GCSE Science podcasts - Inheritance, variation and evolution

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the first episode in a series on Inheritance, Variation and Evolution.

Or in other words, how you get things from your biological parents, how everyone is a little bit different, and how life forms have changed over billions of years.

We’re starting at the beginning with reproduction. There are two types of reproduction you need to know about: sexual and asexual.

Now, I’m going to be using a few more specialised terms. You’ll need to have a good idea of what meiosis and mitosis are. We did a whole episode on meiosis and mitosis in our series on The Cell, so if you need a little memory jogging, it might be a good idea to jump back to that episode on BBC Sounds before coming back here.

But if you’re ready, let’s get into it. First up is sexual reproduction, which involves two parents. When I say parents here, we’re talking about your biological parents, not necessarily the people who bring you up.

The two biological parents, the mother and father, combine their genetic material, DNA, to make children, or – for your exam – offspring.

These offspring are genetically different from their parents and aren’t clones of them. I said in the first series on The Cell that you might have your dad’s chin? Well you might have his chin but you won’t look identical to him.

This is all down to a process called meiosis, which is a special type of cell division just for sex cells.You’ll need to remember some details here, so grab a pen and write this down.

Meiosis is a type of cell division that creates sex cells, known as gametes. Gametes in animals are sperm and ova, or eggs. Sexual reproduction happens in plants too. Their gametes are pollen and ova. Easy way to remember: pollen are plant sperm and ova are plant eggs.

Each gamete contains half the number of chromosomes that are found in a typical body cell. So remember, typical body cells in humans have 46 chromosomes, in 23 pairs (that’s known as diploid) but in gametes there are half of this number so only 23 chromosomes. This is known as haploid. So you’ve got haploid and diploid.

Now gametes only need half this number of chromosomes because they are going to do something interesting. They’re going to join forces with another gamete to get to the full 46, and this happens during a process called fertilisation.

In humans, fertilisation happens when the male sperm cell fuses with the female egg cell. This combines the genetic material of both parents to form offspring called a zygote.

When the parents’ genetic material inside the gametes is combined, the zygote has the full number of chromosomes again, so that’s 46 chromosomes in 23 pairs, which is known as diploid.

This process of combining the genetic material of two parents leads to genetic variation, it's why everyone looks different. The genetic material is randomly shuffled, which is why even if you have the same two biological parents, siblings don’t look alike, unless you’re identical twins. So you might get your dad’s chin and your mum’s nose, your sister might get your mum’s chin and your dad’s nose.So that’s sexual reproduction - now let’s look at asexual reproduction.

What you need to understand is that asexual reproduction only involves one parent to create offspring. How?

Let’s talk about potatoes (trust me here). Have you ever seen a potato that’s past its best? They have little growths coming out of them, kind of looking like little shoots?

Well, if you sat there and watched this old potato, you’d notice that this little shoot would continue to grow. It would get bigger and bigger, and eventually a bud would form. From this little bud a new potato would grow.

No mention of gametes fusing together, nothing like that. This potato would be genetically identical to its parent.

It would be a clone of that old potato and that is an example of asexual reproduction. Asexual reproduction involves the process of mitosis, not meiosis, mitosis. And that’s a type of cell division where cells create identical copies of the parent cells.

A good example of this are bacterial cells. In terms of plants, apart from potatoes, strawberries also reproduce asexually. And, you might not know this, so do jellyfish!

I’m Dr Alex Lathbridge and this is Bitesize Biology.

To find the rest of this series and more from our Bitesize podcasts, you can search Bitesize on the BBC Sounds app.

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is episode two of a seven-part series on Inheritance, variation and evolution. In this episode we’re going to talk about genetic inheritance. That means we’re talking about chromosomes, genes and alleles.

So do you remember that cells have a nucleus? If not, go back and listen to our series on The Cell.
Here’s a quick recap: The nucleus is sort of like a library, containing lots of books, or chromosomes.Within the books there are lots of recipes, or genes, that have instructions, and the recipes are written with letters, known as bases. This is the DNA code.

Each gene, or recipe, codes for a sequence of amino acids, which code for a specific protein. Lots of coding there I know. Now, those genes, the instructions, they determine what characteristics you inherit. Often, characteristics are determined by several genes interacting with each other. But there are a few examples where characteristics are controlled by just one gene, like eye colour in humans.

But before we get into that, I’m going to take you through the key terms you need to know, so grab a pen and write this down: Chromosomes are contained inside the nucleus of a cell and contain lots of different genes. Chromosomes are found in the nucleus as pairs, one is inherited from each biological parent.

DNA is found in our genes and everyone’s DNA is completely unique to them. It is the basic genetic material inherited from our biological parents and determines our characteristics.

Genes are sections of DNA. And alleles are different versions of the same gene. They exist as pairs.Now we’ve not talked about alleles before, but they are really important. Why?

You get half your genes from each biological parent. For almost every gene you get one allele from each parent.

So if a gene is a recipe in a book, the alleles are the variations. The recipe from your mum might say use blue icing and the one from your dad might say use red icing. The same gene (which is icing) but different variations, red or blue.

Real world example that isn’t to do with cake: we all have different eye colours. Some of us might have brown eyes, green eyes or blue eyes. These different versions of the same gene for eye colour, these are the alleles.

You inherited two alleles for eye colour and the cell chooses which one to use.I’m going to say this word “allele” a lot in this episode so be sure to get your head around it before we go any further.

I am sorry because I'm going to keep throwing some more terminology at you. You really need to have that pen in hand.

Dominant alleles. These the alleles that are always used by the cell. No matter what characteristic they cause and no matter what other allele is paired with it, they will always be chosen by the cell. Even if there is only one dominant allele in the pair. Dominant alleles are written as big capital letters, so if its dominant make sure you give it a capital letter. The alleles for brown eyes are dominant.

The opposite of that: recessive alleles. They won’t be chosen by the cell in the presence of a dominant allele. A characteristic caused by a recessive allele will only be chosen if the individual has two recessive alleles, one from each biological parent, and does not have a dominant allele. These are written in non-capital letters. The allele, for instance, for blue eyes is recessive, so if you have blue eyes, that means you’ve had to inherit two recessive alleles on the eye colour gene from each of your parents.

We’ve got homozygous alleles. Those are two alleles that are both identical for the same characteristic. An organism might have two alleles from their mother and father for a particular trait, eye colour for example, that are the same, so if you receive this allele for brown eyes from each parent you will definitely have brown eyes.

And then we have heterozygous alleles, these are the opposite of homozygous alleles, these are two alleles that are different for a characteristic. An allele for blue eyes from one parent, and brown eyes from the other. And no this doesn’t mean that the person will have two differently coloured eyes.It’s easier to remember these two long words if you think that homo means same, and hetero means different, homozygous, heterozygous.

A genotype is the combination of all the alleles that an individual has. So in effect, it’s their genes.Genotype = genes. It’s everything.

A phenotype is the characteristics that an individual has, as a result of their genes.
So phenotype = physical.

So, you might have a genotype containing two recessive alleles for eye colour, and the phenotype would be your blue eyes. Or two dominant alleles (like brown) and your phenotype would be brown eyes.

But you could also have a phenotype, so physical characteristic, of brown eyes with a heterozygous genotype (so that would be one blue allele and one brown allele) you wouldn’t be able to tell the difference just by looking at someone’s face.

So two people with brown eyes, one of them could have homozygous alleles, they might have two dominant alleles for brown eyes. Whereas someone else could have heterozygous alleles, they could have inherited one allele from one parent for recessive brown eyes, versus one from another parent for dominant brown eyes.

Just quickly I want to give you two examples of inherited health disorders: Cystic fibrosis is an inherited disorder that affects the lungs and digestive system. It is caused by a recessive allele. So to be born with cystic fibrosis, the offspring inherits two copies of this faulty recessive allele, one from each of their parents.

And also Polydactyly is an inherited condition where a person has extra fingers or toes and it’s caused by a dominant allele. So it can be passed on by just one allele from one parent.

Genes also play a role in determining the biological sex of offspring. This is called sex determination.Typically, but not always, people have 23 chromosomes pairs. For example, people with Down’s Syndrome have an extra copy of chromosome 21.

The first 22 are for characteristics, things like eye colour, and the 23rd pair is for biological sex: whether offspring are male or female.

Because during fertilisation, there is a 50% chance that a sperm containing an X chromosome will fuse with the egg (an egg is always X); this would result in a baby girl, XX.

There is also a 50% chance that that sperm could be containing a Y chromosome. And that will fuse with the egg; this would result in a baby boy, XY.

Because this is GCSE-level, all of that is a huge simplification on biological sex but it’s what you need for the exams.

(In actual scientific fact, sex differentiation beyond the rigid XX = male and XY = female – there’s some diversity in the middle. You can have XY chromosomes but appear female. Or XX chromosomes but have male physical characteristics.

Basically, what I’m saying is that while the sex chromosomes are very important parts, human development is a complex web of genetics, hormones, and lots of different biological factors that biologists are still discovering today.)

And remember, sex and gender are fundamentally different. Even though we use the same words (male and female), gender is more about perception and society, not about cells.

I’m Dr Alex Lathbridge and this is Bitesize Biology.

Catch up with the rest of the series on BBC Sounds.

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is episode three of a seven-part series on inheritance, variation and evolution. In this episode we’re going to talk about variation: why there are differences between individuals in one species.

I’ll be going through two causes of variation, genetic and environmental, and a combination of both.

A species is a group of organisms that’s able to interbreed and produce fertile offspring.

My dog Ollie is a mix of lots of different breeds of dogs. His father was a working golden retriever and his mother was a cross between a Labrador and an English Springer Spaniel. So that means he’s technically a Golden Springador, which is really funny because his fur is black. Dogs – no matter how different – are all examples of one species, which is why they can be crossed to make amazing individuals like Ollie.

Ok more about my perfect baby boy in a bit but let’s talk about something far less interesting: humans.

Humans are all the same species, but like dogs, individuals within the same species still have lots of variations. Humans look similar to each other, but we’re not identical.

Some variation is caused by genetics, what you inherit from your biological parents, and some variation is caused by the environment, how an individual grows up and interacts with its surroundings. And some variation is caused by a combination of both genetics and the environment.

Let’s first have a look at genetic variation. Grab a pen and write this down.

It’s the combination of genes from the two biological parents that gives us genetic variation. We talked about this in the first episode of the series, and it’s really important so a quick recap:

Each of the human sex cells, or gametes, contain 23 chromosomes – this is half of the full number of chromosomes in a normal cell, which is 46. So sex cells are known as haploid.

The sperm and egg, the sex cells, fuse together during fertilisation to form offspring. And the parents’ genes are combined in that new cell, which has the full 46 chromosomes, and is known as a zygote, and it's a diploid cell, meaning it has all 46 chromosome, 23 pairs.

So, which characteristics are caused by our inherited genes and lead to genetic variation?Things like eye colour, hair colour, blood groups, skin colour, even if your ears are lobed or not.And as I mentioned in the last episode, your biological sex, whether you’re physically female or male, is genetically determined.

Let’s take a look at variation caused by environmental factors, the type of environment an individual lives in, which can lead to differences, or variation, within a species.

For instance, I have scar on my little finger from when I was six years old because I wanted to test the hypothesis that if an electric pencil sharpener could make a pencil sharp – could my finger also be made into the ultimate weapon? Suffice it to say, my first scientific experiment was terminated very quickly.

Characteristics of animals and plants can be affected by lots of different environmental factors, whether it’s hot or cold, the amount of food they have, life circumstances.Plants are really affected by environmental factors. Every so often, I lie to myself and say that I can take care of one of those basil plants that you can buy at the supermarket. But every time, a week later after I’ve brought it home, I’ve either watered it too much or not enough and its leaves have turned yellow or maybe it’s not had enough sunlight (because the UK doesn’t get a lot of that in winter) and then it wilts, and I feel bad.

In humans, variation might be caused by environmental things like your diet, or an unfortunate injury might lead to scars or missing teeth, or your accent can be determined by where you live.
This has nothing to do with genetic factors – it’s all about environment. These factors won’t be inherited, your children won’t have a scar just because you do. So variation is caused by genetic factors and environmental factors.

But, the majority of variation in animals and plants is really caused by a combination of the two.For example, your mum might be really tall, and you inherit her genes for height. But in this hypothetical example, you might also have had a poor diet growing up, meaning you don’t grow very well. So even though genetically, you might be predisposed to being tall, the environmental factor of not having a proper diet also affects your height.

Likewise, if Ollie was compared to any of his brothers or sisters from the same litter, chances are that he might be taller than some, able to jump higher than a few, or have a better sense of smell.All because they’ve all grown up in different environments, despite having incredibly similar genetics.

Before I go to take Ollie to the park, I want to tell you about when genes change – also known as mutations.

You might hear the word mutation and it sounds bad, right? It’s really not, mutations are what make the world so different.

Mutations are random, spontaneous changes in the structure of a gene or chromosome. They lead to genetic variation within a species. But not all mutations will affect the characteristics that an individual has.

Because sometimes only a tiny part of a gene is altered, and so the characteristic that the gene represents doesn’t change. In this instance, a mutation has changed the genotype (the genes), but the phenotype (the characteristic) has not been affected by the mutation. Genotype = genes. Phenotype = physical characteristic.

In other situations, the mutation of a gene might have a small effect on the phenotype, for example an eye colour might be slightly more brown, so the both the genotype and phenotype are altered by the mutation.

In very rare cases, a genetic mutation will have a big effect on the phenotype. Some genetic health conditions like sickle cell are caused by a genetic mutation that affects how red blood cells develop. So if you have sickle cell disease, your normally round red blood cells are shaped differently. Meaning that they can get stuck in blood vessels, which causes a lot of pain and health issues.

I’m Dr Alex Lathbridge and this is Bitesize Biology.

All episodes are available on BBC Sounds.

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the fourth episode in a seven-part series on inheritance, variation and evolution.

In this episode we’re going to be talking about evolution. We’re going to look at what evolution is, and a process called natural selection, the main accepted theory of how evolution works.

We spoke about genetic variation in the last episode, Episode 3. And if you need a refresher, you’ve got time to listen to it again.

I’m going to keep it really simple to begin with. What is evolution?

Evolution is a change in the heritable characteristics of a population over time, from one generation to the next. That’s it.

Right, but how does evolution work? Because millions of years ago, life on earth was limited to single celled organisms that survived in water.

So how did we go from that, to now, with humans, dogs, whales, sunflowers, pigeons, lettuce, pandas, naked mole rats, and every other living thing under the sun?

What drove that change? Was being a single celled organism that bad?

Well, let’s get into it.

Charles Darwin was a scientist who spent a lot of time travelling around the world in the 1800s, studying fossils and animals.

Exploring lots of different places by ship, he noted that there were different characteristics that helped organisms to survive, depending on their environment.

Collecting lots of evidence during his travels, he came up with the idea that normal variation in certain heritable traits meant that populations can adapt and survive in different environments and so a species gradually changes.

And given enough time, these small changes can add up in a population, to the extent that a new species altogether can evolve altogether.

Or more simply, he came to the conclusion that evolution is driven by the natural process of gradual change over time and that is Darwin’s theory of natural selection.

You need to understand the four stages of Darwin’s theory of natural selection, so grab a pen and write this down:

1. Variation. Individuals within one species show lots of variation. This is caused by differences in genes.

2. Environment. Because of this large amount of variation, some individuals will have a useful characteristic, or trait, that makes them better adapted to their environment, and this gives them an advantage.

3. Survival. Individuals who possess this useful, advantageous trait are more likely to survive.

4. Offspring. The individuals that are more likely to survive, are more likely to successfully produce offspring.

These offspring will inherit the genes from their parents and so will also possess the advantageous, useful trait.

So that’s variation, adaptation to the environment, survival, offspring.

So, over a very long period of time, the useful characteristic can become very common within the population, and so a new species may evolve.

And remember there are also individuals who are poorly adapted to their environment, and are less likely to survive and reproduce to create offspring. And their genes are less likely to be passed on to the next generation.

And that’s how natural selection works. Let’s see what it looks like in the real world.

Imagine a herd of plant-eating animals living somewhere that they can thrive, our environment is a land with lots of bushes, shrubs, trees, and whatnot.

Now, some of those individuals might be a taller than others, thanks to differences in their genes. That’s variation.

Taller individuals can eat from the trees that the shorter individuals can’t reach (that’s an adaptational advantage) they’re more likely to survive and breed.

This means that their offspring are likely to inherit that trait and also be tall, because this is the trait that increased their parents’ chances of survival in the environment. And that’ll happen again and again and again.

So, what does that process look like over millions of years?

Yes, that’s right. The evolutionary journey resulting in the tallest land animal on the earth, the giraffe, through the natural selection of genes producing the long neck it needs to survive in the savannah.

But how do all of these random, useful, advantageous characteristics suddenly appear in individuals that make them better suited to their environment? How did giraffes go from ancestors with short necks to end up with the long things that they have now?

It’s all down to mutations.

Mutations are randomly and constantly occurring within a population, some good, some bad and many are just neutral.

Animals aren’t choosing to have mutations, in the same way that we’re not choosing to have mutations. It’s just down to luck.

Mutations cause changes in genes, the genotype, and this can lead to changes in characteristics, the phenotype.

So, in natural selection, a mutation that is beneficial to individuals can make them more likely to survive in an environment.

Therefore, more likely to reproduce, and that selects the useful gene naturally to be passed on to future generations of offspring.

So, one specific butterfly might have a random mutation that enables it to camouflage really well in its environment, meaning it is less likely to be eaten by other animals.

The lucky butterfly has a better chance of surviving due to this mutation, it then goes on to successfully breed and the offspring will inherit gene that camouflages them.

Over time, the gene for the useful characteristic will become very common within the population, because of survival.

The genes are doing the hard work here. So, you shouldn’t write down in an exam “the advantageous trait” gets passed on.

The genes that produce that trait are passed on from parent to offspring.

I’m Dr Alex Lathbridge and this is Bitesize Biology. You can listen to the rest of the episodes on BBC Sounds.

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the fifth episode of a seven-part series on Inheritance, Variation and Evolution.

In this episode, we’re going to talk about what evidence exists to support the theory of evolution.

That word theory causes a lot of issues for some people because they get confused by the meaning.

In common usage, the word theory is said in the same way you would say “idea”, but to a scientist, something that’s a theory has evidence.

With that in mind, we’re going to cover three bits of evidence today: fossils, bacteria and extinction.

Because scientists are obsessed with fossils, we’re going to start there, and they’re really good for your exams.

What are fossils? Fossils are the preserved remains of dead organisms that lived millions of years ago. They’re generally found in rocks. Because they are the remains of animals that lived long ago, they give us a snapshot in time, they can show us how populations have changed over the ages, giving evidence for the theory of evolution.

If fossils are found in rocks, how can you tell the difference between a fossil and a rock? This won’t be on your exam but a mate of mine who’s a palaeontologist – a fossil scientist – told me that, because bones that have been fossilised are dryer than the rock around it, you can sometimes use your tongue. Because some fossil bones are filled with lots of pores, so if you lick it, they suck up the water and it sticks to your tongue like really tiny Velcro.

Anyway, back to passing your exams so grab a pen.

Fossils can be preserved in three main ways:

1. Hard body parts. these are things that don’t decay easily like skeletons, bones and teeth.

2. No decay. This happens in environments like amber, tar pits, ice glaciers and peat bogs. The microbes that decay organisms can’t survive there, so decay can’t happen.

3. Buried traces. This usually occurs when an organism is buried in a soft material like clay, like casts, burrows, footprints and impressions.

So how do fossils provide evidence for the theory of evolution?

Fossils are like snapshots; they are biological photographs over millions of years that demonstrate organisms gradually getting more and more complex over time. Fossils of the simplest organisms are found in the oldest rocks from billions of years ago, and in the rocks found more recently, more complex organisms can be found.

This supports evolution where simple life forms gradually change into more complex ones over time.

Scientists can study fossils and learn how much organisms have changed since life began on Earth.

This information is known as the fossil record.

But there are gaps in the fossil record because the very earliest forms of life that existed were soft-bodied, and so they left very little fossil evidence. This is why scientists cannot be absolutely certain about how life began. The fossil record is incomplete, there isn’t a fossil of every animal and plant that ever existed.

Let’s go from fossils to antibiotics.

So antibiotics are things used to fight infections caused by certain species of harmful bacteria (and not viruses).

But over time, bacteria become resistant to these antibiotics, which is something doctors and vets need to be very aware of, but it’s also evidence for evolution.

How?

Bacteria reproduce at a very fast rate and random mutations in the genes produce new strains of bacteria that become resistant to certain antibiotics.

Bacterial cells with mutations that make them resistant to antibiotic medicine, mean that antibiotics no longer work to destroy the bacteria. So these resistant bacteria are therefore more likely to survive and reproduce, increasing the population size of the antibiotic-resistant strain of bacteria.

This is an example of natural selection, where a mutation has caused an organism to have a beneficial trait, and the genes for that trait are inherited by the subsequent generations of offspring.

And because bacteria are so rapid at reproducing, and generations can happen again and again and again really, really quickly, it means that they evolve new resistant strains rather rapidly – and yes, this is bad news for humans and animals who don’t want to be ill.

An example of a bacteria that has become resistant to antibiotics is MRSA. It's really, really hard to get rid of, as its resistant to multiple types of antibiotics.

How do you reduce the rate of antibiotic-resistant strains developing?

There are three ways:

1. Antibiotics shouldn’t be prescribed inappropriately, such as treating non-serious infections or those caused by viruses.

2. Antibiotics should have restricted use in agriculture, like farmers not treating lots of animals before they get sick.

3. People should always finish all of the antibiotics that they’re given, otherwise some bacteria might still survive and form resistant strains.

This is all really important, because scientists are constantly trying to develop more and more effective antibiotics.

This is evidence for evolution as it demonstrates the process of natural selection.

So we’ve got fossil records in the past and bacteria today. So what’s the final piece of evidence for evolution?

Extinction

So what’s extinction? Extinction is when there are no remaining individuals of a species alive.

Organisms that don’t have useful traits or are poorly adapted to their environment, and therefore less likely to survive, reproduce and so on and so forth, may become extinct.

There are five things that you need to remember that might lead to extinction:

1. New diseases.

2. New predators.

3. New competitors.

4. Changes to the environment, such as climate change.

5. A single catastrophic event, like a volcanic eruption or an asteroid collision.

How do we know this? The fossil record contains evidence of lots of species that have sadly gone extinct, things like the dodo, dinosaurs and big woolly mammoths. Its only really because of the fossils that we know that they ever existed but have now gone extinct. This is evidence of evolution.

I’m Dr Alex Lathbridge and this is Bitesize Biology – subscribe on BBC Sounds

Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the sixth episode of a seven-part series on Inheritance, Variation and Evolution.

In this episode we’re going to talk about two methods where humans deliberately change animals and plants to give them desired traits: selective breeding and genetic engineering.

In episode four of this series we talked about natural selection, where genes for beneficial traits become more common within a population. It's important that you understand natural selection, before jumping into this, because today we’re going to be talking about another kind of selection.

And to do so, we’re going to talk about my favourite animals: dogs.

Humans first co-existed with wolves tens of thousands of years ago and the eventual domestication led to what we know today as dogs.

In the wild, animals like wolves would adapt to the environments that they’re in. For instance, the Arctic Wolf has long, white fur because it lives in the frozen reaches of the arctic circle. Whereas, the Arabian Wolf is smaller and adapted to living in the changing weather found in the desert.

These are traits that they have due to natural selection. That’s just a couple of different of wolves, but dogs have over 400 different breeds and that seems like a lot more variation. Why?

Because humans bred different dogs for different uses. And so selected different desired traits.

What does that mean?

Well, want to hunt an animal? You pick the dogs with the best sight and speed and breed them together.

Need a guard? Breed the ones that are the most protective.

Need some dogs to herd your sheep? You breed the most trainable ones.

Want a companion? Breed the dogs that show the least aggression and so on and so forth, all across the world, eventually resulting in different breeds.

This is an example of selective breeding, where humans deliberately animals (or plants) for selected characteristics that we decide are useful or desired.

As you can guess, selective breeding happens over generations. It takes a while to successfully select for certain traits.

Apart from dogs, examples of selective breeding are:

Farmers breeding cows for lots of meat, or large volumes of milk.

Or farmers breeding plants to create crops that are resistant to disease or that grow large flowers.

Okay back to dogs now.

So imagine you’re responsible for breeding your ideal dog.

There are four stages of selective breeding you’ll need to remember so grab a pen and write this down:

1. What are you breeding for? You’ve got to decide which characteristic is important and what you want to select for future generations.

2. Find the parents in the stock that best show this characteristic. You then breed them together.

3. From the offspring of these two parents, you then find the best offspring with the desired characteristics to be the parents of the next generation.

4. Repeat the process again and again continuously over many generations, until all the offspring show the desired characteristics.

Basically, you keep breeding the best parents to get the best offspring with the useful characteristic.

It’s like natural selection, but it’s not happening naturally in nature. Its artificial selection.

We’ve been doing this stuff for thousands of years with plants and animals, so why? What are the benefits?

Advantage one: new varieties. Selective breeding can lead to new varieties of crops or animals, that produce more or better-quality food. Corn, for instance, was selectively bred over thousands of years from an ancestor that was ten times smaller than it is today and wasn’t yellow.

Advantage two: gentler animals. Selective breeding can create animals that do not cause harm, for example cattle without horns.

If you’re thinking that this sounds a little bit dodgy, you’re right. Because plants and animals aren’t choosing this, humans are the ones artificially selecting desired and useful traits, it has to be done correctly.

Like a wise person once said, “with great power there must also come, great responsibility.”

This is because when desired traits are deliberately and selectively bred over time, future generations of the animals and plants will share lots of the same genes, reducing the genetic variation.

Unfortunately, some dog breeds are perfect examples of reduced genetic variation.

Because certain dogs are desired for looking a specific way, rather than having a specific purpose.

There are irresponsible breeders who will use one dog to father many litters of puppies.

When dogs from these litters come to be mated, some will be paired with dogs that share the same father from other litters. Over generations, more and more dogs across a particular breed are related to one another and the chances of relatives mating increase.

Why is this bad? Because the animals will be very closely related genetically, and this is known as inbreeding.

Why is inbreeding bad?

Risk one: disease. Reduced genetic variation can lead to animals or crops being more susceptible to a new disease. If one of them is killed by the new disease, it’s likely that many of them will be affected.

For instance, Cavalier King Charles Spaniels have a higher risk of heart disease. Bulldogs risk painful bacterial infections if their face folds aren’t cleaned regularly.

Risk two: health problems. Reduced variation and inbreeding unfortunately means there’s more chance of plants or animals inheriting harmful genetic defects.

A high percentage of Dalmation dogs are deaf in one or both ears. And do not get me started on the ethics of intentionally breeding dogs that can’t physically give birth naturally because their bodies aren’t large enough to push out their puppies' large heads.

But it’s the 21st century, surely there’s got to be another way that humans can change an organism to give it desired traits?

Yes - genetic engineering.

This involves modifying the genes of an organism by transferring a gene from one organism to another, resulting in it having a desired characteristic.

Genetic engineering has four stages:

1. Select the desired characteristic.

2. The useful gene is removed from one organism using enzymes and is inserted into a vector.

3. The vector is a small circle of DNA made of plasmids (if you don’t know what plasmids are listen back to our Cell Structure episode if you need a refresher)

That small circle of DNA made of plasmids is transferred to the organism that you want to develop the characteristic.

4. You then replicate the modified organism with the gene for the desired characteristic.

Let’s think about some real-world examples where genetic engineering is used:

1. Diabetes. Diabetes is a health condition where the body doesn’t regulate its blood sugar levels, so glucose in the blood is too low or too high. Insulin is a hormone that regulates blood sugar. Genetic engineering can be used to genetically modify bacterial cells that make human insulin, which is then taken by people with diabetes.

2. Crops. Genetically engineered crops have been modified to be resistant to diseases or insects, or to make bigger fruits.

Like selective breeding there are advantages and disadvantages to genetic engineering:

Advantage one – improving yields. Yield means the amount of something made. Genetic Engineering can improve crop yields or crop quality, which can reduce hunger in developing nations.

Advantage two – improving nutrients. Crops can be genetically engineered to contain a specific nutrient.

An example of this is a type of rice called “Golden Rice” which is genetically modified to include beta-carotene. A deficiency in beta-carotene can cause blindness so beta-carotene is pretty important.

But there are risks to genetic engineering:

Risk one – causing harm to humans. Some people think genetically engineered crops aren’t ethical or safe for humans. Because we don’t know the long-term effects of genetically modified food. For example, humans could develop allergies to them.

Risk two – transfer to the rest of nature. What benefits one plant, may harm other plants or animals. For example, pollen taken from genetically modified plants could harm insects that carry it between plants.

I’m Dr Alex Lathbridge and this is Bitesize Biology – subscribe on BBC Sounds

Hello. I’m Dr Alex Lathbridge and this is the Bitesize Biology.

This is the last episode in our series on Inheritance, Variation and Evolution.

In this episode we’re going to talk about classification. We’re going to take a look at three different types of classification: the Linnean System, the three-domain system and the binomial system.

Look I know, it sounds dry. But these are fundamental to how we view the world and also they give a little insight into how much things have changed over the last few centuries.

Before the invention of flat pack furniture, one of the biggest things to come out of Sweden was the Linnean System.

One of the earliest classification systems of organisms, it was proposed was by a man called Carl Linnaeus in the 1700s. The Linnean System classified organisms into different groups depending on their structure and characteristics. Basically, the biological equivalent of “you lot look kind of similar – are you related?”

Classification like this allows you to group organisms together that share similar characteristics. The largest groups are very general, and then classification of different organisms allows you to divide them down into smaller and more specific groups.

Ok, grab a pen, write this down.

The Linnean classifies organisms into seven smaller and smaller groups like this:

Kingdom, Phylum, Class, Order, Family, Genus and finally Species.

I’ve got an easy phrase to remember this order: Kings Play Chess On Fancy Gold Squares, where the first letter matches with the different groups: Kingdom, Phylum, Class, Order, Family, Genus, Species. Kings Play Chess On Fancy Gold Squares.

Species is the final classification stage of the Linnean System, where organisms are grouped into their specific species so they are really similar.

A species is a group of organisms capable of interbreeding and producing fertile offspring. So all dogs, despite their different breeds, size difference and everything, can interbreed to have fertile offspring, because they’re all one species.

But on the other hand, zebras are one species, and horses are a sperate species, but if you do breed them together you get a hybrid, and one name for it is a “zorse.” And yes, luckily, its infertile.

Remember: Kingdom, Phylum, Class, Order, Family, Genus, Species.

Kings Play Chess On Fancy Gold Squares, that’s the Linnean system.

Next up is The Binomial System, this is using two Latin names to classify species.

And if you’re thinking “oh no, do I have to learn Latin now?” don’t worry, all you need to know is that this system uses Latin words.

And it's in two parts, the first word is the genus and the second word is the species. So it uses the last two stages of the Linnean system.

So you might have heard the name homo sapiens for humans, this is an example of the Binomial system.

The first word ‘homo’ is the genus, and the second word ‘sapiens’ is the species.

Or Felix catus for cats. Felix is the genus, catus is the species.

The binomial system is helpful because it allows scientists to name and identify individual species.

Remember, the Linnean system was invented in the 18th century. This was way before we understood genetics like we do now, so for a very long time it was like:

“Has it got wings and feathers? Cool – it’s a bird. That thing – does it have scales? Oh it does? OK that’s a reptile.”

Sounds good, yeah? Well, no, it's not. It’s not a joke when you’re sitting in a university lecture, having to come to terms with the fact that a red panda is called a red panda, because apparently, if you looked at the face markings, it looks sort of like a panda (you know, the regular, big black and white ones). Imagine the surprise when we actually progressed further than sticking to a concept developed before the invention of matches, the bicycle, and chicken nuggets, and found out that genetically, they’re not really closely related to pandas. Or bears. They’re closer to skunks. Mind blown.

Thankfully, advances in science through time have led to the development of microscopes. This enables scientists to look at organisms in much more detail, investigating their internal structures and cells.

With all these scientific advances going on, new classification systems have been proposed, which leads us to the final system you need to know about: the Three Domain system.

The Three Domain System was developed by Carl Woese in 1990. This was based on scientific evidence from chemical analysis, so looking at the chemical reactions that happen inside organisms.

From his research, he found that some species that were traditionally thought to be closely related, were actually quite different and shouldn’t be grouped together.

In this Three Domain system organisms are divided into (you guessed it) three domains:

1. Archaea. These are primitive organisms that live in extreme environments, for example one is known as Pyrococcus furiosus, and it lives in extreme environments inside a volcano.

2. Bacteria. So these are things like E. coli, the prokaryotes.

3. Eukaryota. Remember Eukaryotic cells from our first series on The Cell? Everything else pretty much falls into this category, so fungi, plants, animals and protists.

These are then categorised into the usual smaller groups we are now familiar with: Kingdom, Phylum, Class, Order, Family, Genus, Species.

And with the study of DNA sequences, scientists can see how closely related organisms are genetically.

Evolutionary trees are tools that scientists use. It’s a bit like family trees but instead of grandparents and aunts and uncles, it shows how different species are related, and what common ancestors they share from millions of years ago.

If you think of a tree shape, at the very end of each branch there is a species. Species on branches that are close together indicate that they are closely related. And where the branches meet, there will be a species that is related to both of them: a common ancestor.

Species that are separated by lots of branches mean they are not closely related, even if at first glance you think they might be, like dolphins and sharks, because dolphins are mammals and sharks are fish. Or another example, red pandas and pandas.

I’m Dr Alex Lathbridge and this is the Bitesize Biology, to hear more search Bitesize Biology on BBC Sounds.