IB Biology 2 / IHS
Name: _________________________________________________ Per. __________
Post-Lab: Using a SNP to Predict Bitter-Tasting Ability!
Revised 2015-16
1.
Write the names of your teammates in the order (from left to right) they loaded their DNA in your gel. Consult
the gel map/ a teammate if you don’t remember.
____________________________ , ____________________________ , ______________________________ , _______________________________
2.
Determine your teammates' PTC phenotypes (same order as in # 1). First taste a strip of control taste
paper in the center of your tongue for several seconds. Note the taste. Then, remove the control paper and
replace it with one strip of PTC taste paper. Again, taste for several seconds. How do you and your teammates
describe the taste of the PTC paper as compared to the control: bitter or no taste (other than paper)?
_____________________________ , ____________________________ , ______________________________ , _______________________________
3.
Analyze your teammates PTC genotypes. Observe the photograph of the stained gel containing your PCR
digest and those from your teammates. Orient the photograph with the wells at the top. The red line in the
annotated gel approximates the location of the uncut PTC gene DNA: 221 base pairs. Use the ideal gel
photograph to help interpret the band(s) in each lane of your gel.
a) Locate the marker DNA lanes on either side of the gel. Our gels ran a little longer than the ideal gel, so
the bands of DNA appear more spread out than they are in the ideal gel. The marker was a plasmid called
pBR322 that was digested with a restriction enzyme called BstNI. Locate the bands corresponding to each
restriction fragment: 1,857 bp, 1,058 bp, 929 bp, 383 bp and 121 bp. The 121-bp band may be very faint,
sometimes invisible.
How many bands of marker DNA are visible on your gel? __________________ .
b) Locate the lanes containing each student’s undigested PCR product (U). There should be one
prominent band in this lane with a size of about 221 bp. Compare the migration of the undigested PCR
product with that of the 383-bp and 121-bp bands of the marker. How many students can see their
undigested DNA on your gel?
____________ out of ____________ .
c) Decide which of three patterns each student's digested PCR products (D) shows:
t t nontaster (homozygous recessive) - a single band in the same position as the uncut control.
TT taster (homozygous dominant) – ideally, two bands: one 177 bp and the other 44 bp. The 177-bp band,
being smaller, migrates ahead of the uncut control; the 44-bp band may be invisible due to primer dimer at
around 50 bp*. If you see the 177-bp band, but not the 44 bp band, you can still score the genotype as TT.
(*Primer dimer is a lab phenomenon in which the primers, being in such large concentrations, bind to and
amplify each other.)
Tt taster (heterozygous) ideally, three bands: the 221-bp band from the "t" parent, and the 177-bp and 44bp bands from the "T" parent. The 221-bp band will be brighter (more intense) than the 177-bp band. (If
the 221-bp band is less intense than the 177-bp band, suspect an incomplete restriction digest of genotype
TT.) The 44-bp band may be invisible. If you see the 221-bp and 177-bp bands, but not the 44-bp band, you
can still score the genotype as Tt.
Additional faint bands at other positions occur when the primers bind to chromosomal loci other than the PTC gene
and give rise to nonspecific amplification products. (We tried to minimize this by placing our PCR tubes on ice until
we could start the PCR machine.)
4.
Record your teammates' PTC phenotypes (same order as in # 1) here:
_____________________________ , ____________________________ , ______________________________ , _______________________________
5.
Re-write team results in Table 1, below. Use tally marks.
Table 2. Correlating PTC genotype with phenotype for my lab group
Genotype from gel photo (#
Phenotype from taste test:
Phenotype from taste test:
Numbers refer to length in DNA base pairs
taster (# 2 above)
nontaster (see #2 above)
TT
2 bands, 177 & 44 (but 44 might be invisible)
Tt
3 bands because t allele is uncut, 221; and, T allele is
cut into 177 and 44 (but 44 might be invisible)
t t (1 band, 221)
No genotype data
6.
How well does the TAS2R38 genotype predict PTC-tasting phenotype? Use your team data in your response.
7.
What is one kind of evidence that would indicate whether the ancestral state of TAS2R38 is to be a taster?
8.
How did the HaeIII enzyme discriminate between the single nucleotide polymorphism in TAS2R38? Be
specific.
9.
Why wasn't the HaeIII enzyme (within the Ready-to-Go Bead) denatured by the hot temperature stage of PCR?
(Hint: think evolutionary tree!)
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10. How does having a “G” at position 145 in both his alleles of the TAS2R38 gene render a person unable to taste
PTC? Explain in detail.
11. In a forensic crime lab like CODIS (the Combined DNA Index System of the FBI), SNPs are not used to exonerate
or incriminate suspects. Instead, they use thirteen STR (Short Tandem Repeats) loci in their DNA analysis.
Some of these loci have more than 20 alleles in the population!! Explain what an STR is by consulting Neil 9e
(pp 420-421) or by going online at
http://www.biology.arizona.edu/human_bio/activities/blackett2/str_description.html.
A diagram might be useful.
12. A 2001 study of 344 populations worldwide by Guo and Reed* estimated the frequency of the recessive PTC
nontaster allele at 0.48. Estimate the frequency of the nontaster allele in our class by using our value of q2.
(Hint: our class is not in HWE, but we'll pretend that it is in order to answer this question.) Show your work.
13. What is one plausible, scientific reason that could explain why
a) …a student has no bands in either his U and D lanes?
b) …a student has the 221-bp band in her U lane, but no bands in her D lane?
14. What did you like best about our PTC tasting experiment?
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