Lesson video

Welcome to this lesson from the unit Inheritance, Genotype, and Phenotype.

And the title of today's lesson is Models of Single Gene Inheritance: Punnett Squares.

What we're gonna be looking at today is how alleles and genes are inherited, and how we can show this on a diagram in order to work out the probability ratio of offspring receiving particular genotypes and phenotypes.

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 interpret and construct your own Punnett squares and to show the inheritance of alleles of a single gene.

So we've got some keywords for today's lesson.

And our keywords are gamete, genotype, Punnett square, ratio, and probability.

So I'm gonna put the definitions up.

So if you wanna write them down, if you pause the video, and if not, we'll be going through them as we go through the lesson.

Okay.

So our lesson today is in two parts.

The first one is constructing Punnett squares.

So how we build those.

And the second one is how we use them to predict inheritance.

So let's get started with the first part of our lesson, which is constructing Punnett squares.

So just a reminder that genetic information is passed from parents to offspring during reproduction, and the gametes contain half of the chromosomes of each parent.

So therefore, the resulting zygote will have two copies of each chromosome.

And therefore, two copies of each gene, which forms a pair.

In humans, there are 46 chromosomes.

So 23 from each parent.

So they're shown here in pairs.

So I've only drawn four pairs of chromosomes here, but there are 23.

And I've also shown them in two different colours, so blue and pink, to show that they've been inherited one from each parent.

So one from the father and one from the mother.

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

So if we take a pair of chromosomes there to look at, we can see that I've modelled four different genes.

In actual fact, chromosomes contain over 20,000 genes altogether.

So each individual chromosome is gonna carry a lot of genes.

So these four here, we can see the genes are in the same locations, but they may be different versions of those genes.

So we've got different alleles.

So alleles are different versions of the same gene.

So they code for the same characteristic, but they code for different versions of that characteristic.

So you may get a different phenotype.

So cells with the nucleus have two copies of each gene, and as I've said, these can be different versions of the same gene.

And different versions of the same gene are called alleles.

So a gamete will carry one of these.

So when the two gametes come together, therefore, the offspring will have two.

So in this case, they both have an eye colour gene, but these are different alleles for those same genes.

So the combination of alleles that an organism has is called it's a genotype.

And we can see in this example here that we have a genotype for each gene, and we're just focusing here on this top gene at the moment.

And this will determine the characteristic that this particular gene codes for what phenotype it will produce.

So we can see the genotype here is bb, two little Bs, same allele.

And then we can see for this one, we've got two capital Bs, again, the same allele from the mother and the father.

And in this last version here, they received a different allele from each parent.

So scientists like to indicate an organism genotype using a pair of letters, pair of the same letters.

So we're looking here at a different gene now.

We've just moved down the chromosome.

And we're looking at one that relates to ear wax.

And we can see, again, this person has inherited a different allele from each parent.

So because we express alleles as either dominant or recessive, so dominant with a capital letter or recessive with a lowercase letter, we can see this person has one dominant and one recessive in its genotype.

The genotype for one gene could be two of the same alleles or two different alleles.

So for this particular example, again, B, we can see we have got three different options, okay? Two different alleles in the genotype is termed heterozygous.

Hetero meaning different.

So that would be our combination on the end that has one capital B and one lowercase B.

Or we could have two of the same allele.

And then that is termed homozygous, homo being the same.

So both of the two pairs on the left-hand side, the two lowercase Bs, and the two capital Bs are both homozygous.

So we would say that one's homozygous recessive and one's homozygous dominant.

and then the other one is heterozygous.

So there we go.

Homozygous recessive, homozygous dominant, and heterozygous.

So choose the correct statement.

An individual always has two recessive alleles for each gene, an individual always has two alleles for each gene, an individual always has two dominant alleles for each gene, or an individual always has one recessive and one dominant for each gene.

So you decide which is the correct answer, and then we will check back.

Okay, bit to read on those ones.

So did you manage to choose the correct one? The correct one is this one.

An individual always has two alleles for each gene, but you can have different combinations in your genotype, but it's always a combination of two.

So here we go.

We've got a mother's genotype here, so two recessive alleles.

So she's homozygous recessive.

And we've got the father here who's heterozygous.

So he's got one dominant allele and one recessive allele.

And, therefore, when they make gametes, the gametes will receive one of each type.

So we can see at the bottom, all of the eggs are going to receive a recessive allele because that's what the mom has got, two recessive alleles.

But in the father's case, the sperm, half of the sperm are going to receive that capital dominant allele, and then the other half of the sperm are going to receive the recessive allele.

So the gametes, as well as receiving one chromosome each, are gonna receive one of those genes each, but the version of that gene that they have is determined by what the parents have in their genotype.

So now we can model this out with a Punnett square to have a look at what the offspring might inherit from these two parents.

So a Punnett square is a model of inheritance of alleles from parents gametes to their offspring.

So we put the mother's genotype at the top there, and then the two eggs just to show.

And we can see that she has got the two recessive alleles.

So each egg has a recessive allele, and then down the side, we put the father's gametes there.

And again, half of his sperm will have the dominant allele and half the sperm, the recessive allele.

Now what we do is we cross them together.

So we say if that first egg went with that first sperm, this is the combination that the offspring would receive in the zygote.

So they would have one dominant allele and one recessive allele.

Now if that second version of the egg, which is actually the same, receives, sorry, is fertilised by the first sperm, then we would receive same again.

That offspring within that zygote would have one dominant allele and one recessive allele.

So it's genotype is heterozygous.

Now let's look at the bottom.

So half of the father sperm have this recessive allele.

So if the recessive allele sperm meets the recessive allele egg, then, therefore, the offspring will have two recessive alleles.

So, therefore, they'll be homozygous recessive.

And that is also the case for that end one, where we've again got the sperm with the recessive allele, with the egg with the recessive allele.

So, therefore, the genotype of this offspring, again, is homozygous recessive.

So a Punnett square shows all possible combinations of genotypes in the offspring from the genotypes of the gametes.

So we've changed the mother's genotype now, so she's got a different one, so she's heterozygous as well.

So, therefore, in this case, half of her eggs will have the dominant allele and half of them will have the recessive allele.

So let's cross these together and let's have a look at the possible combinations we could have in our offsprings.

In this one, we've got a offspring that has two capitals, so two dominant alleles.

So it is therefore homozygous dominant.

And, of course, these two together, we've got a heterozygous offspring.

And then these two together, and we have got a dominant and a recessive.

So we've got heterozygous.

And again, this time we've got recessive and recessive.

So we've got homozygous recessive.

So in this case, we can see that outta four possible combinations, we've got one homozygous dominant, one homozygous recessive, and two heterozygous.

Now, this does not mean that the parents would have four offspring or have to have four offspring.

It means these are the possible combinations that you can receive from the gametes of those particular parents.

Those are your options and those are the ratios that you will get them in.

Now, that's what we will come to next.

So to construct Punnett square, when we are drawing them ourselves, we don't need to draw the gametes on them.

Those were there just to show you where these particular letters come from.

So we would draw them like this.

So at the top there, we've got a mother's genotype.

This time, it's heterozygous.

And, therefore, the father here, you can see in this one is homozygous recessive.

So we're gonna cross them together.

So you cross down from the top and then across.

Oh yeah, there we go, heterozygous and homozygous recessive.

These are our gamete's genotypes.

So here we go.

There's one, and it's heterozygous.

Then we cross across with our father's, one of our father's gametes to our mother's egg, and we get ee, which is homozygous recessive.

Then we cross down and across, and we get another heterozygous.

And then we cross across, and we get homozygous recessive.

So we can see that in the possible combinations of genotypes for the offspring of these two parents are as below.

They can either be heterozygous or they could be homozygous recessive.

Those are your options.

So let's see if we can do one together and then see if you can do one by yourself.

Okay, so a single gene A codes for the colour pigment in a flower.

So a Punnett square shows the possible genotypes of the offspring from one homozygous dominant and one homozygous recessive parent.

So let's have a look.

So homozygous dominant at the top there with the two capital letters, and homozygous recessive down the side.

And then what we want to do is we want to cross them together.

So we cross across there, we get heterozygous and heterozygous.

Again, you might see where we're going here.

Heterozygous again, heterozygous again.

So the possible offspring genotypes are all heterozygous.

So all of the offspring would be heterozygous.

Okay, time for you to have a go at one of these now.

So I would like you to construct a Punnett square to show the possible genotypes of offspring from two heterozygous individuals for the same gene A.

Okay, so two heterozygous parents.

So draw it as I've drawn it there in the model.

We'll pause the video and we'll come back and we'll see how you've got 'em.

Okay, let's see then.

So we should have drawn something that looks like this, with our heterozygous parents on both sides.

And then when you cross them together, you should have got homozygous dominant there, heterozygous, and then heterozygous again, and then homozygous recessive there.

So lots of different combinations there.

So three possible genotypes of the offspring in that particular example.

Okay, time for a quick check.

So I would like you to identify the errors in the construction of this Punnett square.

So anywhere where it might have gone wrong.

So pause the video while you decide, and then we'll come back and we'll see if you found them all.

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

So one error.

That's incorrect.

So we shouldn't have a two recessive.

Well, one, because one of the parents doesn't have a recessive.

But, two, if you cross it across, you can see that that's incorrect.

And number two, you can see here that's been done incorrectly as well.

That should be two capital A, so two dominant alleles in that genotype.

So if you found those two, then well done.

Okay, time for a quick task.

So wet earwax is a dominant characteristic, and it's only one allele for wet earwax.

So dominant capital E is needed in the genotype for an individual to have that characteristic.

So what I would like you to do is I would like you to construct Punnett square to show the possible offspring genotypes for a heterozygous parent and a homozygous recessive parent.

And then I would like you to construct a Punnett square to show the possible offspring genotypes for a heterozygous parent and a homozygous dominant parent.

Okay, so remember to keep you nice and neat, pencil and a ruler to draw your Punnett squares, and come back and we'll see how you've got them.

So let's see how you got on with that one then.

So the first thing is to construct a Punnett square to show the possible offspring genotypes for heterozygous parent and a homozygous recessive parent.

So, hopefully, you drew this.

So we've got a heterozygous parent, it doesn't matter which way round you put them on the Punnett square.

And then these are your possible outcomes.

So two heterozygous offspring and two homozygous recessive.

And then for your second one, we should divide a heterozygous parent on one side and a homozygous dominant on the other.

And your options for your offspring are two homozygous dominant and two heterozygous.

So if you drew those out correctly and denoted them correctly in the offspring, then well done.

So it's time to move on to the second part of our lesson, which is predictions of inheritance.

So we're gonna be using these Punnett squares in order to predict the possible genotypes of our offspring with numbers.

So Punnett Square can be used to make mathematical predictions about the genotypes and the phenotypes of offspring.

So here's a particular Punnett square, we have the inheritance of a single gene, we've got one heterozygous parent, and one homozygous recessive parent, and those are the possible combinations for our offspring.

So in this example, there are two possible genotypes for the offspring, heterozygous and recessive, homozygous recessive.

And the heterozygous contains a dominant allele.

So, therefore, the offspring with this particular genotype would have the dominant phenotype.

And then the one that is homozygous recessive has two recessive alleles.

So, therefore, the offspring would have the recessive phenotype.

So the Punnett square can be used to predict the likely proportion of each genotype and phenotype in the offspring.

So here's an example of a prediction.

So in this case, half, or 50%, of the offspring have the genotype that's heterozygous and will express the dominant phenotype.

So half of all of the offspring would have the dominant phenotype.

And half, or 50%, of the offspring would have homozygous recessive.

So, therefore, they would show the recessive phenotype.

So predictions of possible offspring genotype and phenotype can also be shown as a probability ratio.

So a ratio shows how much one value there is compared to another value.

So the probability of the genotypes occurring in this offspring are as follows.

So the heterozygous compared to the homozygous recessive is 2:2.

So we can see at these four, two have the first genotype and two have the second.

But we would simplify this down to 1:1.

So that is our ratio.

So what is the ratio and percentage probability of an individual offspring inheriting the heterozygous genotype in this example? Okay, so the example's already there.

What's the percentage and what's the ratio of receiving the heterozygous genotype? So pause the video while you decide, and then we'll come back and we'll see if you've got it right.

Okay, so the percentage of having the heterozygous genotype would be 50% and the ratio would be 1:1.

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

So what is the ratio and percentage probability of an individual offspring having the homozygous recessive genotype in this examples? So a different one this time.

Again, pause the video while you decide, and then we'll check.

Okay, so we are looking for the homozygous recessive this time.

So the percentage chance here is 25% 'cause just one out of those four boxes has got the two recessive alleles.

And, therefore, the ratio would be 1:3.

So three don't, and one does, 1:3.

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

So pea plants can produce purple or white flowers, and it's controlled by a single gene.

And the purple allele is dominant capital P, and the white allele is recessive.

And this is what this would look like in a Punnett square.

So we've got one parent who has got two purple alleles and one that has got two white alleles.

So, therefore, if we cross them together in the offspring, we can see that all of the offspring or heterozygous.

So, therefore, 100% of the offspring will have the genotype that's heterozygous, capital P, little P, and the purple flower would be the phenotype as they all contain one dominant allele.

And if you have a dominant allele in your genotype, then that is the feature that you will express in your phenotype.

So two heterozygous purple flower pea plants now.

So maybe the offspring from the previous Punnett square are bred together.

And then this is what we get for our possible genotypes in our offspring.

So we've got the dominant homozygous, recessive homozygous, and heterozygous.

So let's have a look now.

The genotype ratio here is 1:2:1, okay, for the different genotypes.

But the phenotype ratio is 3:1 because you've got three purple and one white flower.

So the phenotype and genotype ratios are the same each time fertilisation takes place.

So just because you have four offspring, it doesn't mean that one of each of those offspring would have one of each of the genotypes.

It's like when you toss a coin, each time the toss a coin, even though it's a 50:50 chance or a 1:1 ratio probability, it doesn't mean that you'll get heads, tails, heads, tails, heads, tails.

And that's exactly the same with these probabilities.

The Punnett square gives you the possible genotypes, but this is the probability every time fertilisation takes place.

So you could have these two flowers that breed together that are bred together, and you could get all of the offspring could end up with dominant homozygous, or all of them could end up white, okay? You just don't know.

But the chances are the more offspring you have, the more likely you are to get close to this ratio, which is the same with tossing a coin.

The more times you toss the coin, the closer that you are likely to get to having that one-to-one ratio of heads to tails.

So we say that each instance of sexual reproduction is independent, and so the ratio stays the same each time it takes place.

So time for a quick check.

I would like you to match the genotype to the phenotype description.

So pause the video while you do this.

Okay, let's see how you got on.

So we've got our heterozygous one there.

So that's gonna give us a dominant phenotype.

And then we have got homozygous recessive, which is gonna give us our recessive phenotype.

And then we've got our dominant homozygous, which is also gonna give us our dominant phenotype.

So we've got those right, then well done.

Let's move on to a practise task.

So in pea plants, round peas are produced by a dominant allele and wrinkled peas by a recessive allele.

So what I would like you to do is to construct a Punnett square to show how wrinkled offspring, so, remember, that's the recessive condition, could be produced by a round pea parent and a wrinkled pea parent.

And then I would like you to calculate the probability of the parent's producing an offspring with the wrinkle pea.

What was the probability of that occurring? And explain why this probability would stay the same for every offspring produced by these parent plants.

So there's quite a lot to do here so you'll want to take it one step at a time.

So pause the video.

And then when you come back, I'll give you some feedback.

Okay, so let's see how you got on with this one then.

So in pea plants, round peas are produced by a dominant allele, which is we're gonna give the capital R, and wrinkle peas by recessive allele.

So we need to now construct Punnett square that shows how we could get wrinkle peas.

So we know wrinkle peas, 'cause it's recessive, need to be homozygous recessive, which means both parents need to be able to give a recessive allele, which therefore means that that round parent must be heterozygous.

Otherwise, it would not be able to pass on a recessive allele to its offspring.

So we've got a heterozygous parent there, which is the round pea plant, and then we've got the homozygous recessive, which is the wrinkle pea.

And we can see if you've done your Punnett square like this, that you've got a 50% chance of having a round pea, which would be heterozygous, and you've got 50% chance of having a wrinkle pea, which would be homozygous recessive.

So if we look at that in terms of probability, we've got 50% or 1:1, okay, for the parents producing an offspring, those particular parents with wrinkled peas.

And explain why this probability is the same.

So every time they have offspring, why is it always a 1:1 ratio that they would have wrinkled peas? Because the probability is the same every time fertilisation occurs because each instance of sexual reproduction is independent.

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

And we come to the end of our lesson.

So well done today with your work.

Let's go through our summary.

So most characteristics are influenced by multiple genes, but some are determined by just one.

And an individual will inherit two alleles for each gene, one from each parent.

And the alleles for a gene may be dominant or recessive.

A Punnett square is a model of the inheritance of alleles from parents gametes to offspring.

It shows all possible combinations of a genotype in the offspring.

These could be homozygous dominant, homozygous recessive, or heterozygous.

A Punnett square can be used to predict the likely proportion of each genotype and phenotype in the offspring and to calculate the probability of each genotype and phenotype occurring in the offspring.

Each instance of sexual reproduction is independent, and so it does not affect the probability for the subsequent offspring.

So well done today and I'll see you soon.