Transcript PTC Tasting and Evolution

PTC Tasting and
Evolution
Why you don’t like to eat veggies and other
bitter foods
Why do we have taste
preferences?
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Is it in our genes?
If so why?
Can it influence dietary preferences?
How do I determine if data is valid?
Vocabulary
• SNPs – single nucleotide polymorphism or a
single change in a gene that allows for
differentiation in that gene (an A is changed to
another nucleotide)
• Locus – location on the chromosome
• Haplotype - a group of alleles of different genes
on a single chromosome that are closely
enough linked to be inherited usually as a unit
(Merriam-Webster Online Dictionary 2008)
example: red hair and blue eyes are on the
same chromosome and are inherited together.
Who came up with the
PTC paper test?
• Fox, 1931
• Olsen et.al. (1989) data fitted best a 2-locus
model in which one locus controls PTC tasting
and the other locus controls a more general
taste ability.
• Are there differences in population ability to
taste?
• Who came up with the PTC paper test?
• The incidence of inability to taste PTC is much
lower in blacks than in whites in North America:
3% as compared with 30%
PTC Factoids
• It has long been proposed that a relationship of
PTC nontasting to cretinism exists (e.g.,
Shepard, 1961).
• PTC is structurally similar to 6-n-propylthiouracil
(PROP) and the naturally occurring anti-thyroid
substance l-goitrin; all members of this class of
chemicals have anti-thyroidal activity and are
not tasted by PTC non-tasters (Shepard, 1961).
• This means that PTC non-tasters could develop
thyroid problems because they cannot taste
alkaloids as readily
Why can I taste PTC?
• There has been a suggestion that PTC tasting
in humans is related to a component of saliva:
Cohen and Ogdon (1949) claimed that PTC
tasters can taste PTC only if it is dissolved in
their own saliva
• If the tongue is dried and then presented with
PTC dissolved in someone else's saliva, it is
tasteless. Jones and McLachlan (1991)
described a technique for fitting mixture
distributions to data on PTC sensitivity.
How is PTC taste related
to natural selection?
• Tepper (1998) discussed the classic explanation
for the persistence of the PROP polymorphism,
i.e., a selective advantage for avoidance of
harmful compounds in the environment that are
often bitter tasting
• PROP and PTC are chemically related to the
isothiocyanates and goitrin, bitter-tasting
compounds that are present in cruciferous
vegetables such as cabbage, broccoli, brussel
sprouts, turnips, and kale.
How is PTC related to
natural selection?
• When eaten in large quantities, these
compounds interfere with iodine
metabolism, producing thyroid
enlargement and goiter-like symptoms.
Tepper (1998) noted that the incidence of
thyroid deficiency disease is relatively rare
among PTC tasters.
Modern Society?
• In modern society, however, avoidance of
bitter-tasting foods may have health
disadvantages, since epidemiologic
studies indicate that diets low in fruits and
vegetables and high in fat may be
associated with increased risk of heart
disease and cancer
Possible effect on dietary
preferences in modern times
• Children carrying one or two bittersensitive regions of a gene prefer higher
concentrations of sucrose solutions and
have a stronger preference for sweettasting food and beverages than did the
bitter-insensitive kids.
(http://www.sciencedaily.com/videos/2005/
0508-the_taste_gene.htm)
• How could this affect obesity problems?
Why aren’t PTC genes
lost in a population
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If PP = homozygous PTC tasters
If AP = PTC tasters who are heterozygous
If AA = homozygous non tasters, then
If AP mates with AP, what percent of their
children will be
– Tasters
– Nontasters
Genetics of PTC
• Fisher et al. (1939) hypothesized that the
pervasive phenotypic variation in PTC taste
perception is due to balancing natural selection,
which may have favored heterozygotes.
• Wooding et. al (2004) analyzed the entire coding
region of PTC (1,002 bp) in a sample of 330
chromosomes from different populations (62
African, 138 Asian, 110 European, and 20 North
American) by use of statistical tests for natural
selection that take into account the potentially
confounding effects of human population growth.
Alleles and PTC tasting
• Two haplotypes of intermediate frequency
corresponding to 'taster' and 'nontaster'
phenotypes were found.
• Supported the hypothesis of Fisher et al.
(1939) and suggested that balancing
natural selection has acted to maintain
'taster' and 'nontaster' alleles at the PTC
locus
SNPs and Phenotypes
• Kim et al. (2003) cloned the TAS2R38 gene
(607751) and identified 3 coding SNPs giving
rise to 5 haplotypes worldwide that completely
explain the bimodal distribution of PTC taste
sensitivity (super taster vs. taster).
• Distinct phenotypes were associated with
specific haplotypes, which demonstrated that
the TAS2R38 gene has a direct influence on
PTC taste sensitivity and that sequence variants
at different sites interact with each other within
the encoded gene product.
Hypothesis 1
If tested for the ability to taste PTC paper,
there will be a greater number of tasters
than non-tasters in a random section of
the community population.
Postulate: Tasting alkaloids allowed humans
to detect the presence of poisonous
substances.
Hypothesis 2
If tested for the ability to taste PTC paper,
there will be a greater number of tasters
than non-tasters in the females tested in a
given population.
Postulate: Tasting alkaloids allowed women
to detect the presence of poisonous
substances in foods they gathered.
Hypothesis 3
If tested for the ability to taste PTC paper, there
will be a greater number of tasters than nontasters in the Caucasian vs. non-Caucasian
members of the tested population.
Postulate: Foods such as bitter melon, collard
greens, kale and other bitter tasting foods are
eaten by Caucasians who do not have the
allele.
Hypothesis 4
If tested for the ability to taste PTC paper,
there will be a greater number of tasters
than non-tasters in the non-Caucasian
(blacks, Asians, Hispanic) members of the
tested population.
Postulate: Foods such as bitter melon,
collard greens, kale and other bitter
tasting foods are eaten more frequently by
non-Caucasians.
Groups
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Group 1 – Tyler, Steph, Hyeon
Group 2 – Claire, Kelly, Michael
Group 3 – Hallie, Sara
Group 4 – Adam, Ryan
Group 5 – Connor, Meghan, Morgan M
Group 6 – Morgan L., Patrick,
Group 7 – Josh, Rachel
Group 8 – Chris, Jessie
Jobs within groups
1. Researcher(s) – researching information
from the questions
2. Data analyzer – runs T-test and creates
histogram based on the hypothesis
chosen
3. Everyone choses a part of the lab to
write.
What you will do
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Get into groups
Collate data for the various subgroups (Caucasians, AfricanAmericans, Asian- Americans, Hispanic-Americans)
Put data on the main data sheets on the computer; copies will be
generated
Have one person select a hypothesis for your group to prove or
disprove based on the data
Conduct an unpaired T-test on the data for your hypothesis
Create a histogram or bar graph of your data for comparison
Use the Analysis questions to help you in your discussion
PTC lab.doc
What you will hand in
• Group PPT, with each person’s work clearly
identified
• Standard Lab PPT or Prezio
– Introduction, with hypothesis
– Method of Testing (explain how you conned people
into tasting the papers)
– Data Analysis, containing T-test results and
histogram
– Discussion, including validation or refutation of
hypothesis and data to back up your claim, as well as
inclusion of answers to research questions from
handout
– Conclusion, for your hypothesis