Alleles and Genes

Summary

This video explores the genetic trait of tasting PTC, a chemical found on paper strips. Some people can taste its bitterness, while others cannot, due to their genetic makeup. The script delves into how alleles and genotypes determine this ability, highlighting the complexity of genetic inheritance and the rarity of certain traits.

Takeaways

• 📝 The script discusses the genetic ability to taste PTC, a chemical found on paper strips, which some people can taste as bitter while others cannot.
• 🧬 PTC tasting ability is a genetic trait, determined by alleles inherited from both parents, illustrating the basics of genetic inheritance.
• 🔬 The script mentions that the ability to taste PTC is likely influenced by more than one gene, indicating the complexity of genetic traits.
• 👅 The inability to taste PTC is described as a recessive trait, meaning individuals with the 'tt' genotype do not experience the bitter taste.
• 🧬👨‍👩‍👧‍👦 The narrator's parents can taste PTC, suggesting they have the Tt genotype, as the narrator, with the tt genotype, cannot taste it.
• 🧬🧵 Chromosomes and DNA are highlighted as carriers of genetic information, with each person inheriting 23 from each parent, forming 23 pairs.
• 🔠 Alleles are represented by letters, with uppercase indicating a dominant allele and lowercase indicating a recessive one.
• 🌱 The dominant trait of tasting PTC (TT or Tt genotype) is more common than the recessive non-taster trait (tt genotype) in the population.
• 📊 The script introduces the concept of Punnett squares as a tool for predicting offspring genotypes and phenotypes, though it is saved for another video.
• 🤔 The video script challenges the assumption that dominant traits are always more common, providing the example of polydactyly as a counterpoint.
• 🔬 The Amoeba Sisters encourage viewers to stay curious about genetics and the complexities of how traits are inherited and expressed.

Q & A

What is PTC and why is it used in genetic classes?

PTC stands for phenylthiocarbamide, a chemical that can be sold on paper strips and is popular in genetic classes. It is used because some people can taste it as bitter, while others taste nothing. This variation is based on genetics.

How does the ability to taste PTC relate to genetics?

The ability to taste PTC or not is based on genetics, specifically related to a gene that codes for taste receptors on the tongue. This gene can be dominant or recessive, affecting whether an individual can taste PTC.

What is the significance of the PTC paper test in genetic studies?

The PTC paper test is significant in genetic studies as it demonstrates a clear genetic trait. It helps illustrate how genetic variations can lead to different phenotypic expressions, such as tasting or not tasting PTC.

What is an allele and how does it relate to the ability to taste PTC?

An allele is a variant of a gene. In the case of PTC taste sensitivity, each individual inherits two alleles, one from each parent. These alleles determine whether the person can taste PTC or not.

What are the possible genotypes for the ability to taste PTC?

The possible genotypes for the ability to taste PTC are TT (both dominant alleles), Tt (one dominant and one recessive allele), and tt (both recessive alleles). Only the tt genotype results in the inability to taste PTC.

Why might someone not be able to taste PTC despite their parents being able to taste it?

If both parents can taste PTC but have the genotype Tt, they can pass on a recessive allele to their child. If the child inherits a recessive allele from each parent, their genotype would be tt, resulting in the inability to taste PTC.

What is a phenotype and how does it relate to genotypes?

A phenotype is a physical characteristic that results from the expression of an individual's genotype. In the case of PTC tasting, the phenotype is either being a taster or a non-taster, which is determined by the genotype.

How does the dominance of a trait affect its expression in a population?

A dominant trait is expressed in an individual if at least one dominant allele is present. However, the prevalence of a trait in a population is not solely determined by dominance; the frequency of the dominant allele in the population also plays a role.

What is a Punnett square and how is it used in genetics?

A Punnett square is a diagram used to predict the genotypes of offspring based on the genotypes of the parents. It helps in understanding the probabilities of certain traits being expressed in the offspring.

Why might the dominant trait of tasting PTC not be more common than the recessive trait?

Even though the dominant trait of tasting PTC is expressed with just one dominant allele, the frequency of the dominant allele in the population might be lower. This can result in the recessive trait being more common, as seen in some cases of polydactyly.

Outlines

00:00

🧬 Genetics of Taste: The PTC Phenomenon

This paragraph delves into the genetic basis for the ability to taste PTC, a chemical found on paper strips used in genetics classes. It explains that some individuals can detect its bitterness while others cannot, highlighting this as a genetic trait. The narrator, unable to taste PTC, explores the concept of dominant and recessive alleles and how they determine one's ability to perceive the taste. The discussion also touches on the complexity of genetic traits and the inheritance of genes from both parents, using the PTC example to illustrate how genetic variation can result in different phenotypes, such as the narrator's inability to taste PTC despite both parents being able to do so.

05:00

🥼 Understanding Genotypes and Phenotypes: The PTC Taster Dilemma

The second paragraph continues the genetic theme by explaining genotypes and phenotypes in the context of PTC taste sensitivity. It clarifies that tasting PTC is a dominant trait, meaning that individuals with at least one dominant allele (TT or Tt genotype) can taste it, while those with a homozygous recessive genotype (tt) cannot. The narrator, identifying as a non-taster with the tt genotype, ponders the genetic makeup of their parents, concluding that they must both be carriers of the recessive allele (Tt genotype) to have a non-tasting child. The paragraph also dispels the misconception that dominant traits are always more common, providing the example of polydactyly to illustrate that dominant alleles can be rare despite their expressiveness in phenotypes. The summary ends with a reminder to stay curious about genetics.

Keywords

💡Tastebuds

Tastebuds are the sensory organs on the tongue and other parts of the mouth that allow us to perceive different flavors. In the video, the concept of tastebuds is central to understanding the genetic variation in the ability to taste PTC, a chemical that can be tasted by some individuals but not by others, highlighting the theme of genetic diversity.

💡PTC

PTC, or phenylthiocarbamide, is a chemical compound that comes in the form of paper strips and is used to test for genetic differences in taste perception. The video discusses how some individuals can taste PTC as bitter while others cannot, illustrating the genetic basis for taste sensitivity and the video's focus on genetic traits.

💡Genetics

Genetics is the study of genes, heredity, and variation in living organisms. The video script uses genetics to explain the variation in the ability to taste PTC, emphasizing how traits like taste sensitivity are inherited and can vary among individuals due to genetic differences.

💡Alleles

Alleles are different forms of a gene that can exist at the same place on a chromosome. In the context of the video, the script explains that the ability to taste PTC is influenced by alleles, with the dominant allele allowing for the taste and the recessive allele resulting in no taste perception.

💡Dominant and Recessive Alleles

Dominant alleles are versions of a gene that will be expressed if at least one copy is present, while recessive alleles are only expressed in the absence of a dominant allele. The video uses the terms to explain how the ability to taste PTC is a dominant trait, with the script providing the example of the narrator being a non-taster with a tt genotype.

💡Genotype

A genotype is the genetic makeup of an individual in terms of the specific alleles they carry for a particular trait. The video script discusses genotypes such as TT, Tt, and tt to explain how different combinations of alleles for the PTC taste sensitivity gene result in different phenotypes.

💡Phenotype

A phenotype is the observable physical or biochemical characteristics of an individual, determined by both the genotype and the environment. The video script uses the term to describe the expression of the ability to taste PTC, with 'PTC taster' and 'non-taster' being the phenotypes resulting from different genotypes.

💡Chromosomes

Chromosomes are thread-like structures made of DNA and protein that carry genetic information. The video script explains that humans inherit 23 chromosomes from each parent, and it uses the concept to illustrate how the alleles for the ability to taste PTC are inherited.

💡Locus

A locus is a specific location on a chromosome where a particular gene or genetic marker is found. The video script refers to the locus to explain where on the chromosomes the gene for PTC taste sensitivity might be located.

💡Punnett Squares

Punnett Squares are a graphical tool used to predict the genotypes of offspring based on the genotypes of the parents. Although not the main focus of the video, the script mentions Punnett Squares as a method to determine the probabilities of certain genotypes, which relate to the theme of genetic inheritance.

💡Polydactyly

Polydactyly is a condition characterized by the presence of more than the usual number of fingers or toes. The video script uses polydactyly as an example to illustrate that dominant traits are not always more common in a population, which relates to the broader theme of genetic variation and trait frequency.

Highlights

The speaker discovered they cannot taste PTC, a chemical often used in genetic classes.

PTC is a chemical that can be sold on paper strips and elicits a bitter taste in some individuals.

The ability to taste PTC is a genetic trait, with some people tasting bitterness and others tasting nothing.

The lesson on PTC taste sensitivity has a lasting impact on the speaker's understanding of genetics.

Traits like PTC taste sensitivity can be influenced by more than one gene, similar to eye color.

Genes are portions of DNA that code for specific traits, such as the ability to taste PTC.

Alleles are variants of a gene, and they can be dominant or recessive, influencing the expression of traits.

The speaker's parents can taste PTC, suggesting a complex genetic inheritance for this trait.

Humans inherit 23 chromosomes from each parent, making up 46 chromosomes in total.

Each chromosome pair contains one chromosome from each parent, contributing to genetic traits.

The PTC taste sensitivity gene is located at a specific locus on the chromosomes.

The speaker is a non-taster with the genotype tt, which is a recessive phenotype for PTC.

The ability to taste PTC is a dominant trait, expressed with at least one dominant allele.

The genotypes TT and Tt can taste PTC, while tt cannot, illustrating basic Mendelian inheritance.

The speaker's parents must have the Tt genotype to have a non-taster child.

Dominant traits are not always more common in a population, as allele frequency can vary.

The Amoeba Sisters encourage viewers to stay curious about genetics and scientific discoveries.

Transcripts

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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.