Lesson video

Hello and welcome to this lesson from the unit: inheritance, genotype, and phenotype.

The title of today's lesson is "Alleles, genotype, and phenotype." What we're going to be looking at is different versions of genes that we call alleles.

How we denote these alleles, the combination that we receive in our genotype and how this combination affects our phenotype, which is the physical characteristics of an organism.

My name's Mrs. Barnard, and I'm going to be taking you through today's lesson.

So by the end of today's lesson, you should be able to explain how the genotype affects the phenotype of an organism.

I've got some keywords in today's lesson, and our keywords are "allele," "dominant," "recessive," "genotype" and "phenotype." I'll put the definitions up so you can pause the video if you want to write them down, but otherwise, we'll be going through them as we go through the learning today.

So our lesson today is in three parts.

The first part of the lesson is alleles, so that's different versions of genes.

The genotype, which is how we denote the different combinations of alleles that we inherit.

And finally, the phenotype, which is how the genotype will affect the physical characteristics of an organism.

So let's get started with the first part of today's lesson, which is alleles.

So hopefully, to start with, this is a little bit of revision.

I'm just going over some ideas that you already know.

So DNA is organised into structures called chromosomes.

So chromosomes are big packages of DNA, and sections of these chromosomes are genes.

And that's because a gene is a short section of DNA that codes for a particular protein.

It gives an organism its physical features, but also helps to control its biological processes.

And because short sections of DNA are called genes, if we've got this really, really long molecule DNA and it's all bundled up into our chromosome, as you can see on this image below, then that means that we can mark on the chromosome roughly where that gene actually is located within that package.

It's quite a specific place.

The genome project that took place and was completed in 2003 mapped where all of these genes were on different chromosomes and what they coded for, and that was the human genome product.

Since then, scientists have mapped lots and lots of different organisms including microorganisms, and the knowledge they've had about genes and their location in the chromosome has really helped with lots and lots of treatments for various diseases and genetic disorders.

So humans have 46 chromosomes in the nuclei of their cells.

So that's what all of you have, 23 from each parent.

Now, different organisms have different numbers of chromosomes, so this is only true for humans.

Each chromosome stores thousands of genes.

So there are over 20,000 genes that humans have.

So because you've got 23 unique chromosomes, that means that those genes have to be distributed through all of those chromosomes.

So you can imagine that lots of genes are on each chromosome or within each chromosome, and each gene is different.

So in the example in this image here, I've just drawn four genes 'cause it would get confusing if we drew all of the ones that there are there.

So four different genes.

And if we were to unbundle the DNA inside that packaged chromosome, this is what it would look like in a particular gene.

And it's got that particular sequence.

So that sequence of bases that determine the order of amino acids in a protein.

So that's our genetic code that's in there.

So DNA is packaged into chromosomes, and the reason that we get these, we inherit these is because they're transferred to offspring by gametes.

So each gamete will carry half of the chromosomes required to make the offspring.

So we can see here the two gametes of humans.

We've got egg cells and we've got sperm cells, and each of those will carry chromosomes.

They'll carry one of each pair, so they're gonna have 23 chromosomes each.

And inside those chromosomes is all that packaged up DNA and those individual genes that we just saw on the previous image.

Now, the reason that we can get all of these different chromosomes into our original cell, which is a zygote, is through fertilisation.

So the nucleus of the sperm cell will fuse with the nucleus of the egg cell.

And when they do, we get this nucleus in the zygote, which contains the 23 chromosomes from each of those gametes.

So those come from each parent.

So therefore, we have two copies of each chromosome in the zygote, which means that we have two copies of every gene in the zygote too.

We tend to therefore look at chromosomes as pairs.

And you can see in this image that I've shown them as pairs.

Now, there's only four, and you'll see in the images going through this deck that we can't put all of the chromosomes on there 'cause we wouldn't be able to see everything or all of the genes.

So you've got to imagine there's lots more than are shown in these images.

So in humans, there's 46 chromosomes.

So we would have 23.

And I've shown them as blue and pink here to denote whether they've come from the sperm, the male gamete, or whether they've come from the female gamete.

They carry the same genes stored in the same locations on the chromosome.

And you can see that in that picture there on the end.

So we've got a pair of chromosomes, they're matching chromosomes, so one's come from the sperm and one's come from the egg.

And when you look at that matching pair side by side, you can see that they carry the same genes in the same locations.

So time for a quick check.

Select which of these statements are true.

So pause off while you decide, and then we will check back and we will see how you've got 'em.

Okay, so let's see.

So offspring inherit two copies of each chromosome.

That is true, one from their mom and one from the dad.

Offspring inherit different genes from each parent.

That is not true.

Offspring inherit two copies of each gene.

That is true.

And offspring inherit half their DNA from each parent.

That is also true.

So if you've got those three correct, then well done.

Let's move on.

So cells with the nucleus have two copies of each gene, as we've already said, because they have two copies of each chromosome.

But these can be different versions of the same gene, and different versions of the same gene are called alleles.

So you can see an example here, the same gene, so we've got this pink band shown in the same place on the chromosome, but you can see from the letters that are on there, they're different versions of the same gene.

So we denote them with the same letter.

But you can see in this case, one is a capital and one is a lowercase.

So this might be an example.

This gene codes for a pigment involved in eye colour, but you've got different versions of that protein, that pigment that's involved in eye colour on these two genes.

So you've got two different versions of this same gene.

The gene is still involved in the pigment for eye colour, but there's two different versions of it, so therefore, they are alleles.

So what is a different allele? So an allele is a genetic variant of the same gene.

So they still affect the same characteristic, but they have different base sequences, and those different base sequences have occurred through mutations.

So you can see an example here of a substitution mutation, where that C base has been changed for a G, so cytosine to guanine, and it's led to a different triplet code, which means it would lead to a different amino acid being coded for or may do.

And because they are different versions of the same gene, so still involved in the pigment for eye colour in this example, we would give them the same letter, but they are slightly different.

So during reproduction, the offspring will receive two of each chromosome.

So for each gene, you will receive two copies.

You might have two copies of the same allele, or you might have two different alleles.

So these are your options at the bottom.

So you might get two of the same.

You can see there we're just focusing in on that gene shown by the pink band there at the top, but this could be the case for all of the bands shown and for the many thousands of genes that are on that chromosome.

It's kind of hard to get your head around like how many genes there are, so how many different combinations that you could actually have.

So we're just focusing in on one gene.

So these are our two options here.

So we can see we've got two here that are recessive, two that are dominant, and the last one has got one recessive and one dominant gene.

So three different combinations that you can receive for the same characteristic, for the same gene.

So true or false: alleles and genes are the same thing.

So first of all, I want you to decide whether you think this is true or false, and then which of the statements below do you think best justifies your answer.

So pause the video where you decide, and then when you come back, we'll see if you've got it right.

Okay, let's see how you got on with that then.

So alleles and genes are the same thing.

This is false.

And the reason is because alleles are genetic variants of the same gene.

They code for different versions of the same characteristics.

So they're not coding for different characteristics, they're not different genes.

Same gene, different version of the same characteristics, so different alleles.

So if you got that right, well done.

Let's move on.

So we've got a bit of a practise task now.

So these images show the possible combinations of chromosomes that could be inherited based on one gene.

So label the three images below to show your understanding of alleles, genes, and chromosomes.

So use the following words: allele, gene, chromosome, same, different.

And label up your picture.

Now, we'll probably add to this picture in task later on in the lesson, okay? So make sure you give yourself plenty of space when you're drawing this.

So pause the video while you do it, and we'll check that you've got it right when you come back.

Okay, let's see how you got on with that then.

So here is a picture.

Here's the picture and these are the things you should have labelled on.

So at the top, we've got the same alleles in both of those two versions.

And then we've got different alleles in the third chromosome pair.

We've got the different genes labelled down the side, and we have got the fact that it's a pair of chromosomes.

You might have added something on there, you might have said that one from each parent, or one comes from the mother, one comes from the father, okay? You might have said that the genes are in the same location.

Okay, anything extra is absolutely fine, but if you've got this basic done, then well done.

So let's move on to the second part of our lesson, which is genotype.

So the combination of alleles that an organism has is called its genotype.

And an individual has a genotype for each gene.

And this will determine the version of the characteristic that that gene codes for.

So you can see here there's lots of genes on each chromosome.

So we've only shown four, but imagine the thousands of genes that's on each chromosome and then the different versions of alleles you could get for each gene.

You can see why there's so much variation between different individuals.

But let's just focus in on one gene for now.

So still looking at this gene here, and this one has got the genotype that is the same, so we've got two lowercase Bs, we've got another one that's received the same alleles, so it's got two uppercase Bs.

And in this third version, there's one of each type of allele, and therefore, it's got an uppercase B and a lowercase B.

So scientists indicate the genotype using a pair of the same letter.

Now, that letter is usually associated with the characteristic that that gene is involved in coding for.

So alleles are either dominant or recessive.

And we show the dominant wall with a capital version of that letter, and we show recessive with a lowercase version of that letter.

So looking at a different gene here, so we're going to the second one down, and we're gonna have a little look at earwax.

So in this version, the genotype of this person is capital E, little E.

They've got one of each allele in their genotype.

So just a reminder that different alleles are genetic variants.

They are different versions of the same characteristics.

They're not different genes.

So it's the same gene.

And we get different versions, genetic variants of genes because of changes in the base sequence, and that occurs because of mutations.

And the mutation modelled here is a substitution mutation.

So you can see that one of the bases, that's been changed for another one, and therefore, the genetic code changes.

So we're still coding for the same characteristic, but there's going to be a slightly different outcome in both cases.

So let's look at earwax.

So wet earwax is dominant, so most people do have wet earwax.

It's a dominant characteristic.

So therefore, in order to have this characteristic, you only need to have one allele for wet earwax.

So if you have two dominant alleles, you'll have wet earwax.

And if you have one dominant allele and one recessive allele, then you'll have wet ear wax.

So that's what you need in your genotype in order for the individual to have that characteristic.

For an individual to have dry ear wax, because this is recessive, you would need two recessive alleles in your genotype to have this particular phenotype, this dry ear wax, and you can see how those genotypes have been denoted there below the pictures.

So for the wet ear wax, it's either two capital Es or a capital E and a lowercase E.

And for the recessive it's two lowercase Es.

So let's look at plants now.

So in pea plants, a purple flower is dominant, so therefore, we give it a capital P.

And a white flower is recessive, so we give it the lowercase P.

So in the genotype, if the dominant allele is present, then the flower will be purple.

So that could either be capital P, capital P, or capital P, little P.

The genotype must have two recessive alleles, so two little Ps, two lowercase Ps for the recessive characteristics to be expressed.

In this case, it would be the white flower.

So what I would like you to do is to match the genotypes, so the denotation with the correct description.

So pause the video while you do that, and then we'll come back and we'll see how you've got up.

Okay, so hopefully, you got that right.

So the first one, we have got two dominant alleles, so two capitals.

Then the next one, we've got two recessive alleles, so two lowercase letters.

And the next one, we've got one recessive and one dominant allele.

So one capital and one lowercase.

So if you got that right, well done.

Let's move on.

So another check.

So two dominant alleles must be present in the genotype for the organism to have the dominant version of the characteristic.

Now, you might want to read that again slowly in your own time.

And then I'd like you to select whether you think that is true or false.

And once you've decided whether it's true or false, which of the statements below do you think best helps you to explain your true or false choice, helps you to justify it? So pause the video and then we'll see how you've got on after.

Okay, so the correct answer is false, and the justification for that is because you only need to have one dominant allele in your genotype for the organism to have the dominant characteristic.

So if you got that right, well done.

So time for practise tasks.

So Izzy and Alex are discussing the genotypes of pea plants.

And Izzy says, "The genotype of the white flower plant is WW capitals, and the purple flower is PP." Again, capitals.

And Alex says, "No, the white flower is to lower case Ws because it's recessive, and the purple is two capital Ps." Now, they're both wrong.

So I would like you to, number one, explain why they are wrong.

And number two, can you write the correct possible genotypes for these two pea plant flower genes, please? Okay, pause the video while you do that, and then we'll come back and we'll give you some feedback.

Okay, let's look at Izzy and Alex's suggestions again.

So they are wrong because the same letter should be used for both the dominant and the recessive.

So you've got to pick one.

So usually, you'd pick the dominant ones.

In this case, P.

And the dominant characteristic could also be present if there is a one dominant allele in the genotype.

So you could have purple flowers that could be capital P, little P.

That would also produce purple flowers.

So if you had those in your answer, well done.

And then the genotypes would be as follows.

The white flower pea plant would have a little P, little P, so it would be recessive, 'cause it would have two recessive alleles.

And the purple flower pea plant would be dominant.

So it would be PP, two capitals, or it could be a capital P and a lowercase P.

So if you've got those right, then well done.

So it's time for us to move on to our third part of our lesson, which is phenotype.

So the genotype that we've just discussed is a combination of the alleles that an individual has for a certain characteristic.

And in our genome, we've got our genes and versions of these genes which will be alleles, and we've got non-coding DNA between them.

So alleles are versions of genes that carry the genetic code to make proteins.

Sometimes, it's easy to forget that genes, their role is to code for proteins, the structure of which give us our physical characteristics and control our processes.

So therefore, the genetic code within a gene will determine the order that the amino acids are bonded together, and then that allows it to fold to make a protein.

And because alleles have differences in their codes, this may lead to differences in the order of amino acids, which therefore will mean differences in the protein.

And differences in the protein could lead to differences in the phenotype, because our genetic code there is on the bases, and they code in threes for the order of amino acids, which is the first stage in protein structure.

And the protein that's produced will determine the phenotype, and therefore, the phenotype is produced due to the alleles that are present.

So what alleles do you have coding for what proteins, which is going to lead to what phenotype? So therefore, for example, we've got a protein structure, and one of the phenotypes that it could code for is ear shape.

There's loads of different other phenotypes that it could code for.

So examples of phenotypes that we've looked at already are our purple and white flowers and our wet and dry ear wax.

And we can see our genotypes are underneath there, and then the phenotypes are shown below.

So the actual physical characteristic.

Now, only features that result at least partly from the genetic code of an organism's genes are called the phenotype.

So for example, variation in flower colour and eye colour are phenotypic because they result from the different combination of alleles that you would inherit, whereas things like damage or scars are not phenotypic because they are results from the lifestyle or the environment.

So not from the genes or the combination of alleles that are inherited.

So time for a check.

I'd like to fill in the blanks.

The phenotype of an organism is determined by its what? And this is a combination of what that it has.

So pause while you decide the missing words, and then we'll check if you've got it right.

Okay, so the phenotype of an organism is determined by its genotype, and this is the combination of alleles that it has.

So if you've got those right, then well done.

So many alleles for a gene will lead to different combinations.

So there's not just two alleles in some cases.

Sometimes, there's many different alleles.

And if you get different combinations in a genotype, although you'll only ever receive two alleles for each gene, it would produce different phenotypes.

So different alleles code for different proteins, which are involved in controlling the colour of pigments in your eyes.

So you get lots of different eye colour phenotypes depending on the combination of alleles that are inherited.

Also, changes to structural proteins like keratin in your hair and functional proteins.

So they're like enzymes and things to do controlling colour pigment.

This leads to variation in our phenotypes.

And we can see here we've got many, many different alleles that exist for the phenotype that's to do with your hair, and therefore, there's many, many possible genotypes and many possible outcomes of that.

So you can see that in these hair colours and structures in these images.

So a combination of genotype and lifestyle and environment can affect your phenotype.

So for example, muscle shape and size will be affected by your genes and the combination of alleles that you inherit, but also by your lifestyle.

So the exercise that you do.

And some phenotypic characteristics you can't see.

Some are easy to see, like muscles, but others involved in processes like digestion or germination, okay? You won't be able to see those.

And they're controlled by genes and the combination of alleles and also the environment.

Some phenotypic characteristics are determined only by the genotype, so therefore, they won't be changed by the environment or the lifestyle.

So let's look at a couple of examples.

So first of all, we've got our eyes that even if you wear coloured contact lenses, your genotype, and therefore, your inherited eye colour will always remain the same.

And your blood type, that is determined by the alleles in your genotype that's involved in the formation of red blood cells, and therefore, it cannot be changed.

It's coded for.

So true or false: all phenotypes are affected by the combination of alleles.

Once you've decided whether that is true or false, if you can decide which of the statements below best justifies that, please.

Okay, so all phenotypes are affected by the combination of alleles.

This is true.

And the statement that best justifies this is that the phenotype of an organism is determined by the combination of its alleles always, but also, in some cases, by the environment.

So if you chose those ones, then well done.

So many genes code for proteins involved in biological processes.

So therefore, you might not see them in a physical characteristic.

And changes to the code of these genes, so different alleles can affect the order that the amino acids are put together, and therefore, the 3D structure of protein.

So we've got an example here of an enzyme, and you can see the 3D structure of the protein is very important, because a protein is adapted for its function, and therefore, if you change the structure of it, you might make it not functional.

So we can see here a picture on the side.

So a picture of an enzyme with an active site labelled.

And again, a molecular drawing, which is done by computer of how the protein is made up.

And enzymes have an active site that is a very specific shape that is complimentary to substrates.

And therefore, it's really important it has that shape so that the substrate can bind.

So a non-functional proteins produced as a result of alleles can lead to genetic conditions and affect people's health.

So if you do get a change in the sequence of bases in your genes leading to different alleles, those alleles may lead to a protein that is non-functional.

So here's an example.

So phenylketonuria or PKU is a condition where an enzyme that usually digests a particular chemical, a particular amino acid in the diet, isn't functional.

So it means that the substrate doesn't fit into the active site.

And you can see that in that image there that the active site has changed shape, because now that protein's 3D structure has been affected because the amino acids in its sequence were changed due to a change in the genetic code.

And because that substrate no longer fits in the active site, that means that the chemical that it's supposed to break down builds up in the body and it can lead to health complications.

Here's another example.

So changes in enzymes that produce the brown pigment protein melanin may lead to something called albinism, and changes to the structure of proteins in the lungs that are involved in mucus consistency may lead to cystic fibrosis, which may be a condition that you have heard of where mucus can build up in the airways and it stops the easy passage of air and oxygen into the blood.

It also can lead to infection, because bacteria can get trapped in those air tubes 'cause that mucus is not easy to move along.

So changes to proteins due to changes in the genetic code of genes, i.

e.

alleles, the combination of alleles in the genotype will therefore determine the phenotype of the individual.

And in some cases, this can lead to genetic disorders.

So true or false: different alleles can code for changes to proteins which can affect a person's health.

Now, when you've decided that's true or false, which statement do you think best justifies your answer? So pause while you decide and you read them through, and then we'll see if you've got it right.

Okay, so the correct answer is true, and the best statement to justify this is because alleles can cause changes to proteins involved in biological processes that can affect a person's health.

So if you've got that right, then well done.

So time for a practise test task.

PKU is a recessive phenotype.

It is a condition where an enzyme that usually digests a particular chemical in the diet is non-functional.

Therefore, the chemical builds up in the body and it can lead to serious health problems. So we've got some images here.

We've got a couple of chromosomes.

We've got the DNA there, a section which is called a gene, then an image of the PKU protein, and then just an image to show a child who has PKU.

So I would like you to use the images to explain why this child has the condition PKU.

So those are the sort of stimulus images.

And in your answer, can you please ensure that you use the following key words: "recessive," "genotype," "allele," "base sequence," "amino acid," "protein structure," and "phenotype." So pause the video.

This is a bit of extended writing, so come back and we'll do some feedback after you've had a chance to finish.

Okay, hopefully, you got an okay with that task.

So we've still got the images there.

So this is what I would've hoped to see in your answer.

It doesn't have to be exactly the same but it includes all of those key words.

So the child has the phenotype, pp.

Now if you chose a different letter but you chose, that's fine, but those two letters were shown in the image.

She has two recessive alleles for the gene that codes for the protein enzyme.

The base sequence codes for the amino acids, and that has changed the protein structure and it's made it non-functional.

So therefore, the child's phenotype is to present the PKU condition or to have PKU.

So if you wrote that or you got all of your words in and wrote words similar to that, then that's really good, well done.

It's difficult stuff.

Okay, we've come to the end of today's lesson, so well done for your work.

So an individual inherits two copies of each gene in a pair of chromosomes, one from each parent during reproduction.

Different versions of the gene are called alleles.

And if there are two for a particular gene, they can be recessive or dominant, indicated with the same letter, capital for dominant, and recessive, lowercase.

The combination of alleles that an organism has is called its genotype, and different alleles of a gene are associated with different versions of the same characteristic, which is called the phenotype.

Dominant phenotypes are shown if there is one dominant allele in the genotype.

Recessive characteristics are only shown if there are two recessive alleles in the genotype.

Some phenotypes are determined only by its genotype.

For example, eye colour or genetic conditions.

Others are also affected by the environment.

So well done today, and we'll see you soon.