplification products in both PCR reactions (Figure 2).
Thus this method provides an easy and efficient way to study the
inheritance of the PTC sensitivity among family members.
T NT
T NT T
NT
Taster Primers
Non-Taster Primers
Tester (T) or Non-Taster (NT)
control DNA Templates
Background
Variation in PTC sensitivity was discovered in the 1930s, and it has since
been widely used in genetic and population studies. It has enjoyed great
attention among geneticists and in class rooms as an informative and
easily typed genetic marker. Particularly because variations in PTC
sensitivity is regardless of sex, age, and race, and is impossible to predict
the phenotypes before testing. Yet when tested, the outcomes can be
drastically different even among siblings. For majority of the population,
the inheritance appears to follow simple Mendelian rule. Thus it has
traditionally been viewed as one of the best human trait for classroom
teaching of genetic inheritance.
Figure 1. Bitter Taster Detector Kit contains two primer pairs.
The Taster prime pair will only amplify the taster allele, while the
Non-Taster primer pair will amplify the non-taster allele.
A
T
NT
T
NT
T
NT
T
NT T NT
The PTC Sensitivity Gene-TAS2R38
The gene responsible for PTC sensitivity was eventually identified in
2003, more than 70 years after its original discovery. TAS2R38 encodes a
G protein coupled membrane receptor protein. Analysis of SNPs (single
nucleotide polymorphisms) along the coding region of TAS2R38 revealed
that taster and nontaster alleles differ by just 3 amino acid changes.
Position
Taster
Nontaster
145
Pro
Ala
785
Ala
Val
886
Val
Ile
Table 1 Polymorphisms and Amino Acid changes in TAS2R38
Typing PTC Sensitivity by Allele-Specific PCR
We have developed a PCR strategy to specifically amplify either the taster
or the nontaster alleles. In this method, genomic DNA is isolated from
buccal swabs. Two PCR reactions are performed to amplify specific
alleles of the TAS2R38 gene, one with taster-specific primers and the other
with nontaster-specific primers. PCR products are resolved by gel
electrophoresis and presence of DNA bands on gel is indicative of the
presence of taster or nontaster alleles (Figure 1).
B
Figure 2. PCR results of a five-member family. A. Father is
homozygous taster. Mother is homozygous nontaster. All three
children are heterozygotes. B. Family pedigree for the inheritance
of PTC Sensitivity alleles.
Conclusion
The Bitter Taster Detector Kit is an excellent educational tool for teaching
basic molecular biology skills including genomic DNA isolation, PCR
amplification, and gel electrophoresis. It greatly enhances students’
interests in such a way that they will be determining their own genotypes,
predicting their own phenotypes, and validating their own DNA testing
results with a simple PTC paper test. The kit includes instruction and all
the reagents needed for DNA isolation, PCR amplification, and PTC paper
test.
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