Lesson video

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

The title of today's lesson is Models of single-gene inheritance: family tree diagrams. And in today's lesson, we're going to be looking at how we can draw diagrams to model inheritance, what shapes we use for males and females, and how we can show how genotype and phenotype are inherited.

We'll be looking specifically at genetic disorders, but also those that are dominant disorders and those that are recessive disorders.

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 construct and interpret a family tree diagram showing information about the inheritance of a feature.

Now we've got key terms in today's lesson as always, and our key terms today are inheritance, genotype, phenotype, family tree, and carrier.

So if you'd like to write the definitions for these, I'll put these up and you can pause the slide.

Otherwise, we will be going through them as we go through today's lesson.

So our lesson today is in two parts.

The first part of the lesson is looking at those family tree diagrams, being able to interpret them, and hopefully maybe draw them yourselves.

And then, we're gonna move on to the inheritance of genetic disorders and how we can show those using these family tree diagrams. So let's get started with the first part of today's lesson, which is interpreting family tree diagrams. So we're gonna start with a little recap.

So just a reminder that each cell has two copies of every gene, because you inherit one from each parent.

And the combinations of the alleles that an individual inherits is called the genotype.

And you have a genotype for each gene, and that will determine the characteristic that that gene codes for.

So from your parents, remember you get 23 chromosomes from each parent.

So 23 from the mother and 23 from the father.

And this picture is an example of one pair of chromosomes.

And you can see each chromosome carries lots of genes.

And if we look at the same gene on these two chromosomes, so one from the mother and one from the father, you can see that there can be different versions of these genes.

And different versions of the genes are called alleles.

So for this example of earwax, we're focusing on this gene here that's shown in purple.

And dominant alleles are shown with a capital letter, and recessive alleles are shown with a lowercase letter.

So therefore, the genotype of this individual, they've got a dominant allele and they've got a recessive allele, so we can denote it at the bottom with a capital letter and the lowercase letter.

So that means that from one parent they receive the dominant allele, and from the other parent they receive the recessive allele.

So the genotype for one gene could be two of the same allele or you could get different alleles.

So if we're focusing on the pink one in this case, the pink gene, you could have two recessive alleles, or you could have two dominant alleles, or you could have one of each.

If you have two different alleles in your genotype, this is called heterozygous.

If you've got two of the same allele, this is called homozygous.

So in these examples at the bottom, these individuals are homozygous recessive, homozygous dominant, and heterozygous.

So this is our mother's genotype, and in this case she's homozygous recessive, and the father is heterozygous.

So when they make their gametes and divide up their chromosomes, one of each pair of their chromosomes is going to go into each of the gametes.

Now remember, this would be actually 23, 'cause they've got 46 altogether.

So 23 into each gamete.

But the gamete receives one of each chromosome, which means that they receive one of each gene.

Therefore, that the allele that they receive may be different.

Now hopefully, you've seen these before, these Punnett squares, and they show the possible combinations of genotypes that the offspring could have from the genotypes of the parents.

So at the top here, we've got the mother in this case is heterozygous.

So you can see that the mother in her individual chromosomes in this pair for this particular gene, she is heterozygous, she's got one dominant and one recessive allele.

That means that half of her eggs will have the dominant allele and half of them will have the recessive allele.

And the father is also heterozygous.

So again, that means that half of the sperm will have the dominant allele and half of the sperm will have the recessive allele.

So when those sperm and eggs come together in fertilisation, then that tells us what possible combinations that we can get in our offspring.

So if this first sperm goes with this first egg then the individual could be heterozygous.

If this first sperm comes with this second egg, then the individual could be homozygous recessive.

And then if we follow this at the bottom, the second sperm and the first egg, then we would have dominant homozygous.

And then finally, at the bottom, another chance to be heterozygous.

So we've got a 2:1:1 ratio for the genotypes.

So those are the possible combinations of alleles that we can get from the genotypes of these two parents.

So let's have a quick check.

So true or false, the offspring receive two alleles for each gene.

So first of all, is that true or is it false? Then once you've decided, can you decide which of the statements below best justifies your answer? So pause the video while you decide, and then we will come back and we will see if you've got it right.

Okay.

So the offspring received two alleles for each gene.

That is true.

And the statement that best justifies that is this first one: they're inherited via the gametes, one from each parent.

So if you got that right, then well done.

So let's move on to looking at family trees now.

So the genotypes and the phenotypes the offspring inherit from their parents can be shown in a family tree.

This is also called a family pedigree.

Now this is a very simple example here.

So we've got females are always shown as circles, and we've got males as shown as squares.

And then, a line that runs between a male and a female is the reproduction line, and that's a horizontal line and it runs between two parents.

And then, the offspring are shown in the next row down, And you can see that they are joined up to the reproduction line that's between the two parents.

And in this case, we've got two generations shown 'cause we've got two rows.

So we've got males as squares, females as circles, reproduction line between parents, and the offspring come down from that reproduction line on a separate row.

So time for a quick check.

So how many males are shown in this diagram? Okay, so the answer is 3 'cause we've got three squares there in this diagram.

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

Another check.

How many generations are shown in this diagram? So the correct answer here is 3, 'cause we've got three rows.

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

So let's move on.

So now let's talk about the shading.

So the shadings of the circles and squares is changed to indicate what the genotype and the phenotype is of those individuals.

So we've got a key here.

So we've got homozygous dominant is shaded in grey, and in a box, that would mean it was a male.

And then if it's not shaded in, if it's white, it's homozygous recessive.

That's the same shading for a female.

So shaded in grey for homozygous dominant, and for homozygous recessive, white.

If they're heterozygous, then we've got this stripy shading that is grey or white.

So an example of an interpretation here.

So we've got two parents here.

One is a male who's heterozygous, one is a female who's homozygous dominant.

There's a reproduction line between them.

And their offspring there, they've got a male who's homozygous dominant, a female who's homozygous dominant, and a female who's heterozygous.

And here we go, I got another example.

So often, with these genetic tree diagrams, these family tree diagrams, we will number the individuals.

It makes it much easier for us to be able to talk about them when we're interpreting the family tree so that we understand what we're talking about, and so that other people understand our interpretation.

So in this example, we've got three generations shown.

We've got the parents, 1 and 2, at the top.

And their offspring are 3, 4, and 5.

Now we can see that because we've got a line coming down from the reproduction line between the two parents.

And on that line, we've got the three offspring, 3, 4, and 5.

Now where people sometimes get confused is that they think that 6 is also the offspring of these two parents, but it is not, because you can see it doesn't join onto the same line as the offspring 3, 4, and 5.

This is of the same generation.

So this female number 6 is of the same generation, but it has reproduced with individual number 3, which is one of the offspring of 1 and 2.

And then, you can see that those as parents, 6 and 3, have had offspring 7 and 8.

So have a look at these, and what I'd like you to do is choose the incorrectly drawn family tree.

So look carefully at the shapes, but also look at the genotypes that have been written on those, and choose two that are incorrect.

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

Okay, so how did you do with that one? So the first one may be a bit tricky to see.

So in this one, we can see that the female, at the bottom, that's been given the genotype dominant homozygous, two capital Bs, has actually got that shading, so that means that she is heterozygous, so she should have a capital B and a little B.

Now we can look at the other ones, and we can see as well that the female has been given two little Bs, homozygous recessive, in the middle, and she is shaded in grey, so should be homozygous dominant with two capital Bs.

So that one is incorrect.

The other one that's incorrect is this one, and that is because the males and the females have the wrong shapes.

So the female should be circles, and the male should be squares.

So if you've got those two correct in that you guessed that they were incorrect, then well done.

Let's move on.

So Aisha has started to construct a family tree from the information that she has been provided.

So the information she's got is that two heterozygous parents have got three children.

So she's put her two heterozygous parents at the top.

So those are correct.

And that she's shaded them in as heterozygous.

And one daughter is homozygous dominant, one daughter is homozygous recessive, and one son is heterozygous.

So she's put one of the offspring on there as number 3, but she's got 4 and 5 missing.

And the homozygous recessive daughter reproduces with a homozygous recessive male.

So you can see she's put that on there as number 6.

And they have two sons, 7 and 8.

So what you need to do is take some space on your paper or in your book to make sure that you've got enough space to draw this out neatly, and complete it.

So we need to complete the shapes and the shading, making sure that we put on the sex and the genotype of each of the individuals of the diagram.

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

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

So we've got the information here, so let's fill this in.

So we've got the two heterozygous parents have three children.

One daughter is homozygous dominant, one is homozygous recessive, and one son is heterozygous.

So she's got the homozygous recessive female daughter on there, so she needs to put the male on there that's heterozygous, and the female on there that's homozygous dominant.

So just check your shading is right on those two.

And then, the homozygous recessive daughter then reproduces with a male.

So we've got that one on that side there, which is homozygous recessive.

And then they have two sons, so they should be two squares, and both of them are recessive homozygous.

Sorry.

Both of them are recessive homozygous.

So you should have it filled in like that at the bottom.

So if you got those correct, then well done.

Let's move on.

So we're moving on now to the second part of our lesson.

So we are gonna look specifically at the inheritance of genetic disorders.

So genetic variance of genes, or alleles, can lead to genetic disorders.

So just a reminder that an allele is a version of a gene, and a gene is a short section of DNA that codes for a particular protein.

And it does this because each triplet, each three bases, will code for one amino acid.

The order of amino acids will determine the shape and the structure of the protein.

So we can see here in allele, one version, we've got the triplet codes, which are code for the amino acid which give us the functioning protein.

Remember, this is a computer model of a protein structure here on the right-hand side.

Now if there is a change in that genetic code via a mutation.

So an example is shown below where one of the cytosine bases, the C bases, is replaced by a guanine, base G, and that's changed the triplet code below, which might mean that it will code for a different amino acid.

And if it does, it could change the structure of the protein.

So an example shown in this computer model is that you can see that that blue section at the top of the protein has changed.

Now changing the shape of a protein can often make it non-functional.

And a non-functional protein could lead to a genetic disorder.

So an example of a genetic disorder caused by a change in a protein is cystic fibrosis.

So cystic fibrosis is a genetic disorder that affects mucus production.

The protein affected helps the movement of chloride ions and water, and it makes the mucus less viscous so that it can move more easily and be cleared out of the airways.

So without this protein and this movement of chloride ions and water, the mucus becomes too thick, viscous, and it blocks the small ducks in the body.

So that blocks lots of ducks including in the pancreas and sperm ducts, but also in the airways.

And here's a picture to show you what I mean.

So we've got a normal airway here, and then when you've got this thick mucus that is very, very difficult to move, it restricts the air getting down to the alveoli in the lungs, so therefore it's gonna affect gas exchange.

So cystic fibrosis is a recessive condition, therefore an individual has to have inherited two recessive alleles in order to express this disorder in their phenotype.

So let's have a look at the possible combinations.

So if we take this top gene here and we model that out as the one that codes for the protein that is involved in this movement of chloride ions, and we're gonna give it F.

Now if you have two recessive alleles for this particular gene, then therefore you will have cystic fibrosis.

If you have two dominant alleles, you won't.

If you have a heterozygous and you have one dominant allele and one recessive allele, then you don't show the cystic fibrosis in your phenotype in that you can still move those chloride and the water and you can make sure that that mucus can move, but you still carry that allele.

And that means that you can pass on that allele to your offspring.

So here we go, the phenotype is cystic fibrosis.

No cystic fibrosis for this particular individual.

And this is, even though they don't show the symptoms of cystic fibrosis, they do carry it.

So a carrier of a recessive genetic disorder is heterozygous, and that means they can pass the allele to the offspring.

'Cause remember, when those gametes are formed, one of each of these chromosomes will go into the gamete.

So half of the gametes for a carrier will receive the dominant allele and half of the gametes will receive the recessive allele.

But the people who have this particular genotype, where they have one dominant allele and one recessive allele, don't express this disorder in their phenotype.

So let's look at a family tree for this.

So as each parent passes on one of their alleles to the offspring, the combinations the offspring receives will determine whether they inherit this genetic disorder.

So we can see here we've got two parents and both of them are carriers, so neither of them will have cystic fibrosis, but they both carry an allele for cystic fibrosis.

So in their offspring, their three offspring are numbers 3, 4, and 5.

And in their offspring, you can see that we've got a female that's heterozygous, so has received one dominant and one recessive.

A female that's received two recessive alleles, so must've received one recessive allele from their mother and one recessive allele from their father.

And then we've got a male who's also heterozygous, so received one dominant and one recessive from the parents.

And individual 3 has reproduced with a female 6 who is dominant homozygous, so does not carry the cystic fibrosis allele.

And you can see in the offspring there that that female will have passed on a dominant allele to each of its offspring.

But in the case of the female number 7 it received a recessive allele from its father.

But in the case of the male, it actually received the dominant allele from its father.

So we can track down the alleles and how they're inherited by the conditions that are shown in the phenotype.

So let's try and do one together.

So we're gonna explain how individuals 3 and 4 can have inherited different genotypes from the same parents.

So what we've got here is we've got a male who's got two dominant alleles and we've got a female who has got a dominant and a recessive allele.

So individual 3 has inherited a recessive allele from its mother and a dominant allele from its father.

Individual 4 has inherited two dominant alleles, one from each parent.

So let's see if you could have a go at doing this one yourself.

So explain how individual 3 has cystic fibrosis when neither of its parents do.

So here is your family tree.

So pause the video while you have a go at doing this.

Use the wording that we just used in that previous example where you talk about which alleles have been inherited from which parent, and make it clear which individual you're talking about by using the numbers in the diagram.

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

So individual 1 and 2 are both carriers of cystic fibrosis.

They have one recessive and one dominant allele in their genotype.

Individual 3 has inherited two of those recessive alleles, one from each parent.

So if you got that right, then well done.

So it's time to move on.

So a family tree can be used in combination with a Punnett square to predict the possible genotypes and phenotypes of the offspring.

So for example, if female seven reproduced with a heterozygous male, so we can see female seven here is heterozygous, what would be the probability that their offspring would have the cystic fibrosis disorder? Okay, so let's have a look at what that would look like in a Punnett square.

So two heterozygous parents for the cystic fibrosis disorder means they are both carriers.

So therefore, we can see that the possible combinations for their offspring are that we could have a dominant homozygous, a recessive homozygous, or we could have two heterozygous.

So we've got a 1:2:1 ratio.

So the probability of two carriers of cystic fibrosis having offspring with the disorder is 1:3 or 25%.

So the probability, remember, is the same each time reproduction takes place.

So it could be that they have four children and none of them have cystic fibrosis.

It could be that they have four children and all of them have cystic fibrosis.

It's like tossing a coin, where each time you toss the coin, you won't necessarily get heads, tails, heads, tails.

And that's the same with this.

This is the probability of these genotypes being available in the offspring.

So time for a quick check.

What is the probability percentage and ratio of offspring carrying the cystic fibrosis allele if one parent is a carrier? So in this case, we're looking for the offspring that are carriers.

So pause the video while you fill it in, and then we'll check if you got it right.

Okay, let's have a look at what we've got then.

So we've got a 50% probability.

So we can see those two at the bottom are carriers, and therefore the ratio is 1:1.

So, 2:2 to start with, and then you simplify it down to 1:1.

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

So let's have a look at another inherited genetic disorder.

So polydactyly is a genetic disorder where a baby is born with extra fingers and toes.

And this disorder is also common in animals.

It is a dominant genetic disorder.

So this is different from cystic fibrosis.

So as polydactyly is a dominant condition, an individual with one dominant allele in their genotype will express the disorder in their phenotype.

So let's have a look at what that means.

So again, let's just model it out using this gene at the top.

So the gene is coding for a protein that's involved in digit production.

And we can see here the three different combinations of genotype that you can have.

So we've got homozygous recessive.

Now if you're homozygous recessive, you will not have polydactyly.

Then we've got homozygous dominant, so you will have the poly disorder.

And then we've got heterozygous.

Now in this case, they are not a carrier, they also have the polydactyl disorder.

So that's what makes it different from cystic fibrosis.

So there are no carriers of a dominant disorder.

You either have the disorder because you are heterozygous or you are homozygous dominant, or you don't have the disorder because you are homozygous recessive.

So in the case of dominant disorders, the heterozygous genotype is not a separate category, so therefore we don't have to show the shading, because if they are heterozygous, they will have the genetic disorder.

So we can see in this example then we only have the grey boxes and the unshaded boxes, but still the males and females are shown as squares and circles.

So family trees can be used to determine the genotype of individuals by looking at the inheritance possibilities.

So individuals 2 and 4 do not have the condition, so their genotypes must be homozygous recessive.

So they must have the two lowercase Ps because they've got the two recessive alleles.

So we can tell that.

But what we can't tell from this diagram is which ones are heterozygous and which ones are dominant homozygous, because we can only see whether they have the condition or not.

Individual 4 inherited one recessive allele from each parent.

So if we have a look at individual 4 there, it must have inherited one from each parent.

So we know, therefore, that number 1, parent number 1, must also have a recessive allele in its genotype.

So therefore, that means that its genotype is heterozygous.

So we can work it back based on what has been inherited by the offspring.

So it's a bit of a puzzle really.

So let's practise one together.

So using the information to determine the genotype and phenotype of individual 3 for polydactyly.

So let's look at individual three.

So we know that they have the condition, but we don't know whether they are dominant homozygous or whether they're heterozygous.

So individual 3's genotype is heterozygous, he's inherited a recessive allele from his mother, because that's all that she's got.

She's only got two recessive alleles, and therefore has inherited the dominant allele from his father.

Therefore, that means that he has the polydactyl disorder.

Okay, so you have a go at doing one on your own.

So use the information to determine the genotype and phenotype of individual 6 for polydactyl.

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

So individual 6's genotype is heterozygous.

She has offspring 7 that has the genotype homozygous recessive.

So therefore, individual 7 must have inherited a recessive allele from each parent.

So therefore, that tells you that individual 6 must have a recessive allele as well as the dominant that gives her that genetic disorder.

If you've got that right, well done.

So time for another practise.

So this is a family tree for the recessive disorder cystic fibrosis.

So we're back to cystic fibrosis again, and we can see that we've got the shading.

So what I would like you to do is calculate the probability ratio of individual 10 being a carrier for cystic fibrosis.

10 is there coming from the parent's 5 and 9.

And explain how individual 4 has cystic fibrosis.

So that's the female that's in the middle there of the 2nd generation.

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

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

So first of all, calculating the probability ratio of individual 10 being a carrier.

So we've got one parent that's heterozygous and one that is a dominant homozygous, and therefore carrier and non-carrier is 2:2, which simplifies down to 1:1.

And explain how individual 4 has cystic fibrosis.

Individual 4 has received two recessive alleles in their genotype.

She's received one from each parent.

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

So time for another task.

So this is a family tree for polydactyl.

So we see we haven't got that stripy boxes on this one.

So calculate the probability ratio of individual 10 having polydactyly.

And determine and explain the genotypes and phenotypes of individuals 7 and 8.

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

Okay, so first of all, calculate the probability ratio of individual 10 having the polydactyly phenotype.

So we can use our Punnett square.

So we've got one heterozygous parent and one that's homozygous dominant.

And we have got a 3:1 probability of individual 10 having the phenotype that's polydactyl, 'cause we've got two heterozygous and one homozygous dominant.

And determine and explain the genotypes and phenotypes of 7 and 8.

So we can see that one of their parents is homozygous recessive and one is homozygous dominant.

That means that both of the individuals must be heterozygous.

Individuals 7 and 8 have the genotype that's heterozygous, they've inherited the recessive allele, the lowercase from their father number 3, and the dominant allele P allele from their mother, who is number 6.

So if you got that right, then well done.

And that brings us to the end of today's lesson.

So time for a summary.

So offspring inherit two alleles, one from each parent, the combination of these alleles is called the genotype.

The genotypes and phenotypes of members of a family can be shown in a diagram such as a family tree.

A family tree diagram is a model of inheritance, showing the alleles and whether a characteristic was passed from the parents to the offspring.

And a family tree can be used to work out an individual's genotype or if their phenotype and relatives' genotype are known.

A family tree can be used to determine the history and probability of a genetic disease, for example, cystic fibrosis or polydactyl, occurring in the offspring.

So well done for your work in today's lesson, and we'll see you soon.