Lesson video

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

The title of today's lesson is Explaining Inheritance: Mendel and Beyond.

We're going to be looking at the work of Gregor Mendel today, how his observations from growing plants led to our understanding of inheritance and how his work was built on by other scientists that led to our modern understanding of the role of DNA in inheritance.

My name is 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 describe the role of Gregor Mendel and other scientists in the development and understanding of inheritance and genetics.

So we've got some keywords in today's lesson and our keywords are heredity, trait, recessive, dominant, and DNA.

So if you'd like to pause the video to write down those definitions, then you can do.

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

So our lesson today's in two parts.

The first one is really looking in depth at Mendel's discoveries, and then we are gonna move on to look at further scientists and how they built on the work that Mendel did with his growing plants and how that led to our understanding of DNA.

So let's get started with our first part, today's lesson, which is Mendel's discoveries.

So Gregor Mendel, he was a monk and he was born in Austria in 1822, so here's a picture of him.

And as a monk, he carried out a series of experiments to discover the basic principles of a heredity using peas in the monastery garden.

He was a scientist and he was interested in the production of new varieties of plants.

He's known to have studied the work of past and contemporary scientists such as Charles Darwin.

So he was known to have read the works of Charles Darwin.

Although Charles Darwin didn't actually know who he was.

He did read what he had said and he was trying to build on some of his ideas.

But not everything that Gregor Mendel discovered and concluded on did match what Charles Darwin said.

Particularly things about blending traits together, which we'll come to later.

So Mendel studied pea plants and this picture of a pea plant here being pollinated by hand.

So that's where somebody takes the pollen, usually using some kind of cotton bud from one plant and then deposits it on the stigma of another plant in order to be able to get them to sexually reproduce together so that we can identify and observe the offspring of those plants.

So Mendel studied pea plants as they had distinctive traits, distinctive traits that he had already observed when he'd been growing them in the garden.

And these distinctive traits could easily be identified.

Now a trait is a specific characteristic and at that time because scientists didn't understand about it heredity and also because they didn't really understand about how DNA led to specific traits or characteristics, they were just referred to as traits at that time, modern scientists would refer to these as the phenotype being associated with a particular gene.

So the reason he used pea plants is 'cause they were very easily grown and they could be hand pollinated, not all plants can be.

They're sewn every year.

So therefore you could get a new generation and you could see what the offspring would look like and they had actually been used in similar studies.

Now this is quite important in science because science, if scientists are studying similar things, it makes the conclusions that they draw more valid if you're finding similar patterns and observations from those experiments.

So Mendel studied seven traits.

So these are four of the traits that he studied.

So this was whether they produce smooth or wrinkled peas, yellow or green seed pods, tall or dwarf varieties of the plants and purple or white flowers.

And his experiments went from 1856 to 1863 and they showed that the inheritance of these traits followed a very predictable pattern.

And he wrote all of this down in a book.

There are images available of this book where he wrote down all of the numbers of the plants and what traits that they had.

And then he was looking for patterns in that information.

Now this mathematical approach to the study of heredity was quite new.

People hadn't thought to do this before, so therefore he is now known as the father of genetics.

So Mendel bred plants together with two versions of the trait that he was investigating.

So here is an example.

So with the flowers, the purple flowers, he bred together plants with purple flowers and white flowers 'cause he wanted to see what would happen because if there was blending of traits as there is in some instances with phenotypes, then you would get a colour that was halfway between.

And that does happen sometimes, but in pea plants it didn't.

So when the parents were purple and white, the offspring ended up being purple.

So when you look at that, you think, well what's happened to the white? Where's that trait disappeared to? And that's what Mendel was interested in finding out.

Now often we call this first generation that's bred from purebred parents.

So ones that are completely purple and completely white and have been for a number of generations and not crossbred together.

Then when you breed them together you get this first generation and that's often denoted as the F1 generation.

So Mendel next bred that F1 generation together.

So those offspring that came from purple plants and white plants are the parents.

So we've got the F1 generation there and they are purple flowers.

But we know that they were made from parents that have purple and white flowers.

And in the second generation, which we call the F2 generation, we can see that there's a ratio here.

And that three out of four of the offspring had purple flowers and one out of the four had white flowers.

So that trait has sort of reappeared and that's what Mendel noticed.

And by writing down all of the numbers of all of the plants, he got this common ratio of three to one.

So when breeding together the F2 generation, so that's the generation that came from purple and white parents in their offspring, you get a three to one ratio of the purple to the white flowers.

So he discovered this after repeated it many, many times.

So if you do see an image of his diary and his journal, you can see that there's really, really high numbers of plants that he investigate.

So his sample size was really large.

So at the time of Mendel's discovery there wasn't any knowledge of genes or DNA or alleles.

So Mendel concluded that these traits must be passed on by some kind of factor of inheritance, but he didn't name what it was.

And these factors would be passed on and then they are observed in the traits.

So whatever was passed on from one generation to the next generation allowed that particular trait to observed.

If the trait was not observed after the first generation.

So for example when you have a purebred white flower plant and a purebred purple flower plant together, then all of their offspring are purple, then that must mean that the white trait is recessive.

And then the trait that you do observe in that generation was termed dominant.

So he described these factors and he actually did use a capital letter and a lower case letter, which is exactly the same way as we still denote it today.

So time for a quick check.

So tick the statements that correctly describe Mendel's discoveries.

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

Okay, so let's have a look at the correct answers then.

So pea plants have some traits that show predictable patterns of heredity, that is correct.

Next one, some traits are dominant and some are recessive.

That one is correct.

And an offspring receives a factor from each parent which is unchanged, that is correct.

So this second one down genes are units of inheritance and different versions of the same gene are called alleles.

Even though this is a true statement, it wasn't one of Mendel's discoveries.

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

So modern scientists use our understanding of alleles to explain Mendel's observations.

So this isn't what he knew at the time, but we've used his observations in order to add to them using our current understanding.

So with the inheritance of flower colour as an example, purple is the dominant allele.

So we give it a capital the same way as Mendel did.

And white is recessive, so we give it the lowercase letter.

So here's our example of our parents.

So in our first generation we've got a purple flower bred with a white flower.

Now the gametes there are divided up.

So the purple flower, because it is a purebred, it's got two dominant alleles and then our white one has got two recessive alleles.

So that's what go into their gametes when they're divided.

And then what we do is we can cross those together and we can see what the different combinations that we can get from dominant alleles from one parent and recessive alleles from the other.

And you can see in these examples, you can only end up with plants that have one dominant and one recessive allele 'cause they received the dominant from one parent and the recessive from the other.

And because purple is a dominant trait, that would mean that all of the offspring would be purple.

So then if we have a look at the F2 generation, so we take those purple flowers from the bottom that have got one dominant and one recessive allele, or we would call that heterozygous.

And then again we cross them together and we have a look at what we can get.

So their gat in this case, half of them would have the dominant allele and half of them would have the recessive allele.

And then when we look at the combinations, when we cross those together, this is what we get in our offspring.

So these are our possible genotypes in our offspring, which gives us that predictable ratio that we've already discussed.

So these crosses can also be modelled using Punnett squares.

So the first cross is two homozygous plants for seed shape in this case.

So we're moving away from the coloured plants and we're looking at our seeds.

So smooth seeds are dominant.

So again, we've got this purebred which has got this homozygous dominant genotype.

And then we've got our wrinkled seeds, which is our recessive.

So they are homozygous recessive genotype.

And when we cross those together, the F1 generation are all heterozygous.

And then we can move on to cross that F1 generation together.

So we put both of the heterozygous parents on either side of our Punnett square and then we cross those together and we can see that half of the offspring are heterozygous with smooth seeds.

We can see that one out of the four is dominant smooth seeds.

And then we can see we've got a one out of four that is recessive, has the recessive trait of wrinkled seeds.

So this mathematical relationship was actually repeated for all of the other traits that are controlled by a single gene with a dominant and recessive allele because this isn't always the case.

But for these particular traits, they're controlled by a single gene and they have a dominant and a recessive allele.

So that includes our pea pods, which were yellow or green, with green being our dominant and our plants which are tall and dwarf with tall being our dominant.

So before Mendel's studies, it was incorrectly believed that the traits of the offspring were due to the blending of the traits of their parents.

And this was a conclusion that actually Darwin subscribed to as well.

So Mendel's research was carried out over eight years and it involved thousands of plants, so that made his conclusions more valid.

So let's have a little go at modelling and answer together.

So we'll start with tall and dwarf plants.

So tall and dwarf pea plants are controlled by a single gene.

So we can see that the tall there's got the capital letter.

So therefore that is the dominant trait.

So explain why when we crossed the offspring of these particular plants together, all of the offspring are tall.

So let's have a look at model answers for this then.

So we would say that the tall plant has two dominant alleles and the offspring will inherit an allele from each parent.

And as they inherit an allele from one parent, a dominant allele from one parent, they will be tall because the recessive allele that they will inherit from the other parent.

So if you can now have a go at answering this question, this one is about smooth and wrinkled pea plants, and here's a little image to help you.

So I would like you please to explain why, when crossed, smooth seed plants can produce a wrinkled seed plant.

So pause the video while you have a go at this and then we will come back and check how you've got on.

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

So we should have both parents carry a recessive allele.

So an offspring could inherit a recessive allele from each parent and therefore could produce wrinkled seeds.

So it's all about the alleles that the offspring inherit, and that's what we discuss when we are describing that inheritance.

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

So let's move on to a practise task.

So Izzy and Jacob are trying to summarise the work of Mendel and his contribution to the current understanding of heredity.

So Izzy says, "Mendel discovered that two genes are passed on from parents to offspring." And Jacob says, "Genes were not understood at that time.

Mendel called them heritable factors." Quite a difficult (chuckles) word to say.

And Izzy and Jacob both have some correct ideas.

Use their ideas as a starting point to summarise the discoveries of Mendel.

So you might want to do this as a piece of prose, a paragraph, or you might want to do it in bullet points.

Either way it's gonna take a little bit of writing.

So pause the video and then when you come back I'll give you some feedback.

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

So we are using Izzy and Jacob's ideas and we're building on those to look at the discoveries of Mendel.

So Izzy isn't correct because genes were discovered long after Mendel's work.

So Mendel invested how traits are passed on from parents to offspring in pea plants.

So Izzy knew that something was passed on from parents to offspring.

But in the case of Mendel, it was this idea of traits being passed on.

And Jacob is correct because Mendel was suggesting that these traits might be linked to heritable factors, which were later discovered.

And then they were called genes.

And Mendel did suggest that some of the factors were dominant and some were recessive, which we now know to be true for alleles of genes.

So if you wrote words to that effect or something similar, you might have added some extra things in about the mathematical relationships, and the probability any of that information is correct.

So well done.

So it's time to move on to the second part of today's lesson, which is called further developments.

So Mendel's theories were not widely known or accepted by other scientists in his lifetime.

He wasn't actually a well read scientist, he wasn't famous at the time.

Unlike somebody like Charles Darwin, for example.

His work was published in a journal that was not well known.

He did not have a theory for what the factors of inheritance might be.

So even though he didn't have a theory that was incorrect, he didn't have a theory at all for what these factors might be, so some of his work was dismissed.

And his findings did seem to counter some of the theories of inheritance had already been proposed.

So people who thought that the traits of offspring were blended in their offspring.

In some cases you may see that in a phenotype of an offspring, but it wasn't known that there were these dominant and recessive traits.

Many years later though Mendel's research informed the work of other scientists who were conducting work on heredity and his findings became referred to as Mendel's Laws.

So quick check, Mendel's work immediately changed the way that most scientists explained heredity.

Now first of all, decide whether that's true or false, and then once you've decided which of the statements below best justifies your answer.

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

Okay then.

So Mendel's work immediately changed the way most scientists explained heredity.

That is false.

And the reason is because it took many years for Mendel's work to become widely known and accepted.

And that wasn't unusual for scientists at that time.

So in 1869, the chemical molecule DNA was discovered by a Swiss biochemist called Friedrich Miescher.

So just a reminder that DNA is a chemical molecule, it's made of many repeating units called nucleotides.

And 1869 was quite a long time before the discovery of the structure of DNA, which is the one we talk about more often in biology with Watson and Crick.

But actually the chemical molecule DNA was discovered far earlier.

He extracted the DNA from the nucleus.

Actually, of white blood cells that he was working on.

And he worked out what chemical it was made up of and he realised there was a lot of these chemical groups in this particular molecule and there were new combinations that we hadn't noticed before.

Now his research was disputed again at the time.

You can see a bit of a common theme here, but it was built upon again by many other scientists in the following years.

So this is scientists you might not have heard of.

It was a very, very important discovery.

So in 1943, scientists concluded that DNA was the genetic material in cells that made up genes.

And so that's what Mendel's factors were, that idea of something being passed.

So we then knew that it was DNA, 'cause lots of other chemical molecules in the body at the time, people thought might be the chemical that was passed from parent to offspring.

So it wasn't until 1943 that it was concluded, it is actually DNA is the chemical molecule that's passed from parents to offspring and controls heredity.

So many scientists research different aspects of DNA and they continue to do so, but through the 20th century, including its chemical structure, chromosome structure, and its replications of how the DNA replicates and then what genes are and where they are in the DNA and how they make up DNA.

So an example is in 1953, so Marie Maynard Daly, she discovered that the proteins involved in chromosome structure, which led to the understanding of gene expression.

So whether genes are translated into proteins or not, and how that is controlled.

Now, her work was so important that it was actually cited by James Watson when he received the Nobel Prize.

At the start of the 1950s, Francis Crick and James Watson were trying to discover the structure of DNA and build a model of a DNA molecule.

So that's what they were trying to do.

They knew what chemical grips it was made up of from previous discoveries, and they knew that it was stored in the nucleus and they knew that it was that a heredity factor.

However, another team of researchers included Rosalind Franklin and Maurice Wilkins were also trying to discover the structure of DNA, but they were using a new technique called X-ray crystallography.

And in 1952, Rosalind Franklin made an image of DNA using X-rays and showed that DNA had a helical shape.

The image was shown to James Watson and in 1953, Watson and Crick used the image to help them produce the first 3D model of the structure of DNA.

Now, Watson, Crick and Wilkins received the Nobel Prize in 1962 for working out the structure of DNA.

So you can see that science moves quite slowly sometimes.

So in 1953, they discovered it, but they did not receive the Nobel Prize until 1962.

The controversial part is that Rosalind Franklin was not included or credited for her crucial research.

However, as I said previously, Watson did cite Marie Maynard Daly his work when he was collecting his prize.

So choose the correct statements about the discovery of DNA.

So pause while you read these and then we'll come back and we'll see if you've got it right.

Okay, let's have a look at these then.

So many scientists were involved in discovering the structure and function of DNA.

That is correct statement.

Watson and Crick were the only scientists in involving, sorry, in discovering the structure of DNA, that's incorrect.

The chemical composition of DNA was known before Watson and Crick started their research.

It was known and some scientists worked collaboratively to discover the structure and function of DNA.

That is also true.

So lots of scientists contributed their research to our current understanding of DNA.

So around 120 years after Mendel's experiments in 1988, Professor Alison Smith finally identified the gene and the alleles associated with the trait of smooth and wrinkled peas.

You can see at the top we've got these alleles that code for an enzyme and that converts sugar to starch.

And then that gives us this rounded appearance of the pea.

And we can see underneath the alleles that code for the recessive feature where the enzyme is not working.

So therefore the sugar is not converted to starch gives us these wrinkled piece.

So the story of Mendel and all the further developments show how scientists build on the work of other scientists to improve explanations and theories over time.

Over recent decades, scientists have discovered the importance of studying of the DNA in the genome, not just individual genes.

'Cause remember, the genome is made up of genes and non-coding areas of DNA.

And in this image we can see this is all the chromosomes in the human genome.

So mapping the entire human genome began in 1919, was finally completed in 2003.

And since their scientists have been investigating the proteins that each gene codes for and the role of the non-coding DNA, that is between the genes.

So our understanding of genes, alleles, genomes, and heredity has led to advances in medicine and disease control.

And that includes genetic testing for alleles associated with non-communicable disease, vaccinating against communicable diseases, using genetic material from pathogens.

You might recall that during the COVID pandemic that there was a big rush to try to make vaccines.

And this new style of vaccines was all about using the genetic material from pathogens in order to create the vaccine.

And healthcare that is personalised, using the patient's genetic to decide what course of treatment will be best for them.

So which statements are true? So I'll let you read those and then decide which ones are correct.

And then if you come back, then we'll have a look if you've got them right.

Okay, so let's have a look at these statements then.

So Mendel identified the alleles associated with smooth and wrinkled peas.

That one is not correct.

Our understanding of heredity and DNA has been built up by many scientists, not just Mendel, Franklin, Wilkins Crick, and Watson, that one is true.

Lots and lots of scientists, many who you may never have heard of.

There have been no major developments in our understanding of genetics since the 3D structure of DNA was worked out in 1953.

That is incorrect, as we know that work on is ongoing.

And since Mendel's work, scientists have discovered that non-coding DNA between genes is important.

This is also a true statement.

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

So it's time for a practise task now.

So what I'd like you to do is to create a timeline to show how different scientists have helped to develop our understanding of inheritance and DNA.

You can add images, you can do further research, finding different events, different scientists that were involved.

You can add pictures or drawings.

This is how to start you off.

So you don't need to write loads about each, but it's up to you to develop this as far as you'd like to go.

So we've got 1856, we've got Mendel's discoveries on there, and we've got Miescher on there.

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

Hopefully you got to do some of your own research, but here's some examples of things that you could have included.

So in 1943, DNA is discovered to be the genetic material of heredity.

In 1952, Franklin produced an image of DNA using X-rays.

In 1953, Watson and Crick produced their 3D model of DNA.

In 1953, Daly discovers proteins involved in chromosome structure.

In 1962, Watson, Crick and Wilkins win the Nobel Prize for the 3D structure of DNA.

In 1988, Alison Smith identifies the alleles that are associated with the traits investigated by Mendel.

In 2003, the human genome is sequenced and in the 2020s as increasing use of genetic testing, DNA vaccines, genome editing, and personalised medicine.

Now you might have some specific examples of those.

Now if you've included some of those key points, then well done.

So that brings us to the end of our lesson.

So Mendel investigated the inheritance of traits, e.

g flower colour, and seed texture in pea plants in the 1850s and '60s.

Earlier in contemporary scientists believe that traits blended during sexual reproduction.

Mendel showed traits were dominant and recessive.

Mendel suggested heritable factors caused the traits and other scientists discovered DNA in 1869 and genes in the 1900s.

Theories about heredity developed over time.

And in 1943, DNA was linked with inheritance.

And in 1953, the 3D structure of DNA worked out.

Over recent decades, scientists have discovered the importance of studying whole genomes, not just individual genes.

So the story of Mendel and all the further developments in understanding heredity and DNA show how scientists build on the work of other scientists to improve explanations and theories over time.

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