Alleles and Genes


Captions are on! Click CC at bottom right to turn off. Keep up to date by following us on Twitter (@AmoebaSisters) and Facebook! I don’t remember which grade it was where
I learned something about my tastebuds that can never be unlearned, but the event and
the lesson with genetics has stuck with me forever. For you see, I learned that my tastebuds cannot
taste PTC. Let me preface this with explaining that PTC
stands for this –we’ll stick with PTC—and it’s a chemical that can be sold on these
paper strips. It can be purchased under the name PTC paper,
and it is popular in genetic classes because it has this fascinating quality: some people
put it on their tongue and immediately say, “Yuck, this is bitter!” And some people, when they place the paper
on their tongue…taste absolutely nothing. Well, unless you consider the paper. Does paper have a taste in itself? That’s a debatable question but the point
is…some people can taste PTC. Some people cannot taste PTC. And I was really disappointed, because I remember
that I was the only one there that could not taste it so here was everyone getting this
amazing science experience and I couldn’t taste a thing. Well…. there may have been more than just me that
couldn’t taste it in the classroom that day, but they didn’t seem as concerned by
the fear of missing out of the PTC paper as I was. I remember someone trying to make me feel better by saying, “Oh, but it tastes bitter! You’re actually lucky.” Then they tried to describe what it tasted
like to me. But it’s not the same; I guess I’ll never know for myself what it would have tasted like. Of course, the reason PTC paper is used in
genetic classes is because the trait of being able, or not being able, to taste PTC is based
on genetics! A reminder from our intro to heredity unit
that genes are portions of DNA, and they have the ability to code for a characteristic—
a trait. Like being able to taste, or not to taste,
PTC. Now we do want to point out that many traits
are actually coded for by interactions of more than one gene. Like eye color, which is quite complex, and
determined by interactions of many genes together. In fact, the ability to taste PTC or not,
may involve some other gene interactions. There’s even different ranges for how bitter
the chemical may taste because there may be more kinds of alleles than we’ll mention—more
about that later. But since we do know that the ability to taste
PTC or not taste PTC is at least heavily impacted by a specific gene, it does make it powerful
for genetic classes. One thing I found so interesting is that my
parents can both taste PTC. So why can’t I? Recall that humans have 46 chromosomes. Chromosomes are made up of DNA and protein. It’s a condensed unit of DNA. My whole genetic code is represented by these
chromosomes. You inherit 23 chromosomes from your mother
and 23 chromosomes from your father. Here’s all 46 of them right here. As you can see, there are 23 chromosome pairs. Each pair has one chromosome from one parent and one chromosome from my other parent. If we focus on one of these pairs of chromosomes
where the PTC taste sensitivity gene may be found, we can see an area where the PTC taste
sensitivity gene could be. Let’s assume this is the locus where the
PTC taste sensitivity gene is found—see how it is pointing to a specific area here? That’s because it’s on an area on the chromosomes
that refers to a specific gene that codes for a trait. Now, remember how this chromosome is from
mom. This one is from dad. Each parent contributes an allele—which
is a variant of a gene. An allele is a variety of a gene; a form of
a gene. The alleles could be the same form of the
gene or different forms of the gene—but regardless, in this case, they’re forms
of the gene involved with PTC taste sensitivity. So if PTC taste sensitivity is being used
as a one gene trait example—and as we mentioned it may not be that be simple—- then your
DNA code has a gene related to PTC taste sensitivity. Together the two alleles you inherit, the
forms of that gene, determine the trait of tasting PTC or the trait of not tasting PTC. That gene is involved with coding for taste
receptors on your tongue and the receptors you have can make a difference for whether
you taste PTC or not. The alleles are typically represented by letters. Since this is all about tasting, let’s use
the letter T. But wait—it matters whether I represent it as a capital or lowercase letter! If I use a capital letter to represent an
allele, it means it’s a dominant allele. If one—or both—of the alleles you inherited
for a trait are dominant, then it will be expressed. More about that later. If I use a lowercase letter to represent an
allele, that means it’s a recessive allele. Recessive alleles are typically not expressed
unless there is no dominant allele present. Now remember that you have two allele copies,
so the combinations you can have here could be TT, Tt, or tt. These are called genotypes. Your genetic makeup. Genotypes can help determine a phenotype,
which is a physical characteristic. You’ll notice when writing genotypes, I
put the capital letters first if it contains a capital letter. That’s not because the order matters; it’s
a formatting formality that capitals are written first. It turns out that being able to taste PTC
is a dominant trait. That means the phenotype, which is a
PTC taster, is due to a genotype that includes at least one dominant allele. So which genotypes can taste PTC then? Well TT can; both of those alleles are dominant. So can Tt, because remember it only takes
the presence of one dominant allele. In fact, the only genotype in this simplified
example to not be able to taste PTC would be tt. So obviously that is what I am. I am the tt genotype which results in my non-taster
phenotype. But my parents can taste PTC… So what genotypes would they have to be? Well if they were both TT, that wouldn’t
be possible. If one was TT and one was Tt, that still wouldn’t
be possible. Remember you have to get an allele, a form
of a gene, from EACH parent. If my parents do taste PTC and I do not, then
my parents have the genotype Tt. And their phenotype is PTC taster. Punnett squares can be used to determine the
probabilities of offspring having certain genotypes—which then can be used to determine
their phenotypes. But Punnett squares are for another Amoeba
Sisters video. Before we end, one more thing to mention. In this example, the dominant trait of being
able to taste PTC is more common than the recessive trait of not being able to taste
PTC. And one could jump to an assumption that dominant
traits are more common, especially since it only takes the presence of one dominant allele
to show up in the phenotype. At least, in Mendelian inheritance. But the dominant trait is not always more
common in a population, because it’s possible that the dominant allele itself is more rare. That can be the case with some forms of polydactyly…that is being born with extra fingers. Some forms of polydactyly can be a dominant
trait caused by the presence of at least one dominant allele; however, the dominant allele
may not be as common in the population and the condition of having extra fingers is generally
rare. Well that’s it for the amoeba sisters and
we remind you to stay curious.

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