Lesson video

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

The title of today's lesson is The Inheritance of Biological Sex in Humans.

And what we're going to be looking at today is how chromosomes divide into gametes and how that will determine the probability of the biological sex of the offspring.

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 we've got some key terms for today's lesson.

And our key terms are sexual reproduction, biological sex, sex chromosome and sex hormone.

So I'm gonna put the definitions up.

So if you want to pause the slide while I do that and you want to write them down, then you can do otherwise I'll be going through them as we go through today's lesson.

Okay, so our lesson today is in two parts.

Our first part is sex chromosomes.

So what they are and how those are inherited, and then how sex is determined and how we can model that out using a punnet square.

So let's get started with first part.

Today's lesson, which is sex chromosomes.

Humans reproduce as do lots of other organisms, but we're gonna 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 fertilised exome 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 zygotes, 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, 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 time perhaps to 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 'em.

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 fertilisation.

And the resulting zygote then will have two copies of each chromosome.

So we can see that those in this picture, they're a bit small, but we will, 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 fertilisation.

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 nuclear eye 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 chromosomes, pairs of chromosomes up until that point.

And there 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 or 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 video or you decide, and then we'll check if you've got it right.

So the correct answer is two.

So if you've 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 carrier type.

So you can see this here, you might see in 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 of 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, each one of her exhales will contain an X chromosome.

And then 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, XY would give you the biological sex of male.

So time for a practise 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 Sophia 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 'cause 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 this case was Jun.

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

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

So we're gonna correct that by saying males have an X and a Y chromosome.

Sam said that males only produce sperm with Y chromosomes.

So let's correct.

So half of a male sperm carry an X and half carry a Y chromosome.

And Sofia, all parents have four, I'm sorry, 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've got that one right, then well done and it's time to move on now to the second part of our lesson.

In the second part of our lesson, what we're gonna look at is sex determinism.

Okay? So 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 so 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 can 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 and we've got the uterus and the fallopian tubes and the ovaries, the cervix and the vagina.

And then in the male reproduction reproductive system, we've got the penis, we've got the sperm ducks, 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 colour.

So the ovaries in the female reproductive system, they secrete hormones, oestrogen, 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 let's move on.

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

Now the sperm cell could be carrying the X chromosome or it could be carrying the Y chromosome 'cause 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 fertilises the egg, then we're gonna have an Z zygote, which is gonna lead to offspring with the biological sex of male.

In this case, if it's the X and sperm, so the sperm carrying the X chromosome, and it's gonna 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 gonna 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 and we're gonna get a female and then an X sperm with the other X egg, 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 have 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 fertilises 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 upon its square without all the pictures in the chromosomes now.

So we can use to predict the likely proportion of each sex.

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

So light toss in a coin where you get heads, tails, heads, tails, heads, tails.

That's not necessarily what you're gonna get.

Sometimes you get heads, heads, heads, tails.

Okay, so it's not 50/50.

So every time that 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 punnet 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.

If we simplify that down one to one.

Okay, time for a quick check.

So for each one of these punnet 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 'cause we've ended up with two Y's, 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 practise task is to complete the sex determinism punnet 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 key words 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 the mothers or the father's sex chromosomes on each side, but one on each.

So in this case we've got the mother's sex chromosomes at the top and they're labelled as the mother 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 labelled that these are the gametes, the sperm sex chromosome, those individual ones.

And then we've got the offsprings possible sex chromosomes when we've crossed them together in the punnet square.

And then in your explanation, so explain the one-to-one probability for each offspring being male or female using these key words.

So each female gamete carries one X chromosome.

Each male gat 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 an XY sex chromosomes and therefore have the biological sex male half can inherit an X chromosome from their father.

And so we'll have the XX sex chromosomes and therefore have the biological sex female.

And finally, the probability therefore in inheriting XX or XY chromosomes is one-to-one.

So hopefully you manage 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.

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

And here is our summary.

So sexual reproduction produces offspring bypassing 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 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, and males can pass on X or Y chromosome.

A gene on the Y chromosome triggers the development of testes, which make male sex hormones or androgens.

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

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