Lesson video

Hello and welcome to this lesson from the unit "Inheritance, Genotype and Phenotype".

The title of the lesson is "The Inheritance of Biological Sex "and Sex‑Linked Genetic Disorders."

So what we're going to be looking at in today's lesson is how sex is inherited from parents, i.

e.

how somebody becomes a biological male or a biological female from the chromosomes that they receive, but also how some alleles carried on those sex chromosomes could lead to genetic 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 explain how the inheritance of chromosomes determines biological sex in humans and also the inheritance of sex‑linked genetic disorders.

So as with all lessons, we've got some key terms for you to be looking out for today.

These are our key terms in today's lesson.

So they are sexual reproduction, biological sex, sex chromosome, sex hormone, and sex‑linked genetic disorder.

So I'll put a slide up with the definitions, so if you want to pause to write them down, you can do, but otherwise we'll be going through them as we go through today's lesson.

So our lesson today is in three parts.

The first part is sex chromosomes, so a reminder of what chromosomes are and which ones determine biological sex.

Then onto sex determination, so how we can work out the probability of the sex of the baby, of the offspring of two parents.

And finally sex‑linked genetic disorders where we look at alleles that carry a genetic disorder, those sex chromosomes and how they are inherited.

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

So a little bit of a recap, hopefully.

Humans reproduce, as do lots of other organisms, but we're going to focus on humans.

Humans reproduce using sexual reproduction.

So we've got a male human and a female human here.

And reproduction is producing a zygote.

Now, a zygote is a fertilized egg cell, and it is the offspring of both parents.

So it's the first stage in development of a new offspring.

And it comes from two gametes, the gametes that are received from each of the parents.

And in those gametes, the DNA is passed on.

So that zygote is made up of DNA from both of those parents.

From the zygote we have growth and then we have a baby.

So that is our offspring, our human offspring.

So in sexual reproduction, gametes from males and females carry the DNA, and they carry half of the DNA of each parent.

So that would be carried in a female by the egg cell.

So human females produce egg cells.

Human males produce sperm cells.

In both cases, they carry half of the DNA required in order to make a zygote, which will then grow into a baby.

DNA is packaged into chromosomes, and it is transferred to the offspring by the gametes.

So what we can see here is that the gametes have both got a nucleus, and in their nucleus they have half of the chromosomes that are required to make the offspring.

So in humans, remember that's 23 each.

So they carry 23 each, and then when they come together, we have 46 chromosomes that will be in the final zygote nucleus.

And inside each of those chromosomes is DNA that's packaged up.

And in that DNA we've got genes, which are the short sections that code for proteins.

So as I've already said, humans have 46 chromosomes in the nuclei of their cells, so 23 from each parent.

And each chromosome stores thousands of genes.

And each gene is different.

So out of those 23 different types of chromosome, we've got thousands of genes on each of those chromosomes.

And then you get two copies of each gene because you receive two chromosomes, one from each parent.

So in this model here, we can see that this chromosome is carrying four different genes.

So let's have a quick check.

So true or false, "Chromosomes are structures which contain genes?"

Now, first of all, choose whether that's true or false.

And then afterwards, which of the statements below do you think best justifies whether you chose true or false?

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

Okay, so, "Chromosomes are structures which contain genes."

This is true, and the statement that best justifies this is this first one "Because DNA is packaged up into chromosomes, "and sections of the DNA are genes."

So if you got that right, then well done.

Let's move on.

So in sexual reproduction, the nucleus of the sperm cell will fuse with the nucleus of the egg cell during fertilization, and the resulting zygote then will have two copies of each chromosome.

So we can see those in this picture.

They're a bit small, but we'll look at it in more detail on the next slides.

But we can see we've got an egg cell there, and it's got five chromosomes in and five in the sperm.

And then they come together and the nuclei fuse in fertilization, and we have 46.

In our example here, we've got 10, because it would take way too long.

We wouldn't be able to see it if we drew 46.

So that's our zygote once our nuclei fuse.

So in human cells, we look at it a bit bigger now, there are 46 chromosomes, 23 from each parent, and I've shown them here in pairs.

So one pair of these is the sex chromosomes, and the combination inherited will determine the biological sex of the baby.

So because you've got 23 pairs, actually 22 of those pairs are not to do with the sex of the baby.

It's only one of those pairs that determines that, and that is the sex chromosomes.

Now you can see that these are slightly different in this case because you see one is slightly bigger and one is slightly shorter than the other.

And as you'll come to see, that's because this one would be the chromosome pair of a male.

In all of the cases, chromosomes are generally the same size.

So the sex chromosomes are termed X and Y. So we can see the 22 pairs of chromosomes up until that point, and they are often numbered, so they're easy to see.

And then at the bottom, you've got two options.

You wouldn't have both of these.

You either have X and Y or X and X. So one pair of sex chromosomes for each offspring.

So in this case, if they're the same size, they are X and X, so that would be a female, and if they are X and Y, the Y is smaller, so therefore it would be a male.

So time for a quick check.

"How many sex chromosomes does a human cell have?"

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

So the correct answer is two.

So if you got that right, then well done.

And let's move on.

So a human with two X chromosomes will have the biological sex of female, and a human with an X and a Y chromosome will have the biological sex of male.

So chromosomes can be viewed using a light microscope.

So during cell division, we can see them more clearly.

And they're stained and photographed, and then they're arranged to produce an image that we call a karyotype.

So you can see this here.

You might have seen images that look like this before.

And you can see the chromosomes are lined up after they're photographed in pairs so that we can see them together.

And at the bottom there, the sex chromosomes are usually shown at the bottom, so one pair of sex chromosomes in each offspring.

In this case, we can see we've got one larger chromosome and one shorter chromosome.

So that's X and Y. So this is the karyotype of a male.

So during sexual reproduction, chromosome pairs are halved, like all chromosomes are halved in gametes.

So each gamete will actually only carry one sex chromosome.

So in a female, because a female has two X chromosomes, when those chromosomes are halved, each one of her egg cells will contain an X chromosome.

And then when they're halved in a male, we can see that because a male has X and Y, half of his sperm will have the X chromosome and half of his sperm will have the Y chromosome.

So let's have a quick check.

So, "Select the chromosome combination "which gives the biological sex male."

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

Okay, so hopefully you got this one right.

The correct answer is B. X, Y would give you the biological sex of male.

So time for a practice task now.

So pupils are discussing which sex chromosomes determine biological sex.

We've got four pupils involved.

So Lucas says, "Males have two Y chromosomes, "and females have two X chromosomes."

Jun says, "Males have XY chromosomes, "and females have XX chromosomes."

Sam says, "Males only produce sperm with Y chromosomes."

And Sofia says, "All humans have four sex chromosomes, "two inherited from each parent."

So first of all, what I would like you to do is select the pupil that has the correct understanding, and then I would like you to correct the statements of the other three pupils.

So pause the video while you do that because you'll need a little bit of time to write this out, and then we'll come back and I'll give you some feedback after.

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

So, "Select the pupil who has the correct understanding."

So the person with the correct understanding in this case was Jun.

And let's correct the other statements of the other three pupils then.

So Lucas said that, "Males have two Y chromosomes, "and females have two X chromosomes."

So we're going to correct that by saying, "Males have an X and a Y chromosome."

Sam she said that, "Males only produce sperm with Y chromosomes."

So let's correct her.

So, "Half of a male's sperm carry an X "and half carry a Y chromosome."

And Sofia, "All humans have four sex chromosomes, "two inherited from each parent."

So if we correct that, we would say that, "all humans have two sex chromosomes, "one inherited from each parent."

So if you got that one right, then well done.

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

In males, a gene is carried on the Y chromosome which codes for a protein which triggers the development of the testes.

So let's look at that example there in the image.

So we've got an X and a Y, and the gene for testes development is on the Y chromosome.

There's not a matching one there on the X chromosome.

The testes produce male sex hormones, which we call androgens.

And this is not present on the X chromosome.

So females do not have a copy of this gene.

You see there isn't a corresponding gene on the X chromosome there.

Females do make sex hormones though.

They're just not coded for by this gene.

So a hormone is a chemical messenger that is secreted by a gland, and it travels in the bloodstream to target organs.

So the sex hormones are released by glands in the reproductive system.

And these are the male and the female reproductive systems.

So we can see the female reproductive system here.

Hopefully you will recognize this from previous units of work.

And we've got the uterus and the fallopian tubes and the ovaries, the cervix, and the vagina.

And then in the male reproductive system, we've got the penis.

We've got the sperm ducts.

We've got the testes, and the urethra down the middle of the penis there.

And the glands here are these ones that are shown in this lighter color.

So the ovaries in the female reproductive system, they secrete hormones, estrogen, for example.

And in the male reproductive system, the testes that are coded for by that gene on the Y chromosome, they secrete sex hormones called androgens, so for example, testosterone, which might be a hormone that you have heard of.

So time for a quick check.

So let's select the correct statement.

So, "Sex hormones are chemicals released by glands.

"The testes are glands.

"Only males have sex hormones.

"And sex hormones control the development of testes."

So select the statements that are correct.

So pause the video while you do so.

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

So, "Sex hormones are chemicals released by glands."

This is correct.

This is true.

"The testes are glands."

This is also correct.

"Only males have sex hormones."

This is incorrect.

And, "Sex hormones control the development of testes."

This is also correct.

So if you got that right, then well done.

So during fertilization, one of the egg cells which contain an X chromosome is going to be fertilized by a sperm cell.

Now the sperm cell could be carrying the X chromosome or it could be carrying the Y chromosome because 50% of the sperm carry the X chromosome and 50% of the sperm carry the Y chromosome.

So in this case, if it was the sperm that carries the Y chromosome, that has that gene on it that codes for testes development, fertilizes the egg, then we're going to have an XY zygote, which is going to lead to offspring with the biological sex of male.

In this case, if it's the X sperm, so the sperm carrying the X chromosome, and it's going to go with the X chromosome that's already in the egg, then we would get a female zygote with XX.

Now we can show this using a Punnett square.

And this can show us the inheritance or the chance, the probability chance, of sex inheritance.

So we've got the mother's chromosomes at the top there.

And as we've already said, all of her eggs are going to carry an X chromosome because those are the only chromosomes that she's got.

But with the father, half of the sperm are going to carry an X chromosome and half a Y. So we can put them into this Punnett square in order to determine the sex.

So if we get an X sperm with an X egg, then we're going to get a female.

And then an X sperm with the other X egg, a female.

And then a Y sperm with the X egg, XY.

And then the same again at the bottom.

So we can see that out of the four possible combinations, we have got two that are female and two that are male, which gives us a one‑to‑one ratio.

So the ratio shows us how many males and females that we get in humans.

We can see you've got 50% female and 50% male.

So in all cases, it is the male that determines the sex of the offspring because it's the sperm.

Which of those sperm fertilizes the egg that will determine whether the offspring is male or female, because the mother can only pass on X chromosomes.

So each offspring will inherit an X chromosome from its mother and each one will inherit either an X or a Y from its father.

So we can put it into a Punnett square without all the pictures and the chromosomes now, so we can usually predict the likely proportion of each sex.

However, remember that each instance of sexual reproduction is independent, so the ratio stays the same.

So like tossing the coin where you get heads, tails, heads, tails, heads, tails, that's not necessarily what you're going to get.

Sometimes you get heads, heads, heads, tails.

Okay, so it's not 50‑50.

So every time the sexual reproduction takes place, this probability remains the same.

So in this case, we've got the gamete sex chromosomes, and then we can cross them together in our Punnett square.

And then these are our possible outcomes for our offspring.

So we can see that the ratio of male to female offspring is two to two, or if we simplify that down, one to one.

Okay, time for a quick check.

So for each one of these Punnett squares, can you select the one that's correctly completed for sex determinism?

Look closely.

Pause the video, and we'll come back, and we'll see how you've got on.

Okay, so the correct one is C. So we can see in the other ones, in the first one we've got two male genotypes on the side there instead of a male and a female.

And on the middle one, we can see that it has been crossed incorrectly because we've ended up with two Ys, which would never happen in a genotype.

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

And let's move on.

So what I would like you to do for your practice task is to complete the sex determinism Punnett square and label the key features.

So explain the one‑to‑one probability for each offspring being male or female, but please can you include the following keywords in your explanation?

So gamete, X chromosome, Y chromosome, offspring, and biological sex.

Now it'll take you a little bit of time to write this explanation, so if you pause the video, and when you come back, I'll give you some feedback.

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

So hopefully you managed to complete this Punnett square.

So it doesn't matter whether you put the mother's or the father's sex chromosomes on each side, but one on each.

So in this case, we've got the mother sex chromosomes at the top, and they're labeled as the mother's sex chromosomes, XX.

And at the side we've said that these are the gametes, the sex chromosomes in the gametes, the egg.

And on the other side, we've put the father sex chromosomes.

And at the bottom, we've labeled that these are the gametes, the sperm sex chromosome, those individual ones.

And then we've got the offspring's possible sex chromosomes when we've crossed them together in the Punnett square.

And then in your explanation, so, "Explain the one‑to‑one probability "for each offspring being male or female "using these keywords."

So, "Each female gamete carries one X chromosome.

"Each male gamete carries either an X or a Y chromosome.

"Half of all the male gametes carry an X and half carry a Y.

"All offspring inherit an X chromosome from their mother "and half will inherit a Y chromosome from their father "and so have XY sex chromosomes, "and therefore have the biological sex male.

"Half can inherit an X chromosome from their father "and so will have the XX sex chromosomes, "and therefore have the biological sex female."

And finally, "The probability therefore of inheriting XX "or XY chromosomes is one‑to‑one."

So hopefully you managed to get all of those keywords into your answer.

It doesn't have to be exactly the same as I have written it, but words to that effect.

So if you've managed that, then well done.

So it's time to move on to the third part of our lesson and that is sex‑linked genetic disorders.

So all conditions that are inherited are called genetic disorders.

An expression of a genetic disorder is determined by the combination of alleles you inherit for a certain gene from your parents, and that's called your genotype.

Now we've talked about that previously with the alleles that you inherit that gives you a particular phenotype.

So we're looking specifically this time at alleles that are carried on sex chromosomes.

So if the allele for a genetic disorder is carried on the X or the Y chromosome, not both, then it is called a sex‑linked genetic disorder.

So you can see in this image here there's an example.

So we've got a gene there that's carried on the X chromosome but not the Y chromosome.

So the X chromosome is actually much bigger than the Y chromosome, so it carries more genes.

You see this picture here of some chromosomes that are being shown through a microscope, an optical microscope.

So a little bit fuzzy for that reason.

We can see these chromosomes when a cell is undergoing cell division.

And what a scientist will do is view them and arrange them so we can see all the chromosomes in pairs.

So these are the sex chromosomes as viewed through a microscope.

Because the X chromosome has roughly five times more genes than a Y, roughly a thousand genes on an X chromosome and 200 on a Y, it means that it's got lots of genes that the Y chromosome doesn't have.

So it expresses proteins that the Y chromosome does not.

Therefore, there are more examples of genes that are carried by the X chromosome than are carried only by the Y chromosome.

So time for a quick check.

Which of the statements about sex‑linked genetic disorders are correct?

So, "A, only females can suffer from them.

"B, only males can suffer from them.

"C, both males and females can suffer from them.

"And D, only females can carry them."

Okay, so pause the video while you decide, and then we'll come back and check.

Okay, let's see if we got that one right then.

"Both males and females can suffer from them."

Because as we remember from the previous sections of work in this lesson, both males and females do have an X chromosome.

Females have two X chromosomes and males have one.

So if a sex‑linked disorder is carried on the X chromosome, then both could have that particular disorder.

If it is unusually carried on the Y chromosome, then that would just be males, but females can also have a sex‑linked disorder if it's carried on the X. So red‑green colorblindness is a sex‑linked genetic disorder.

So it's a gene on the X chromosome that controls it.

It's not present on the Y chromosome, so it's only on the X chromosome.

So red‑green colorblindness is a recessive condition.

So therefore, if an individual has a dominant allele in their genotype, then they have normal color vision.

Okay, but if they have two recessive alleles or no dominant allele, therefore they will have the red‑green colorblindness.

So let's have a look at what that looks like.

So here we can see that the males have one copy of the gene on the X chromosome, but females have two copies of that gene because they have two X chromosomes.

So they've got two copies of that gene.

So as red‑green colorblindness is recessive, a female would need to have the homozygous recessive genotype, which is two recessive alleles, in order to have the genetic disorder.

So a female can also be a carrier because she could be heterozygous.

So one of her alleles might be dominant and one of her alleles might be recessive.

So because she has a recessive allele in her genotype, it means that she doesn't express it in her phenotype because she's also got a dominant one as well.

So here are our examples of our possible outcomes for phenotype.

So in a female, we've got you could be dominant homozygous, so two dominant alleles, and then you would have normal color vision and be a female.

Or you could be a female that's heterozygous, which means that you're a carrier for colorblindness but you still have normal color vision.

And finally, if the female is homozygous recessive, then she would have the red and green colorblindness because she's got two recessive alleles in her genotype.

So as a male only has one copy of this gene, he would have red‑green colorblindness only if he has one recessive allele.

So let's have a look at what that looks like.

So a male can't be a carrier.

Therefore, there is a higher probability of them inheriting this disorder because these are the two different genotypes and phenotypes that you could have as a male.

So you would have XY chromosomes, but you could have the X chromosome that's recessive, which would mean that you were red‑green colorblind.

Or you could have the X chromosome that's dominant, which means that you wouldn't. So you can't be a carrier.

You can either have colorblindness or not.

So therefore, it's much more common, red‑green colorblindness, in males than it is in females.

So time for a quick check.

So, "Select the images that show "a female carrier of red‑green colorblindness."

Pause the video while you decide.

Okay, so the correct answer is A. So we can see that there's a female because it's XX, and the genotype is heterozygous, so it would have the dominant phenotype of normal color vision but does carry that red‑green colorblindness allele.

Okay, so if you got that right, well done.

Let's move on.

So the probability ratio for inheriting red‑green colorblindness can be calculated using a Punnett square.

So you can see these on the diagram here.

Now, when we draw Punnett squares and write the genotype, we make sure that we include the sex chromosomes, and we do the allele as a superscript.

So we can see that the mother there is normal vision, okay, but she's a carrier.

So she's got the big C and the little c. And then the father has normal vision, and he has got the big C. And then when we cross those together, let's see what we get.

So we get one possible offspring with two dominant alleles.

We've got another offspring with a heterozygous allele.

Then we've got another male with a dominant allele.

And then we've got another male with a recessive allele.

So as the gene is carried on the X chromosome, the genotype is denoted using the X and the Y chromosome so that therefore we know it's a sex‑linked condition.

And as I've already said, the alleles are shown using superscripts, okay, with the recessive allele being lowercase and the dominant allele being uppercase.

So in this case here, we've got a male with colorblindness.

He's got one recessive allele, and therefore will be colorblind.

Both of our females there, X and X, both have a dominant allele, so therefore will not show red‑green colorblindness.

And then the other male's got a dominant allele, so again won't show red‑green colorblindness.

So what that gives us is a one‑to‑three ratio.

So out of our possible genotypes, there is a one‑to‑three ratio of having colorblindness.

Okay, we can also show this using a family genetic tree.

So we've got hemophilia, which is also a sex‑linked genetic disorder.

And that's where sufferers lack a protein that helps them to clot their blood.

Now that's really important, not just for like scabs on the surface of your skin, but also if any little blood vessels break underneath the surface, like when you bruise yourself.

So therefore it can lead to spontaneous bleeding just out and about, just when you knock yourself, or it can also lead to excessive bleeding when you have been injured.

Okay, so this is a quite serious condition, and it has to be treated regularly.

So the gene for this particular recessive disorder is carried on the X chromosome again, and we can show these on our family tree.

We can see that the females can be carriers, so remember those are the circles, and they're shown with the boxes that are the gray and white stripy boxes.

And they can also not have the condition or have the condition.

So they would be gray if they don't have the condition, because that's dominant not to have it, and they would be white if they have the condition because it's recessive.

But you can see for the males, which are the square boxes, they don't have a gray and white box because they can't be carriers.

They either have the condition or they don't, and you can see that in these examples here.

So time for a quick check.

So is it true or false?

"Only males can suffer from haemophilia."

Now once you've decided, have a look at these statements below and see which one you think best justifies your choice, true or false.

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

Okay, "Only males can suffer from haemophilia."

That is false.

And the justification for that one is this one: Males are more likely to suffer from haemophilia as they only have to inherit one recessive allele, okay, whereas females have to inherit two recessive alleles.

So that's going to happen less frequently.

So if you got that right, then well done.

Let's move on to a practice task.

So we've got a genetic family tree here for haemophilia, like the one that you saw on the previous slide.

So take your time to have a look at this.

So what I would like you to do is to complete the genotypes for female 10 and male 11.

You can see those there with the question marks.

And then the second part of your task is to explain why all females from parents 3 and 6, so find 3 and 6 there, why will all females be carriers and all males will not have the condition?

Okay, so pause the video while you do this because it might take you a little bit of time to work it out, and then we'll come back and I'll give you some feedback.

Okay, let's have a look at how you got on then.

So first of all, completing the genotypes for female 10 and male 11, and that's what we should have.

So we've got a female carrier there and we've got a male who's got the dominant allele, so therefore does not have the haemophilia condition.

And then question number two is an explain question, so, "Explain why all females from parents 3 and 6 "will be carriers and males will not have the condition."

So you can do yourself a Punnett square there to show the two parents.

We've got parent one, the female at the top there that's got two dominant alleles, and then we've got the father down the side with the recessive allele on his X chromosome.

And then when you cross them together, we can see we've got females that are heterozygous carriers, and we've got males that do not carry the condition.

So the explanation would be as follows: A daughter will always receive an X chromosome with the haemophilia recessive allele from her father, as this is his only X. The mother has two dominant alleles, one carried by each X chromosome, so a daughter will always inherit one dominant allele from her mother and therefore be a carrier.

A son will inherit the Y from his father and a dominant allele from his mother, so will not have the condition.

So if you got those statements correct, then well done.

And it's time to summarize our lesson.

So well done with your work today.

The summary of today's lesson is as follows: Sexual reproduction produces offspring by passing genetic material from a male and a female parent via gametes.

The gametes carry half of the chromosomes of the parents, one from each pair, and an individual's biological sex is determined by the two sex chromosomes that they inherit.

In humans, the 23rd pair of chromosomes are sex chromosomes: XX in females and XY in males.

Females always pass on an X chromosome to their offspring because that's all they've got, and males can pass on an X or a Y. A gene on the Y chromosome triggers the development of the testes to make male sex hormones or androgens.

Punnett squares can be used to show the inheritance of sex chromosomes and model probability ratios.

And some disorders are caused by alleles on sex chromosomes and these are called sex‑linked genetic disorders.

So they have sex‑linked inheritance.

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