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Lecture Comments (6)

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Post by David Saver on August 13, 2014

The farthest we have ever drilled down to layers which are supposed to be billions of years old we consistently find oxidized rock.

There is actually no evidence whatsoever that indicates that the early atmosphere had little or no oxygen.

If anything, we find that it may have had even more oxygen than in the present day.

1 answer

Last reply by: Shannen Brown
Thu Nov 7, 2013 9:08 PM

Post by Muna Lakhani on May 11, 2013

In your example, isn't q^2 supposed to be 0.09 to give a 9% of homozygous recessive allele?

1 answer

Last reply by: Dr Carleen Eaton
Thu May 3, 2012 6:25 PM

Post by Tara Ray on May 2, 2012

i need help

Population Genetic and Evolution

  • Evolution is a change in the genetic composition of a population over time. Natural selection is one mechanism for evolution.
  • Genetic drift is the change in allele frequency due random chance and usually occurs in small populations. The founder effect and the bottleneck effect are examples of genetic drift.
  • The movement of alleles into or out of a population is called gene flow and can result in a change in the frequency of alleles.
  • The Hardy-Weinberg Equation, p2 + 2pq + q2 = 1, can be used to predict the frequencies of alleles and genotypes in populations that meet the following conditions:
    1. Mating is random
    2. No mutations occur
    3. No natural selection is occurring.
    4. The population is large.
    5. There is no immigration or emigration.

Population Genetic and Evolution


  • Intro 0:00
  • Review of Natural Selection 0:12
    • Review of Natural Selection
  • Genetic Drift and Gene Flow 4:40
    • Definition of Genetic Drift
    • Example of Genetic Drift: Cholera Epidemic
    • Genetic Drift: Founder Effect
    • Genetic Drift: Bottleneck Effect
    • Gene Flow
  • Quantifying Genetic Variation 14:32
    • Average Heterozygosity
    • Nucleotide Variation
  • Maintaining Genetic Variation 18:12
    • Heterozygote Advantage
    • Example of Heterozygote Advantage: Sickle Cell Anemia
    • Diploidy
    • Geographic Variation
    • Frequency Dependent Selection and Outbreeding
    • Neutral Traits
  • The Hardy-Weinberg Equilibrium 31:11
    • The Hardy-Weinberg Equilibrium
    • The Hardy-Weinberg Conditions
    • The Hardy-Weinberg Equation
    • The Hardy-Weinberg Example
  • Example 1: Match Terms to Descriptions 42:28
  • Example 2: The Hardy-Weinberg Equilibrium 44:31
  • Example 3: The Hardy-Weinberg Equilibrium 49:10
  • Example 4: Maintaining Genetic Variation 51:30

Transcription: Population Genetic and Evolution

Welcome to

We are going to continue our discussion of evolution by talking about population genetics and mechanisms of evolution in addition to natural selection.0002

We are going to start out with a very important mechanism of evolution, and that is natural selection.0014

That was covered in depth in a separate lecture, but we are going to go ahead and review it here.0019

Recall that evolution is a change in the genetic composition or the genetic makeup of a population over time.0024

Natural selection is a mechanism for evolution that allows populations to become better adapted to their environments.0036

According to natural selection, individuals who possess traits that are favorable to survival will have a higher probability of living, producing offspring.0045

And, therefore, the frequency of those traits will increase in the population over many generations.0056

Recall that Darwin made some observations during his studies of animals especially on other organisms in the Galapagos Islands.0065

He observed that individuals within a population vary in the traits that they possess.0075

Second: that species produce more offspring, then, can be supported by the environment.0097

And the offspring must compete for survival; so I will just put "competition for survival".0102

But the underlying idea is that there are limited resources and that populations may produce more individuals than can be supported by the environment.0112

And finally, offspring inherit traits from their parents.0120

And if you look at this photograph here, it shows a butterfly whose wings look pretty similar to wood.0132

So, you could think about how natural selection may have resulted in this trait.0138

If there is an ancestral butterfly, and some of those individuals within a population had wings that were darker, they were brownish,0142

they looked a little more like wood, those butterflies may have had a less chance of being caught by a predator.0151

Greater chance of survival, they, therefore reproduced, and their offspring would carry forward this trait of brownish wings.0158

And overtime, as butterflies were selected for that had wings that looked like wood,0167

over many, many generations, the butterfly population had an increase in frequency in that trait0175

and ended up very well adapted to the environment, able to blend in and able to survive.0183

If the environment changed, then, there would be a selection pressure in a different direction that might alter the wing color over a long period of time.0188

In order for natural selection to occur, there must be a variety of traits in a population in the first place for the selection pressure to even act upon.0198

Remember that an individual has the traits he has. If a butterfly has brown wings or green wings, that is what they have.0209

Natural selection does not occur in individuals. It occurs in populations.0218

Populations can change on average over many generations, in terms of their allele frequencies, their genetic makeup.0222

Individuals do not change in terms of their genetic makeup.0230

Looking at this from a molecular genetics perspective, what is actually happening is an increase0235

in the frequency of the DNA sequences that code for a particular trait within that population.0240

Looking at this type of butterfly versus looking at the butterfly population, 10,000 years ago,0246

the modern butterfly population may have a greater frequency of a DNA sequence that allows the wings to look this particular way.0254

What is very, very important about natural selection is it allows species to adapt to their environment. However, that is not the only mechanism of evolution.0263

Evolution is a change in the genetic composition of a population, and there are other ways in which this can occur such as genetic drift.0271

Genetic drift is a change in allele frequency due to random chance.0282

So, we see that it is a mechanism of evolution because there is a change in the allele frequency of a population, but it is not due to selection.0286

It is due to random chance. Because it is due to random chance, the result of genetic drift is not necessarily that the population becomes better adapted.0294

It could be neutral or the result could even be that the population is less fit for survival because this is just due to chance.0302

Genetic drift occurs in small populations. Just looking at an extreme case, let's say that there are 30 individuals in a population, so a very small population.0312

And among them, we have 25 with brown eyes and 5 with blue eyes, and then, let's say there is a cholera epidemic.0324

It comes along, and it wipes out many, many individuals, and in this population of 30 individuals, let's say 20 died.0342

Of those that died, 15 have brown eyes, and all 5 of the blue-eyed individuals die.0353

That leaves survivors- 10 brown-eyed individuals.0362

Now, the fact that all 5 individuals with blue eyes died, it was not due to selection against people with blue eyes.0377

It is not that the blue eyes made these individuals less fit for survival. It was random chance.0384

Because this population is so small, it just so happened that 5 of the people who died, all 5 of those people had blue eyes.0389

Due to random chance, we have now had a change in the allele frequency in the population.0398

And if these individuals stay isolated and just have mating and offspring within this population,0404

we can see that this result will be a change in the allele frequency in the population.0415

Now, because blue eyes are recessive, there could be some survivors who are carriers for that blue-eyed allele.0422

The blue-eyed allele. It might not have died out completely, but it is possible that it did die out completely.0428

And even if did not, it has decreased frequency now, so genetic drift is a change in allele frequency due to random chance.0435

It could change again. It could increase again, but for now, what we have had is the change in allele frequency.0442

One example of this, a specific type of genetic drift, is something called the founder effect.0448

This is when a small group of individuals becomes separated from the original larger population they were part of, so this is a group of founders.0460

And if the group of founders is small, it is very possible that the allelic frequencies in that little group that breaks off do not reflect that of the larger group.0468

The allele frequencies or the genetic composition of a small group that breaks off0478

and founds a new colony or something does not reflect that of the larger population.0492

Let's say that the population of 10,000 people and 50 of these individuals leave.0517

So, we have 10,000 people, and 50 leave; and they sail up somewhere and set up their own colony.0524

And let's say that within this population of 10,000 people, there is an allele for a disorder, and this allele frequency is 1 in a 1000 in the population.0535

But it just so happens that of the 50 people that leave, 2 of them carry this allele.0546

This is a greater frequency than is found in the general population just by chance.0555

So, when these individual leave, they settle. They have offspring.0563

The frequency of this disorder in their population is going to be greater than in the parent population, and there are examples of this.0567

For example, the Amish are group of individuals in the US mainly living in the Pennsylvania and North-eastern area.0574

And polydactyly, which means having an extra digit, an extra finger, an extra toe,0582

is more common among the Amish population than in the general population, and this is thought to be due to founder effect.0588

Another example: Huntington’s disease in the Afrikaner population is South Africa.0595

Huntington's disease is more common among this population possibly due to founder effect.0603

Recall that Huntington's disease is an autosomal dominant neurological disorder.0609

One type of genetic drift or one example is the founder effect, so this is genetic drift.0617

A second example is what is called the bottleneck effect. It is still genetic drift, but it is due to a slightly different cause.0628

If there is a disaster such as a fire, a flood or an earthquake, that could suddenly reduce the size of a population.0640

We actually saw that here with this cholera epidemic. That was essentially a bottleneck effect.0646

The disaster is the bottleneck, so I think of the individual as being inside a bottle at the neck of the bottle.0651

That is the bottleneck, which is an earthquake, a cholera epidemic, a flood, and many, many individuals die. They are stuck in a bottle.0659

Some, though, make it out to the other side. Those individuals are, sort of, like the founders in the founder effect.0670

Population has been greatly reduced.0677

Whoever happened to survive the disaster, those individuals as a group, may have a different allele frequency than the larger population did.0679

The group that survived is cut off from the original population, but it is not due to distance. It is not due to migration.0691

It is due to death of most of the individuals in the population.0698

And again, it might just be that the survivors have more of an allele,0704

a greater frequency of a certain allele or a smaller frequency of another allele, than the original large population.0710

The bottleneck effect can impact species of animals that are brought back from the brink of extinction.0716

This time, the bottleneck could be due to habitat destruction.0722

Let's say there is a species of rhino, and its habit is destroyed; and we are down to only 20 rhinos in the world.0725

And then, we take those rhinos. We put them in protection programs, and they mate; and they bring their numbers back up.0731

Well, with only 20 rhinos left, we are starting out with a smaller pool of genes.0739

And the bottleneck effect, therefore, could have occurred, where there is a change in the allele frequency compared with the original population.0745

And that might leave this population less able to adapt because of less genetic variation. This is genetic drift.0753

We talked about mechanisms of evolution. Natural selection is one.0762

Genetic drift is two. Another is gene flow.0765

Again, evolution is a change in the genetic composition of a population.0768

And another way to change the genetic composition would be to have individuals come into the population from the outside or leave the population.0771

When we talk about gene flow, another term for this is gene migration.0782

And what we are talking about is the movement of alleles into and out of a population.0790

For example, let's say that there is a species of plant growing along a river.0812

There is this one species of plants, and they are along a river. It has a certain allelic composition in that population.0818

And then, downstream, there is another population of the same plant species growing.0825

Well, wind of insects could carry the pollen from one plant population to the other plant population and introduce new alleles.0831

And therefore, there is gene flow going on between those two populations.0841

Or if a group of individuals leaves their home country, goes and settles in another country and then, marries into that population, produces offspring,0846

now, we have alleles being brought in from one group, and entering the population that was existing in that country already.0856

So that is a form of gene flow and evolution, if it ends up changing the allelic composition of the population over time.0864

We have talked about some mechanisms of evolution, and we have talked about how important genetic variation is in order for natural selection to occur.0873

And we have talked about mechanisms for variation in the previous lecture.0882

What are some ways that we can quantify genetic variation? How widely does a population's genetic composition vary?0888

Well, there is a couple of ways to measure this.0897

One is via average heterozygosity, and the other is by measuring the nucleotide variation in the population.0900

Average heterozygosity, we will cover first. The higher the average heterozygosity is, the greater the genetic diversity in a population.0909

Let's think about this. Remember that being heterozygote means an individual has two different alleles for a trait.0946

So, if somebody is a heterozygote for eye color, they have, say, a blue-eyed allele and a brown-eyed allele.0954

If someone is a homozygote, they have two of the same alleles, either both brown or both blue.0959

Therefore, heterozygotes carry more genetic variation.0964

So the more heterozygotes and for different traits are in a population, the greater the genetic variation.0969

And what we are looking at here is alleles and what is made from those alleles, which is often proteins.0976

So, you could look and see what the individual is producing and measure it that way.0983

Looking at blood types, if somebody has the AB blood type, they are producing both A antigen and B antigen on their red blood cells.0988

You could measure that.0997

Whereas, if somebody is a homozygote for type A, they are just type A blood type, or they could actually be a heterozygote carrying that.0998

Oh, but let's just look at homozygous for A blood type. They are only going to produce the A antigen.1010

Somebody who has only B blood type is only going to produce the B antigen. The heterozygote is going to be producing both.1018

This is one measure of genetic variation. The second measure is nucleotide variation.1023

If we went and actually sequence the DNA in a population, we could, then,1032

look and see how widely those nucleotides sequences vary among individuals in a population.1038

The larger the nucleotide variation, the greater the genetic diversity in the population.1044

For example, let's say that we looked at a population of birds, and we found that on average,1051

if we just took any 2 birds from the population on average, they had a nucleotide variation of 2%.1059

So, let's say 98% of their DNA sequences are the same. 2% is different.1064

Then, I go look at a population of lizards, and I sequence their DNA; and I say "OK, they have a 3% nucleotide variation".1070

So 97% of their sequence is the same. 3% is different.1079

The lizard population has a greater nucleotide variation than that bird population so two different ways to quantify genetic variation.1083

Now, we talked about quantifying it. How does it maintain?1092

Because if you think about natural selection, you would think that it might even wipe out genetic variation.1095

So, we talked about the peppered moths in the previous lecture and that there were light-colored peppered moths1101

that existed in England almost exclusively- not completely but almost exclusively - until about the mid-1800s.1107

These were light-colored moths with some darkish flaps, and they blended in with trees.1115

And predators could not see them that well, hopefully, because they were camouflaged.1121

But then, when the cities in England became industrialized and polluted, and soot covered everything,1125

those moths stuck out and got killed off, and selection favored moths who were dark.1130

Those moths blended in with the tree trunks, so selection favored the dark moths.1137

Now, if genetic variation was not maintained in the population, the moth population could not have adapted to the changing environment.1142

And once the environment got cleaned up, the moth population needed to, again, adapt towards lighter-colored moths, again,1152

who would blend in with the trees that were not covered with soot anymore.1162

So, the only way selection can happen is if genetic variation is maintained. Yet, selection is working against genetic variation, so how can it be maintained?1164

Well, there is a bunch of mechanisms.1173

Genetic variation can be maintained through heterozygote advantage, diploidy, geographic variation, neutral traits, frequency-dependent selection and outbreeding.1174

Starting with the heterozygote advantage, what this is talking about is a survival advantage for heterozygotes.1185

Heterozygotes have a survival advantage.1194

Recall that we just said that heterozygote maintains the genetic variation of a population because they have two alleles.1200

The more heterozygotes you have, the more variation that is because they are carrying two different alleles for a trait instead of the same allele.1209

If heterozygotes have a survival advantage, that will maintain the genetic variation in a population.1216

And there are examples of this- a classic one being that of sickle-cell anemia.1223

Recall that sickle-cell anemia is an inherited disorder that involves the hemoglobin.1231

Individuals with two sickle-cell alleles have sickle-cell anemia, and this abnormal hemoglobin causes red blood cells to sickle.1239

They are not their normal biconcave shape.1250

They, sort of, fold up in this sickle-shape, and the result is that they block blood vessels; and then, there is poor blood flow to tissues and organs.1252

And this obviously is not a survival advantage, having this disease.1264

However, individuals who are heterozygous for sickle-cell have what we call sickle-celled trait.1272

They have one allele for normal hemoglobin and the other allele for sickle form.1286

These individuals generally do not exhibit symptoms of sickle-cell disease.1293

Under extreme conditions like very low oxygen, they could have some symptoms.1297

But it is not in the range of what an individual with sickle-cell anemia would experience.1302

So, for the most part, these individuals are not affected by problems from having the sickle allele, but they have an advantage.1307

They are less severely affected by malaria.1314

We could have an individual...we will say big S here is the normal allele. Little s is the sickle-cell allele.1331

Here, we have a person with this sickle-cell anemia. It would be big S-big S, or excuse me, little s-little s.1341

Here, we have our heterozygous individual with sickle-celled trait. They are big S-little S.1349

And then, we have an individual with just the completely normal genotype and phenotype; and they are big S-big S.1355

This individual, well, this individual up here has sickle-cell disease. They are the disadvantaged.1362

This individual with the normal genotype, big S-big S, is more likely to be severely affected by malaria, so they are the survival disadvantaged.1368

The heterozygote will not have the problems with the individual with sickle-cell disease, and they will not be as severely affected by malaria.1378

They can still get malaria. They are not just resistant to malaria, but they will not get as ill.1387

They will not be as likely to die, and so, this gives them a survival advantage.1392

And in fact, in areas of the world like in parts of Africa where malaria is endemic,1398

we see that the sickle-cell gene is much more common than in areas of the world where you do not see much malaria.1405

There has been a selection for this sickle-cell allele particularly the heterozygotes have an advantage.1412

And that maintains a genetic variation is the population.1419

OK, that is one method of maintaining genetic variation. A second is diploidy.1424

Humans, like many other organisms, are diploid. That means that we carry two copies of each allele, so we are 2n.1428

This allows us to carry the recessive form of a gene without the phenotype being revealed.1441

So, let's say the dominant phenotype is at advantageous. Let's say that individuals with brown eyes are at an advantage.1448

The blue-eyed allele can still be maintained in the population in heterozygotes.1456

This is not heterozygote advantage because if someone has the brown-eyed phenotype, they are at equal advantage.1462

A heterozygote is neither at an advantage nor a disadvantage, so they are not being selected for.1469

What they are doing is they are just carrying this silent pool of genes.1475

So it allows the population to carry a silent pool of alleles; and these are the recessive alleles. Let's look at an example.1478

Let's say there is a lizard population and that lizards can be either green, and that is dominant; or brown, and let's say the brown is recessive.1496

If an individual is homozygous dominant, he is green. If he is heterozygous, he is green.1512

If is homozygous recessive, he is brown.1518

These individuals are at a survival advantage.1522

Now, if the recessive allele is fairly rare, and if being brown makes the individual selected against, let's say there is a lot of greenery.1524

And the green lizards blend in better. They will not be as likely to be killed by predators.1534

The green phenotype is being selected for, but there is still this silent allele being carried around.1539

Every so often, two heterozygotes will meet up and produce an offspring who is brown, who does not have a great survival chance, might get killed off.1548

But, there is still some heterozygotes around just carrying that allele.1555

Now, let's say a lot of the greenery is killed off. The environment changes.1561

Things are just browner and trees and a lot more brown less green.1566

Now, every so often, when two heterozygotes meet up, mate, and, let's say, produce a recessive phenotype offspring,1571

little g-little g, who is brown, that lizard would have a survival advantage.1582

It might leave more offspring, and the allele frequency for the brown phenotype would increase.1586

So, by maintaining this pool of silent alleles, the genetic variation is maintained, so that if conditions change,1593

there is still something to be selected for- individuals with this different variant.1601

OK, we went through heterozygote advantage, diploidy and now, geographic variation.1609

Populations of species may differ in their allelic composition due to geographic isolation.1616

For example, let's say there is a group of wolves, a species of wolf.1623

And if you looked at these wolves in the very northern area of a country, they would, let's say, have a much thicker coat.1629

And as you go south, you notice that the coats are thinner.1637

And this could be due to a difference in alleles that the northern population has more of the thick coat allele and the southern, thinner coat.1641

And ecocline, or just sometimes it is a cline, is a gradient.1652

If a trait changes along a gradient, we say that it is a cline, so it not an either-or situation.1660

Coat thickness, we would say "oh, there is a cline", or coat color.1671

In the northern areas, the coats might be white so that the animals could blend in with the snow.1676

The farther south you go, the darker the coat color might be. That could be a cline.1681

And, so this geographic variation, it maintains genetic variation within that species.1686

Frequency-dependent selection: in frequency-dependent selection, the more common phenotype is at the selective disadvantaged.1696

So more common phenotype is at a selective disadvantage.1708

When would this occur? Well, one example is a predator-prey situation.1722

Sometimes, predators develop an image of what they are looking for in their prey.1728

So they are looking for this picture in their mind of what a prey looks like, and it may have to do with size, the shape of the head, the shape of the body.1736

And they are looking for that, and when they see something that fits the image, they go after it.1747

An individual who has the more common phenotype, who looks like the typical animal of its species might be more likely to be attacked by the predator.1752

The more common phenotype is selected against.1761

Let's say, now, there is an individual who has a different body shape. The predator might not be as likely to attack them.1763

And so that individual is selected for, more likely to survive, pass on his genes, pass on those traits, and those traits increase in frequency.1772

Eventually, though, if that body type becomes the most common, predators will develop an image of that body type.1782

And then, the other body type will be selected for.1788

So, you can see how it can go back and forth between a couple of traits, so it maintains that variation.1791

When one trait becomes too common, it is selected against. This is also called, sometimes, the minority advantage.1796

Alright, finally, outbreeding: outbreeding refers to the mating of individuals who are not closely related.1809

And you probably heard about animals becoming inbreds, so certain breeds of dogs have particular problems like hip dysplasia.1819

And part of it is due to the fact that they have been bred with individuals that they are too closely related to.1827

And then, these alleles become concentrated in the population, these traits.1834

So, with outbreeding, if a population, then, breeds with some individuals from another area from outside their immediate population,1840

that brings in different alleles and maintains the genetic variation.1851

Neutral traits, it is exactly what it sounds like. These are traits that do not confer an advantage, nor do they cause a disadvantage.1857

OK, since we are talking about population genetics,1872

we are going to go on and discuss a very important equation that you should be familiar with called the Hardy-Weinberg equation.1877

And this general topic is the Hardy-Weinberg equilibrium.1884

The Hardy-Weinberg principle states that given certain conditions, if natural selection is not occurring, and these conditions are met,1889

the frequencies of genotypes and the alleles in a population will not change over generations.1898

In other words, the gene pool is in equilibrium.1906

So the Hardy-Weinberg equilibrium says that genotype and allele frequencies do not change over generations if a population is not undergoing natural selection.1909

So, if the population is in equilibrium, and conditions are met,1946

we can use this equation, p2 + 2pq + q2 = 1 in order to predict the frequencies of alleles and genotypes in a population.1953

Now, I have mentioned that we need to be at certain conditions, so here are the conditions.1964

One: mating is random. For a population to be in Hardy-Weinberg equilibrium, we have to have a population in which mating is random.1969

Two: no mutations occur. Now, you can see how in real life this could not happen.1981

We cannot prevent mutations from occurring, but in a large population, you could get close enough to these conditions that these equation is useful.1987

Three: no natural selection is occurring. If natural selection is occurring, the allele frequency is not going to stay the same.2002

Genotypes or phenotypes will be selected for and selected against.2013

We need a large population, so the population is large.2019

Finally, there is not immigration or emigration. We do not have gene flow occurring.2027

The population is isolated.2036

Assuming that these conditions are met, we can use the Hardy-Weinberg equation.2041

So, what does this occasion mean? Let's look at each part of it.2046

p is the frequency of the dominant allele, and we are looking at a situation where we just have two alleles for a trait.2050

q is the frequency of the recessive allele.2061

Now, p + q = 1, and this must be because as you know, in probability, we have to have the frequencies of all the possibilities equal to one.2072

So, if I only have two alleles, let's say brown eye allele and blue eye allele,2085

and the frequency of the brown eye allele is 0.3 - let's say p = 0.3 - well, then, q must equal 0.72090

because there is only two alleles, and they have to add up to 100%.2102

And, of course, you can work in decimals. You can work in percentages just depending on what the questions ask you to do.2106

This is p and q. What, then, is p2?2112

Well, if we look at what p2 is, it is the frequency of the homozygous dominant genotype.2116

q2 is the frequency of the homozygous recessive phenotype.2131

Finally, that leaves us with 2pq. 2pq is the frequency of the heterozygous genotype.2142

So, we have this equation p + q = 1, where p is the frequency of the dominant allele. q is the frequency of the recessive allele.2162

And here, we have p2 that is the frequency of the homozygous dominant genotype.2169

q2 is the frequency of the homozygous recessive genotype, and 2pq is the frequency of the heterozygous genotype.2175

And there is only three possibilities for genotypes, and these are the three; so the frequencies of these three must add up to one.2183

Now, let's put this to use. Let's say that we have a population of 100 people, 100 individuals, and that 9 of these have blue eyes2193

And we are going to talk about simple Mendelian genetics, where there are two alleles. One is brown.2214

One is blue. Brown is dominant over blue.2220

Therefore, p is going to be the frequency of the brown allele. That is my dominant allele.2224

q is going to be the frequency of the recessive allele, which is the blue allele.2234

I have this population of 100 individuals. 9 of whom have blue eyes, and the question I want to figure out is the frequency.2244

I want to figure out the answer to this, the frequency of the genotypes in the population.2254

What is the frequency of each of these genotypes?2262

Well, what I am starting with is 9 individuals have blue eyes. Who is going to have blue eyes?2266

What is going to be the genotypes of individuals with blue eyes?2272

Well, since blue eyes is recessive, the only people who are going to have blue eyes are those who are homozygous recessive.2277

Homozygous recessive is represented by q2.2284

The frequency of homozygous recessive individuals in this population is going to be 9 out of 100 or 9%, so 0.9, so homozygous recessive.2288

The frequency is going to be 0.9 or 9%, actually 0.09 or 9%.2308

Now, what is going to be the frequency of individuals who are homozygous dominant, the homozygous dominant genotype?2321

One way I can figure that out is to say "OK, I know that people is q = 1". I know what q2 is, so q2 = 0.09.2333

So, if I take the square root of q2, that is square root of 0.09, that is going to give me q = 0.3. I, now, know what q is.2351

I have p + 0.3 = 1. Therefore, p = 0.7, so p = 0.7.2366

So, you see where I start out from. I knew that I had 9 individuals with blue eyes.2383

Therefore, q2 = 0.09. Then, that allowed me to figure out, by taking the square root of that, that q = 0.3.2388

So, this people is q = 1. p + 0.3 = 1.2401

1 - 0.3 is 0.7, so p = 0.7. If p = 0.7, then, p2 = 0.72 or 0.49.2408

The frequency of the homozygous recessive genotype is 0.09. The frequency of the homozygous dominant genotype is 0.49.2424

Now, all I have to do is plug back into here.2437

I have got p2 is 0.49 + 2pq, which is the heterozygous genotype, and that is what I am trying to figure out; and q2, which is 0.09.2440

When you calculate that out, you take one; and you subtract 0.09, and you subtract 0.49,2464

what you will come out with is that the frequency of heterozygotes is 42% or 0.42, and this is heterozygotes.2472

What we have ended up with is p2, which is 49% plus 2pq, which is 42% plus q2, which is 9% equalling 100%.2486

OK, this shows you how you can use the Hardy-Weinberg equilibrium to figure out genotypes.2512

We could also talk about the phenotypes that individuals with blue eyes, 9%, and here, 91% have brown eyes.2520

So we could talk about phenotypes, as well, and we could talk about alleles.2532

So, we know the frequency of the alleles in the population, p and q of the genotypes2538

and of the phenotypes assuming that the population meets these conditions.2543

Example one: match the following terms to their descriptions- genetic drift, nucleotide variability, gene flow and frequency-dependent selection.2549

The first is genetic drift. The more common phenotype is at a selective disadvantage.2562

Genetic drift does not refer to a selective disadvantage or advantage. That would be natural selection.2569

Change in allele frequency due to random chance: a measure of the difference2577

in nucleotide sequence within a population, or the entry and exit of alleles in a population.2582

Well, genetic drift is a mechanism of evolution, but it is due to random chance; and it particularly occurs in small populations, so this is actually B.2588

We have a change in allele frequency, but it is not due to selection. It is just due to chance such as through founder effect, so we cross out B.2597

Nucleotide variability, that is not having to do with selection. A measure of the difference in the nucleotide sequence within a population, that is correct.2606

The greater the genetic variability, the greater variation. The greater the nucleotide variability, the greater the genetic variation within a population.2618

Average heterozygosity and nucleotide variability are two ways of measure genetic variation in a population.2629

Gene flow: gene flow refers to the entry and exit of alleles in a population.2636

New individuals may come in to the population. Some individuals may leave, and that can change the genetic composition of the population; so this is D.2642

And finally, frequency-dependent selection refers to the fact that the more common phenotype may be at the2652

selective disadvantage such as when a predator develops an image of a prey and goes after that common or typical-looking prey.2656

And therefore, selection is acting against those with the common phenotype.2666

Example two: 4% of a population is affected by a neurological disorder that is recessive, so recessive, and it is 4% of the population.2672

If the population fits the criteria for the Hardy-Weinberg equilibrium, what percentage of the population will exhibit the normal phenotype?2682

What percentage will be carriers of the disorder?2693

Well, we know that if this is a recessive disorder, and 4% of the population is affected; so what percent is going to be normal?2695

That is pretty easy because it has got to add up to 100%, so those who are affected and those who are unaffected have to total 100% of the population.2704

So, here, I have 4% affected plus those individuals who are unaffected equalling 100%. Therefore, those with the normal phenotype- 96%.2716

Right here, this is 96%. What percentage will be carriers for the disorder?2732

This is a little more complicated. Remember our Hardy-Weinberg equation, p2 + 2pq + q2 = 1.2738

And I know that this is a recessive disorder.2748

So those who have the phenotype that are actually affected by the disorder must be homozygous recessive; so q2 = 4% or 0.04.2750

Now, what I am looking for is carriers. Who is going to be a carrier? Heterozygotes.2770

What this is really asking is the frequency, so heterozygotes.2778

Remember that p is the frequency of the dominant allele. q is the frequency of the recessive allele.2785

A heterozygote is going to be PQ, and the frequency of the heterozygotes or carriers in a population is going to be 2pq.2793

So, we have p2 + 2pq + q2 = 1. I know what q2 is.2809

In order to figure out 2pq, I need to know what q is, so I know q2. I also know that p + q = 1, so since I know q2, I can figure out q.2816

I can just take the square root of q2 equals the square root of 0.04. Therefore, q = 0.2.2830

I know that I have p + 0.2 = 1. That give me p = 1 - 0.2 or p = 0.8.2845

Going back to here, carriers are heterozygotes, so they are PQ. The frequency of the carriers equals 2pq.2857

I have 2 x p, which is 0.8, times q, which is 0.2, so that give me 2 x 0.16 = 0.32 = 32%.2869

The frequency of individuals with a normal phenotype is 96%. The percentage that will be carriers is 32%, so carriers equals 32%,2891

And just to take this a little bit farther, a homozygous dominant is going to be p2,2905

which is going to be 0.82, which is going to be 0.64 or 64%.2914

Another way that I could have figured out - although it would have been more complex - is those with the normal phenotype would be.2921

I can add the frequency of those with the homozygous dominant genotype, which is 0.64 and those with the heterozygous genotype, which is 0.32.2930

And I am going to get 0.96 or 96%, so I am just double-checking that this is correct.2943

Example three: in a population of dogs, brown coat color is dominant over yellow coat color.2952

If the frequency of the dominant allele is 0.7,2958

what percentage of dogs in the population will be predicted to have yellow coats if the population is in Hardy-Weinberg equilibrium?2963

What percentage of dogs will be homozygous dominant for coat color?2972

The dominant allele is p, so the frequency of the dominant allele is p; so that is 0.7.2975

And what they are asking me is what percentage of dogs will have yellow coats.2985

Yellow coats are recessive, so in order to have yellow coats, the individual has to be homozygous recessive; and the frequency of that is q2.2990

That will tell me the frequency of yellow coats in the population. I know that p is 0.7, and I know that p + q = 1, so I end up with 0.7 + q = 1 or q = 0.3.2998

Now that I have q = 0.3, all I have to do is square it to get q2. Therefore, q2 = 0.09 or 9%.3023

So, this is the percentage of dogs in the population with yellow coats. It is going to be 9%.3041

The next question: what percentage of dogs will be homozygous dominant for coat color?3052

The frequency of homozygous dominant is p2 equals the frequency of homozygous dominant, and I know that p is 0.7.3060

So I just need to square p, so p2 = 0.49, which equals 49%.3073

So, yellow coats is 9%, and homozygous dominant is 49%.3082

Example four: if a phenotype is favorable to survival in producing offspring,3092

the allele frequency for that phenotype will increase through natural selection after multiple generations.3096

What are some mechanisms by which a population maintains genetic diversity despite selection pressure?3103

How does a population keep from just ending up with one allele- that favorable allele?3109

Well, there is a bunch of mechanisms. They asked for four.3115

One is heterozygote advantage.3117

As with sickle-cell anemia, if the heterozygotes have a survival advantage,3121

they are going to increase the genetic variation of a population by carrying two different alleles.3125

Diploidy: because we are diploid, we are carrying a pool of recessive alleles that are silent but maintain genetic variation.3131

Geographic variation: populations in different areas may have different allele frequencies.3143

Neutral traits: an example of a neutral trait can be something like blood type.3153

It is not selected for. One type is not selected against, but it maintains diversity in the population.3159

So, that is four, but there are more.3166

Frequency-dependent selection: recall that in frequency-dependent selection,3169

the more common phenotype is selected against, so this favors diversity in the population.3174

Finally, outbreeding: Individuals breeding, mating, with individuals outside their immediate group brings different alleles into the population.3182

This is actually six mechanisms, and you only had to state four.3193

So, that concludes this lecture at

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