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Laboratory Investigation VII: Allele Frequencies

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Allele Frequencies Introduction 0:05
    • Purpose
    • Materials
    • Time
  • Part I 2:12
  • Part II 7:05
  • Analysis 7:51
  • Evolution Connection 10:45
    • Meant to Stimulate How a Population's Allele Frequencies Change Over Time
    • Particular Phenotypes Selected
    • Recessive Allele Keeps Dropping

Transcription: Laboratory Investigation VII: Allele Frequencies

Hi, welcome back to, this is laboratory investigation 7, allele frequencies.0000

Here is the introduction, the purpose is to investigate 0007

how allele frequencies can change in the population during natural selection, over several generations.0010

This is spurring the population towards the point where it is not only allele frequencies would be changing but potentially,0016

over many generations, thousand of generations, eventually,0023

the population will be changed to a point where it becomes a different species, compared to what it once was.0027

That is what natural selection can do along with mutations and other factors.0032

We are going to look at rabbits for color, we have to focus on one particular gene to make this manageable,0037

in terms of natural selection, selecting organs forms of that gene.0042

Let us say that AA, homozygous dominant and heterozygous, codes for grey fur in this rabbit population.0047

With homozygous recessive, it is going to make albino or white rabbits like this little guy here.0055

We are going to say or assume that the white rabbits are being selected against.0062

This is in an arctic area, this is in a smoggy environment with a high latitude and or altitude.0072

We are going to say it is in a temperate deciduous forest, it is not snowing.0079

If you are a white rabbit, you are born with this homozygous recessive genotype.0084

You really stand out to predators, you are going to be picked off quickly.0088

These are being selected for, gradually.0092

To get this done, to represent this lab accurately, you need two color beans.0094

One that is darker than the other and each of the beans is about the same size.0100

You are going to be selecting them randomly out of the cup.0103

You want to be able to see or feel the difference between them.0106

As long as they are relative to the same size, you can use pinto beans and white beans, black beans, navy beans, whatever works.0110

You will see that the dark colored beans represent dominant alleles, the light colored beans represent the recessive.0120

You need a cup and a calculator, to do some division which you could do in your head conceivably.0125

Time required, about 45 minutes.0131

Part 1, assume that the population starts out with 50 rabbits or 100 beans.0133

We are going to assume that there is an equal frequency of the dominant and recessive.0141

The frequency of the dominant allele is 0.5 or 50% for dominant A.0148

It is also 0.5 or 50% for recessive and that equals 1, also known as 100% of the alleles.0156

This is going to change, you are going to see one of them increasing, one of them dropping because of natural selection and0167

affecting which ones are living long enough to pass on their alleles or their genes.0172

Starting out with 50 rabbits which each of them have two alleles representing its fur color, that is 100 beans.0178

We put them all the cup, we will start randomly drawing out two beans at a time from the cup.0184

On a table, you want to separate where the grey ones are and where the white ones are.0189

Here is our separation on the table.0197

Let us say that, we pull out that bean and that bean, heterozygous.0198

Here is homozygous dominant, homozygous dominant.0204

Look, albino bunny.0209

You could see how you would gradually just separate them until all 100 are picked.0218

Here you have got all the gray ones, here you have got all the white ones.0223

When you get to this point where all of the beans are selected, you are going to kill all the white rabbits off.0227

Assume that all of them have died prematurely, to the point where they are not going to be able to pass on their allele to the next generation.0233

The eagles, the hawks, whatever predator, they saw them easily in the terrain0239

and serve this as a daily lunch for those predators, these are gone. 0244

Let us say for just numbers sake, that we end up having 12 beans here which is 6 rabbits that are now gone.0249

Which would leave how many over here, 88.0263

Over here, we have 88 beans which is the same thing as 44 rabbits.0267

Not all of the gray rabbits survived, some of them get picked off by eagles or hawks as well.0277

We are going to kill off 25% of the gray rabbits, we want to do it randomly.0286

You do not want to purposely try to kill off ones that have heterozygous condition or the ones that is a homozygous dominant.0289

Because, the eagles or hawks that was eating them, they do not see their genotype, they just see the pup.0295

The gray ones that are heterozygous and the gray ones that are homozygous dominant look exactly the same.0302

You want to take 25% of them out randomly which means that 11 rabbits die,0307

which means you have got 33 rabbits left or 66 beans.0315

This is 66 beans, let us assume that from that 66 beans, if we count up the remaining alleles,0323

these 33 rabbits or 66 beans, they are still there.0333

In living beings, these alleles can be passed on, they can mate and make babies.0338

We are going to count up the allele frequencies that are remaining.0344

We are going to use that to start the next generation.0346

Of those 66, let us say that 43 are dominant, the dark beans, and then 23 are white beans.0350

If you calculate the percentage, 43 divided by 66 that equals 65%.0364

23 divided by 66 is about 35%.0374

Notice, it is crowded but on the next page, we are assuming that we got 65 and 35 for the next generation.0382

You are going to go back to your bean piles away from the table, where you got your beans from.0391

Now, you are going to start out with 65 black beans in the cup, dark colored beans, and 35 light colored beans in the cup.0396

You will do it all over again.0403

You have already changed in one generation, the allele frequencies by 15% both ways.0405

Over time, you can presume that if those white rabbits keep getting eliminated and0412

cannot pass on those recessive alleles, you will gradually have some changes in those allele frequencies.0417

You are going to use the percentage from generation 1 to put a new number of dark and light beans in the cup.0427

If you had 65% dark beans and 35% light beans, you use those numbers to start over from generation 2, 0432

redo the procedure of randomly selecting the rabbits, separating the two areas like before,0438

kill off all the white ones, kill 25% of the gray ones randomly.0444

Recalculate the percentage again, next time around it could be 71 and 29%.0448

It could be 75 and 25, whatever it might be, based on your calculations.0456

Keep track of your percentages for the dominant and recessive alleles.0462

Do the procedure until you get to five generations.0467

How realistic is this lab activity?0473

It is a great representation for how alleles are passed on from generation to generation.0475

If you start with all heterozygous rabbits, the active picking out two at a time simulates random mating.0479

Rabbits are not as selective about who they mate with, as some other mammals.0487

With a rabbit population where there is a lot of reproduction and a very quick turnover of generations, 0494

with random mating you can simulate that with this bean activity pretty accurately.0502

In the event where it takes place, the white rabbits clearly stand out in the terrain.0509

It is easy for predators to spot, they are not making reproductive maturities.0513

The act of us eliminating all the white ones is pretty accurate.0516

It is possible in real life that some white rabbits in a forest that is very green and brown, that it could survive to make babies.0521

Probably not many of them, they really stand out like a sore thumb.0532

The gray ones that we eliminated 25% of them, it is arbitrary that somebody who invented this lab came up with a 25%.0537

I did not make up this lab, it is given to me by teachers and that is the duty of biology labs, 0545

that they are passed on from teacher to teacher all of the time.0551

I do make some modifications, as I see of it.0554

I think it is a good lab, in terms of accurately trying to show students how allele frequencies can change over time.0557

Through this scenario, the percent of the dominant allele will increase, then the percentage of the recessive allele will decrease.0564

If you were to graph it, graph your data, and I have students do this.0570

On the X axis is the generations or over time here is 0 when you start out with 0.5, 0.5, and 1, 2, 3, 4, 5.0576

From here, from 0 to 1.0, this is your allele frequency.0590

Here is a 0.5, that is where both of them start out, we start out at 0.5.0602

Let us say we use red for the dominant allele.0610

That could be what happens with the dominant.0617

With the recessive, it would tend to be a mere image.0619

That is not perfect but I think you get the point that, for every increment that the dominant allele increases,0626

the recessive allele would decrease the same relative amount.0633

Typically, the recessive allele would not be completely eliminated.0639

I will talk about that on the next page.0643

This lab is meant to simulate how populations of allele frequencies change over time.0648

This population is not in equilibrium.0653

If you remember from previous lessons in this course, Hardy-Weinberg equilibrium is a hypothetical scenario where population is not evolving at all.0656

There is no natural selection, there are no mutations.0665

It is a large population, there is no integration and immigration, etc, random mating.0668

In this case, the title of the lab had to do with allele frequencies relative to natural selection.0674

Since it is not Hardy-Weinberg equilibrium, you are going to get changes in allele frequencies and steps towards evolution.0681

It is in the process of evolving.0688

Particular phenotype are selected for or selected against, based on the environmental conditions, predators, etc.0690

General facts about nature, in terms of like how populations struggle in the wild, 0697

and even our population and civilization.0704

With respect to most genes, there are pressures, there are differences in fitness levels,0708

in terms of passing on those allele to offspring.0715

In reality, a Hardy-Weinberg equilibrium is hard to capture, something that is going on with a lot of genes with a lot of species.0718

It is a hypothetical thing for comparison sake.0727

Generally, there are characteristics that are better to have or not as good to have.0731

The recessive allele will keep dropping, in terms of percentage.0738

It probably will not completely disappear.0741

Why not, even with those white rabbits, those homozygous recessive individuals being picked off and0744

killed by predators prematurely, before being able to pass on their alleles.0753

You still get heterozygous individuals being born.0758

When you have heterozygous individuals being born and they mate, 0763

there is always the chance that they are going to keep having white rabbits.0769

These individuals have 25% chance of having a white offspring.0775

To get rid of that recessive allele completely is tough.0782

When we look at human disorders, those are typically being selected against.0786

Specially if the disorder cuts off someone's life before they make it to adulthood and before they can have children.0790

You would think that, it did not get passed on, why does it keep happening?0799

There are carriers out there, you saw individuals who are heterozygous and0802

have the chances of mating with someone else who is also heterozygous, and passing on the recessive alleles to individuals.0807

For it to be completely eliminated, these are unlikely, not impossible but unlikely.0816

Here is a classic punnet square with two purple flowers, taking pollen from this one and0822

putting it on the pistil or carpal of this one, the female part of the flower.0829

The chance of having the two recessive alleles coming together is 25%.0834

This is kind of the flower example equivalent, relative to the rabbit example from this lab.0841

Thank you for watching