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For more information, please see full course syllabus of Biology
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Lecture Comments (17)

1 answer

Last reply by: Bryan Cardella
Thu May 4, 2017 11:25 AM

Post by Scott Pearce on May 3, 2017

For Epistasis, wouldn't the alleles have to be dominant not recessive ?
Sorry I am confused about that.
Kind regards,
Scott Pearce

1 answer

Last reply by: Bryan Cardella
Mon Jun 29, 2015 3:10 PM

Post by Hossain Khondaker on June 27, 2015

its weird how my eye reacts in sight of the color yellow you used in this video it kind of flashes. Why is that??

1 answer

Last reply by: Bryan Cardella
Sun Dec 7, 2014 9:24 PM

Post by Samuel Eukbay Eukbay on December 7, 2014

Great explanation!!!!! Thank you.

1 answer

Last reply by: Bryan Cardella
Wed Sep 3, 2014 12:34 PM

Post by Ivan de La Grange on September 2, 2014

Is not having allergies or not getting poison ivy considered a recessive trait? I say this because many of your recessive trait examples are expressed in the negative aspect. I say this because I have both traits and it seems to me that people are envious of it.

Thanks Professor Cardella.

2 answers

Last reply by: David Gonzalez
Sun Jun 29, 2014 12:17 AM

Post by David Gonzalez on June 28, 2014

Hi Mr. Cardella, I'm very new to genetics, so I apologize if this is a silly question.

Is the punette square designed to show you what the physical representation of the organism will be like? Or what they will look like on a genetic level? Or both? Thank you!

1 answer

Last reply by: Bryan Cardella
Thu Jun 26, 2014 2:31 AM

Post by Enrique Salinas on June 25, 2014

Can u please explain the following problem?
in a dihybrid cross between bean plants with red (R)wrinkled (w)seeds and white (r) smooth (W)seeds, the F1 progeny is all red and smooth. if the plants are selfed, what proportion of the F2 will also be red and smooth if the genes are linked?


1 answer

Last reply by: Bryan Cardella
Sun Apr 13, 2014 9:42 PM

Post by Aziza Bouba on April 13, 2014

Can a girl have Haemophilia?

1 answer

Last reply by: Bryan Cardella
Sun Apr 13, 2014 9:37 PM

Post by Aziza Bouba on April 13, 2014

concerning autosomal dominant: When a person is for example Hh does he or she have Hungtinton disease?

Genetics, Part I

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
  • Gregor Mendel 0:05
    • Father of Genetics
    • Experimented with Crossing Peas
    • Discovered Consistent Patterns
  • Mendel's Laws of Genetics 3:10
    • Law of Segregation
    • Law of Independent Assortment
  • Genetics Vocabulary #1 6:28
    • Gene
    • Allele
    • Homozygous
    • Heterozygous
    • Genotype
    • Phenotype
    • Hybrid
    • Pure Breeding
  • Generation Vocabulary 13:03
    • Parental Generation
    • 1st Filial
    • 2nd Filial
  • Punnett Squares 15:07
  • Monohybrid Cross 18:52
    • Mating Pure-Breeding Peas in the P Generation
    • F1 Cross
  • Dihybrid Cross Introduction 23:42
    • Traced Inheritance of 2 Genes in Pea Plants
  • Dihybrid Cross Example 26:07
    • Phenotypic Ratio
  • Incomplete Dominance 32:02
    • Blended Inheritance
    • Example
  • Epistasis 35:05
    • Occurs When a Gene Has the Ability to Completely Cancel Out the Expression of Another Gene
    • Example
  • Multiple Alleles 40:12
    • More Than Two Forms of Alleles
    • Example
  • Polygenic Inheritance 46:50
    • Many Traits Get Phenotype From the Inheritance of Numerous Genes
    • Example
  • Test Cross 51:53
    • In Cases of Complete Dominance
    • Test Cross Demonstrates Which Genotype They Have
  • Sex-Linked Traits 53:56
    • Autosomes
    • Sex Chromosomes
  • Genetic Disorders 59:31
    • Autosomal Recessive
    • Autosomal Dominant
    • Sex-Linked Recessive
    • Sex-Linked Dominant

Transcription: Genetics, Part I

Hi, welcome back to www.educator.com, this is the first lesson on genetics.0000

Genetics is one of my favorite things to teach in biology because0006

it is all about how you inherited the pieces of DNA that make you who you are.0009

The environment has a lot to do with the expression of DNA in a person's body or any organism’s body.0015

But without that DNA expressing itself, you would not have those raw materials for the environment to act upon.0020

Thanks to the work of Gregor Mendel, a monk from the 1800s, a very simple man but intelligent man,0026

we got a great head start with figuring out genetics.0035

He is considered the father of genetics because his work is really meticulous, extensive work, really did laid the groundwork.0039

A 19th century Austrian monk who spoke German, he lived in what is now is Czechoslovakia.0047

The world was a little different there, in terms of territory, territories back then in 1800’s.0054

He experimented with crossing peas, pea plants in the garden of the monastery.0062

I have asked my students before, what is the advantage of using peas?0068

If you try this with animals, there would have been a lot of problems.0072

Meaning, getting the animals to mate with the ones you want them to, cleaning up after them.0076

The expense of having animals waiting a long time for the animal to be born,0082

the generation turnover takes a lot longer, the complexity of the genetics of an animal, he got lucky with the pea plants.0087

Pea plants, the species he happened to use have pretty predictable display patterns0093

for how the genetics is actually shown up, how it shows up in the plant.0102

You will see this as we go through some examples with the color of the peas, the shape of peas, the pod etc.0108

This guy, Mr. Mendel, crossed thousands of plants, this took him years to do.0113

If he would just made it a few and saw what happened and he thought he know what is going on,0120

it was the amount of times he did it to prove that this can be something that is replicated time and time again,0126

and you get the same kinds of results.0133

That proved his point, his theory about this.0135

Yes, when we look at his data, he crossed thousands of plants.0137

When we say cross, we mean mate it.0140

He took pollen from one, fertilize the flower of the other.0143

The cool thing about pea plants, in addition to many other advantages that he had is that,0147

every pea plant has male and female parts on it.0151

It makes it very easy to mate them.0155

He discovered some consistent patterns based on the results of the crosses.0157

You will see some interesting ratios of the offspring, what is expected to have happen if he do this crosses.0162

Thanks to the fact he did it hundreds of thousands of times in these various traits, you get those predictable ratios.0169

We will talk more about that, how flipping a coin 10 times, you are not always going to get 5 heads and 5 tails.0176

But, if you flip it a thousand times, you are getting it close to what the statistics tell you is going to happen.0183

There is Mr. Mendel, with his research, there were two laws that have a lot of credence still today0188

but there are some exceptions that I will tell you about.0198

His law of segregation, segregation socially means something very different.0200

If you try to segregate people, you are separating them from each other.0205

Segregation here is talking about gene parts getting segregated.0208

If you watched the cell division lesson on meiosis,0213

you know that there is a separation of duplicated chromatid parts that has to do with making sperm, egg, pollen.0217

That is what the segregation is referring to.0226

Mendel knew that each pair has two units that it could possibly pass on for each trait.0228

And that, these two units were split up in their pollen and eggs, there is a 50-50 chance of inheritance.0233

You are going to see that depending on the genotype and whether it is dominant-dominant, recessive,0241

it will make more sense in a little bit.0247

Only one of them is going to be passed on to the offspring.0249

Remember, from the chromosome explanation in cell division, if you have homologous pairs,0253

one from your mom and one from your dad, they each gave you one piece of that puzzle.0259

Then they leave that, even though he had never seen chromosomes in his life,0264

he knew that there was something of substance that these plants were passing on in their pollen and egg.0269

The law of segregation is true, we know it is true.0275

The exception is for when non disjunction occurs.0279

Non disjunction would be, when you have failure of separation during meiosis.0283

Failure of homologous chromosomes to separate or the choramatids to separate.0290

You end up with too few or too chromosomes in sperm or egg, or pollen and egg, that does not happen that often.0294

Non disjunction would be the exception to the law of segregation actually happening.0302

The law of independence is a little bit different.0307

Mendel also knew that each trait is inherited independently from every other trait.0309

Example, just because a pea is green does not mean it will be round.0314

The inheritance of color in peas has nothing to do the inheritance of shape.0319

The size of the plant, whether it is tall or short has nothing to do with the peapod color or peapod look.0323

All those things are inherited separately.0330

The one exception is linked genes.0333

We now know that, when look at complex inheritance patterns that we know more about today,0337

there are these linked genes where they are found in the same chromosome and tend to be inherited together.0344

One example I can think of is people with red hair are much more likely to get freckles than people with other hair colors.0349

That could be due to linked genes having to do with getting that particular hair color,0359

impacting the expression of the chance of getting freckles.0364

But there are lots of examples of linked genes.0368

Mendel did not actually get to see that.0371

Those very simple genetic pattern with pea plants which I think was a good thing.0375

He got to cover some slack, he was doing this work in 1800’s, did not have a lot of technology.0379

He really found out some awesome stuff.0385

Some genetics vocabulary, this is the first set of vocabulary I want to show with you.0389

As we go to the examples of these different genetic phenomenon and these patterns,0393

that we can use these terms and you know what I’m talking about.0398

A gene, most of you know that, it is a segment of DNA that codes for a specific protein.0401

We all know that we get our genes from our parents.0407

What it actually is, is a segment of DNA and if you watch the lesson on RNA,0410

you know that RNA is made from DNA that RNA is translated into protein.0417

The proteins is what makes the trait.0421

Sometimes these traits are visible on the surface of an organisms body, sometimes they are not.0425

There is some manifestation of that DNA in the trait.0430

That is what the gene is actually is.0436

An allele is a component of gene.0438

Organisms typically have two for each gene, they pass on one to their offspring because of that segregation phenomenon.0441

When we talk about alleles, alleles are usually called dominant or recessive.0448

What that means is, if it is a dominant allele, it pertains to the allele that can overshadow or mask another.0452

If I inherit two alleles from my parents and this one is not expressed but this one is,0462

we would call this one dominant because it is masking, it is not allowing this DNA to be expressed,0468

this is the one that is being expressed.0473

Recessive is the one that can be overshadowed.0475

These are relative terms, you are going to see there are exceptions to this dominant-recessive thing,0481

when we talk about more complex patterns that is coming up a little bit later.0485

Typically, dominant alleles, they use a capital letter.0489

If we are talking about gene A, dominant alleles would be A.0494

For recessive, it would be lower case.0501

Homozygous and heterozygous, when you inherit two alleles for a gene,0505

that combination is called homozygous or heterozygous.0509

Homo meaning same, zygous coming from that word zygote which means that first cell of life.0512

The sperm or the egg coming together, or the pollen and the egg coming together.0519

Homo meaning same, they have the same allele for the gene that is now in that zygote.0524

Homozygous could be this or it could be this, or it could be this.0533

I’m giving you a few different genes.0542

For gene A, they are both dominant alleles, each parent gave one dominant allele and for gene r, both recessive alleles.0544

These will be expressed, this is the case where recessive, since no dominant alleles have been inherited, those will get expressed.0552

Here is another example, we would actually call this one homozygous dominant.0560

Just to tell us that, the ones that are the same is a dominant one.0567

This one we will call homozygous recessive because the ones that are the same are the recessive ones.0570

Those were the two that were inherited.0577

Heterozygous is having the opposite form of alleles for a gene.0579

Here is heterozygous, this is heterozygous, and that is heterozygous.0584

The two opposite forms, the dominant or recessive inherited together.0592

Mom or dad gave each of them, that is how it is.0595

You would not use the term heterozygous dominant or heterozygous recessive, that does not make sense.0599

Heterozygous tells you that you have inherited dominant from one parent and recessive from the other.0604

More vocabulary for you, we just looked at genotypes in the previous slide.0611

A genotype is the specific combination of alleles for a gene.0615

Some examples of genotypes is that, that, how about this one, this one.0618

I will give you one that is going to look weird right now, but later on it will not look weird.0634

This is also a genotype pertaining to blood type.0639

These should look familiar, based on what you saw on the previous slide.0643

They are all combinations of alleles that have been inherited.0646

These are forms of DNA that have been inherited in that new individual.0649

The genotype gets expressed, and the actual physical expression the genotype is known as phenotype.0655

Sometimes, phenotype can be seen, when we are talking about something like skin color, whether not someone has a widow's peak.0663

If you are unfamiliar with a widow's peak, if your hair comes down to a little point here in the middle of your hair line, that is a widow’s peak.0675

As far as I know that is caused by a dominant allele, if you have it.0687

Another one could be blood type, this is one that you cannot tell by looking at someone.0692

There is a physical manifestation, a physical expression of the genes that made these.0699

It is shown in the surface of red blood cell.0705

But all of these are phenotypes, the physical expression of what inherited up here.0708

Hybrid, there are multiple definitions, the one we are going to use more with this particular lesson is,0714

it is an individual who was the result of crossing two sexually reproducing parents.0719

I'm a hybrid of my two parents, that is how we could use it.0724

It also can mean a combination of two closely related species.0727

For instance, a mule is a sterile hybrid of the donkey and horse.0731

That hybrid thing will come up in biology occasionally, but with this lesson0738

we are going to talk about hybrid as a combination of two sexually reproducing individuals.0743

Pure breeding also known as true breeding.0749

Pure breeding, the individuals homozygous for a gene, only one kind of a allele to pass on for a trait.0751

You are going to see it coming up when we talk about peas, a pure breeding pea.0757

If it is yellow, it is only going to pass on the alleles that cause its offspring to be yellow.0761

Same with the green pea, if green pea is true breeding, it only has that certain kind of allele, maybe it is wrinkled.0767

It is true breeding or pure breeding for that wrinkled allele type.0776

Vocabulary associated with generations, when we cross one individual and get a second group,0785

cross the second then you will get the third group, there are names for that.0791

Mendel started the process for categorizing generations of organisms, when tracing inheritance.0794

a have kept notes on this to keep it straight, we knew what generation leads to which.0800

The P generation, sometimes called P1, but the 1 is a little bit redundant.0805

P means parental generation.0810

If we plant these peas, we are going to get the next groups of the parents are here.0815

We are saying, this parent is all yellow, I know it is in black and white, it is I know it is hard to tell,0821

but these are all yellow and these are all green.0825

The parental generation, if we take pollen from one and put on the flower the other,0828

the offspring are going to be F1 from the P generation.0834

F1 is 1st filial, filial meaning it is of the family from the previous generation.0839

When we cross F1, their offspring are F2, and that pattern continues.0847

If you were to cross F2, the 2nd filial, they may get three.0852

You cross F3, they may get four.0856

It keeps going on until F whatever.0858

You could think of the P generation as grandparents.0862

The F1 is the parents and the F2 are the grandchildren.0866

It is all relative, F2 is telling you it came from two previous generations, that is why I wrote and so on.0871

You can keep going and going, if you want to see future patterns coming from all the way back from P.0881

You are going to see this on the next couple slides, this will make more sense what is going on with this expression.0888

That in the F2, the grandchildren, that 1P looks a lot like the grandparent up here.0895

But in the F1 generation, that green phenotype was hidden and it is masked.0901

Punnett squares, there was a scientist by the name of Punnett,0908

who came up with this way of using a square to have a visual representation for the probability of allele inheritance in offspring.0912

Let us say in the P generation, we have a heterozygous individual and0921

we are crossing this individual with another heterozygous individual.0929

You are going to see this a lot through this lesson.0932

This X means we are mating those two and these are their genotypes.0935

The phenotype does not really matter here, I just want to show you how the gene inheritance occurs,0939

with respect to this Punnett square.0946

You put one individual on one side and you separate the alleles here and here.0948

This is kind of acting out that segregation phenomenon.0953

During meiosis, you get a splitting of those chromosomes.0958

This is saying that, if that is the male individual, half the sperm are going to have inheritance of this,0961

half the sperm are going to have this other allele in it.0967

Or a pollen, if we are talking about plant, half the pollen grains have this, half the pollen grains have that.0970

The other parent is the same, we put that parent’s alleles here and here.0975

This is just how it is, one parent is on the top of this four square Punnett square and the other parent is on the left.0979

You do not put it here or here, that is how it is.0988

It does not matter where you put the male and female.0992

The male or female could be here and vice versa.0994

And then, you show how they come together, it is accounting for all the possibilities of inheritance.0997

These two gametes, pollen and egg can combine, these two combine, and so on.1004

What this is saying is, there are four possibilities, there is a 25% chance of the offspring being homozygous dominant.1014

There is a 50% chance, 2 out of 4 of it being heterozygous, 25% chance of being homozygous recessive.1024

The genotypic ratio for the F1, if these is the P generation which are here,1033

this is telling you what is going to happen in the F1 that come from them.1043

The genotypic ratio is 1 to 2 to 1 that is like saying 25% -50%-25%.1046

Here is what I’m saying, 1 to 2 to 1, this ratio will come up again and again.1056

Some people like rating fractions, it is just another way of representing the chances of this happening.1073

Keep in mind, if you were to have only four offspring, you are not always going to get half of them with heterozygous.1078

I mentioned this earlier, if you flip a coin 4 times are you always going to get two heads and two tails, no.1087

But if you keep flipping that coin, statistics tell you that, as time goes on you are more likely to get the 50-50 ratio.1095

If we are doing this with peas and we planted 4 peas and see what happens, it is possible that we get the perfect one, the 2-1.1103

But every time inheritance happens, every time a pollen grain and egg combine,1113

you get this being reenacted over and over again, in terms of the probabilities.1118

If we planted 100 peas or 500 peas, it is much more likely that is going to be close to that 1-2-1 ratio.1124

Monohybrid cross is telling us, when we are focusing on one gene, monosingular.1133

Hybrid, combination crosses is the mating.1138

It is tracing inheritance of one gene.1142

Mono started by mating pure breeding peas in the P generation.1145

Homozygous dominant for this color gene, this is focusing on color this time.1152

We focus on other traits, the size of the plant, etc.1159

We are focusing on the color of the pea.1163

We are going to call it the Y gene, it happens a lot.1166

Geneticists will assign a letter based on the dominant phenotype.1170

You do not have to, if you want you can use A for this gene.1175

We are using Y for yellow.1179

Pure breeding means, this plant, all of its peas are going to look yellow.1181

It is only going to pass on dominant alleles.1187

This plant, all of its peas are green, and because there is no dominant allele,1190

those heterozygous they will be y, it would look yellow because this allele dominates that.1196

When you mate them, I want to use green because I do not want to confuse you about the green pea thing.1203

Here is the P generation, this is a P generation of peas.1214

When you do a Punnett square, they are all going to be heterozygous.1222

What will they look like, they will look like this parent because at least one Y allele,1243

the big one, the dominant one, will mask this recessive one.1250

The heterozygous individuals look just like this one.1255

They are all heterozygous and they are all going to look yellow, 100% chance.1259

We are going to take these F1 individuals, the F1 all yellow, all heterozygous, and mate them.1273

I realize that their pea plant siblings, that is not a big deal when it comes to plants.1279

It is a different kind of thing, when it comes to humans.1285

You are going to mate the F1 generation and see what happens.1289

If the F1 cross, two heterozygous were made, all of them are going to be heterozygous looking yellow.1292

It is going to be a little bit different this time around.1309

25% chance of homozygous dominant, 50% chance of heterozygous, and 25% chance of homozygous recessive.1321

In the F2, the genotypic ratio is what?1334

It is 1 to 2 to 1, one homozygous, 2 out of 4 are going to be heterozygous, 1 out of 4 is homozygous recessive.1345

However, the phenotypic ratio is going to be a little bit different.1356

It is going to be 3 to 1, why, because these three all look yellow but this one looks green,1369

like its grandparent pea plant.1381

That is the interesting thing that, now when we look at the genotype versus phenotype, there are some differences.1385

Because in this particular case with heterozygous, only the dominant allele is being expressed and the recessive one is being ignored.1391

You are going to see it in the future, in this lesson,1401

there are some cases with the heterozygous individual, the recessive is also expressed.1404

That is not complete dominance like we are seeing here.1409

There are your ratios in the F2 generation, the grandchildren peas from the P generation.1412

It is not a hybrid color cross.1419

With the dihybrid cross, we are going to trace the inheritance of two genes at the same time,1424

that is why it is dihybrid.1428

Mendel did this, he simultaneously traced pea color and shape.1431

I’m going to introduce a new designation for you.1437

Now, as a shortcut, instead of writing more than this genotype, I’m just going to write Y blank meaning,1440

that blank whether it is a dominant Y or recessive y, same thing, in terms of what phenotype you get.1447

Because that one dominant is going to be expressed and whether the other one is dominant or recessive, it is going to look yellow.1455

Like we also saw before, both recessive homozygous, recessive codes for green, the phenotype.1462

With shape of pea, getting at least one dominant allele for the r gene is going to make the peas round,1468

like we are used to seeing when we eat peas.1475

If no dominance are inherited and it is homozygous recessive, you are going to have wrinkled peas.1477

Let us see what happens when you mate the pure breeding plants.1484

First to this, he knew based on previous observation, that this plant is completely pure breeding.1488

Regardless of what I mate it with, time and time again, it is always making yellow round peas, and he mated it with this one.1497

This one always yellow round, here is the phenotype and this one will look wrinkled.1508

It is always making that wrinkled green one.1519

When you mate them together, we are not going to have to do a Punnett square for this,1522

you are always going to get this, heterozygous for both genes.1528

What it is going to look like, check this out, look back here.1535

It is going to look like this, like that parent, even though the genotypes are different.1542

Here is what you have in the F1, all of the F1 generation looks like that and has these genotypes.1548

What happens when you mate F1, when it is heterozygous for both genes?1557

You need do a very large Punnett square for that, that is coming up next.1562

Here is the dihybrid cross example, in terms of mating those F1 individuals.1569

Like we said, we are mating this times two.1574

I’m writing that kind of small because I need more room1583

to write the different pollen and egg allele combinations that we are going to get.1585

If we account for everything, here is what ends up happening in the pollen grains.1593

That is all the possible combinations.1608

The way you can keep track of that is, this is just makes sense in my head.1611

Since, you have to account for every possibility, I just make two little dots next to every letter.1617

Once there are two dots, you know you are done.1624

Here is what I mean, you can have that one and that one combining, that is one pollen grain possibility.1627

We could also have this one and that one.1635

Now, we know we are done with the dominant Y.1640

We could also have this one combined with this one.1643

We are also going to have this one combined with this one.1648

Those are all the different possibilities.1652

Since the other parent has the same genotype for both the color of gene and the shape of the pea gene, it is the same.1654

It would be really exhausted to write out the genotypes for all of these.1672

Instead, let us just keep track of the phenotypes.1677

For the first one, first couple, I will write the genotype.1683

This one is going to look like that grandparent because having that pollen grain combine with that egg,1687

we are having this pollen grain combine with that egg, you are going to get that.1693

What are they going to look like?1697

It will look like that.1702

For the rest of them, up on the top row, because of this, the fact that you are always in this part of the Punnett square,1704

you are always going to get a dominant Y and dominant R.1713

The rest of these are all going to be round and yellow, regardless of what they inherit here.1717

Even though, this particular combination is going to be heterozygous like those two parents,1723

it is still going to be yellow and round.1730

This one, its genotype would be like this, still they all look identical.1733

On the same token, this whole left column, all yellow and round.1739

So far, a lot of yellow and round, for the same reason, when this particular gamete is inherited with these, all yellow and round.1747

Hold on, we will get some varieties, this one right here.1756

You definitely have the yellow color, but r and r, homozygous recessive for shape.1759

It looks almost like popcorn, that is your wrinkled pea.1769

This one here, yellow and round, another wrinkled yellow because there is no dominant R but there is a dominant for Y.1772

Next up, round and yellow, how about this one right here.1787

It is also a little popcorn looking one, has a dominant allele for color but not for shape.1796

Here is where we finally see some green peas, check it out.1804

No dominant allele for the Y gene but it is round, so we are going to have a green round pea like I'm used to eating.1809

Here when we have that round and green, different genotype for this one but same phenotype.1820

Here as well, the last one completely recessive for both genes, wrinkled and green.1828

Look at that, it is kind of pretty.1840

Writing a genotypic ratio for this will be kind of waste of time.1843

It will be a lot of numbers and a lot of colons, it is more meaningful to talk about the phenotypic ratio.1846

The F2 for this dihybrid cross, the phenotypic ratio equals,1853

How many yellow round peas, 9.1866

How many yellow wrinkled peas, 3.1875

How many round green,3.1882

How many wrinkled green, 1.1884

9 to 3 to 3 to 1, the number is going to equal 16 because that is all the possible combinations.1887

This is a famous ratio that Mendel came up with.1892

He did dihybrid crosses with a lot of different traits, not just pea color, pea shape.1894

He got it done with flower position and pea plant size, he got it done for peapod shape, peapod color.1899

The point is that, when you do this with the dihybrid cross1906

with both particular individuals being heterozygous in the parental generation,1911

or in this case the F1 generation, you are going to get this interesting phenotypic ratio.1916

We just talked about complete dominance previously, meaning when you get this, it is going to look yellow.1924

This one is not even expressed, that is known as complete dominance.1933

That does not always happen in genetics.1936

Sometimes, inheritance of a heterozygous genotype, both alleles are expressed together, that is called incomplete dominance.1939

Blended inheritance is another term for what it is.1948

Blending of alleles or blending of genes.1952

A good example is snapdragon flower, here is a pink snapdragon flower on a snapdragon plant.1955

We are going to start with the P generation.1961

The P generation is going to be, these are for red, this is what the color.1965

The P generation, we have pure breeding, red with pure breeding white.1974

That looks red, this one looks white.1984

Normally, if you cross this with complete dominance, all the offspring will be heterozygous and red.1990

In this case, that is not what happens.1995

The cross with the P to make F1, all of F1 is heterozygous and pink.1997

They are all pink like this one.2021

This one heterozygous, it is definitely a blending.2023

Just like if you combine red and white paint, you get pink.2027

Both parts of the DNA on each chromosome are expressed and you get this collective pigment combination that equals pink.2030

What about when we take the F1 pink, take pollen from one and egg from the other and combine them?2041

Here is our F1 cross.2049

Like we said before, once again, genotypic ratio 1 to 2 to 1, no surprise there.2064

What is the phenotypic ratio here?2073

Is it 3 to 1, no it is not because these individuals are pink.2075

Both the genotypic and phenotypic ratios for F2 are 1 to 2 to 1.2081

1 red to 2 pink to 1 white, there is incomplete dominance.2090

This actually happens a lot in human genetics as well.2094

There is a blending of traits from both parents to have something this kind in between in the offspring.2098

Epistasis, this is a phenomenon in the genetics where a gene has the ability to completely cancel out the expression of another gene or genes.2107

It trumps the other genes.2118

Like in certain games of cards, you have a trump card where that you can beat every hand, I can beat every card with this.2120

With albinism, the gene that causes albinism to occur, it cancels out all the other genes that relate to skin color.2129

Let us assume there are 4 genes for skin color.2139

Let us say skin colored genes A, B, C, and D.2144

The epistatic gene, the one that can cause albinism, you can see an example here is GEE for epistasis.2159

If you are wondering literally what is albinism, it is the inability to make melanin.2172

This individual no melanin, that is the pigment that is in your skin, hair, the irises of your eyes.2178

They just cannot make it in their melanocytes, the cells that we normally produce that pigment.2191

It is because they have inherited with this gene, this genotype.2197

Homozygous recessive can cancel out all of those, it does.2203

Regardless of how much dominant alleles you got with those 4 skin color genes.2208

I’m going to give you 3 examples of people with inheritance of these 5 genes and we will talk about what they look like.2214

Person number 1, with skin color, they have inherited this.2220

Remember, these are the names of the genes but you inherit two alleles for every gene.2232

There is all 8 alleles for the 4 genes and they have inherited this.2236

What do they look like, are they albino?2243

No, because of the fact that this is homozygous dominant and not homozygous recessive,2246

you are not going to get those turned off.2251

What is this skin color like for this person, fairly fair skin, not as fair as if they are all recessive.2253

But, there is a lot more recessive alleles than dominant alleles.2260

Maybe close to my skin color, it is hard to say.2264

This person is not albino.2269

How about this individual?2285

Darker skin than the first individual, no albinism.2288

They do not have the homozygous recessive case that would cause them all to be canceled out.2293

You get expression of these four genes, both of these people not albino.2298

I do want to introduce you to one other term that is going to come up again later in this lesson.2305

The fact that this individual has that one recessive allele, they are carrier for albinism.2310

If they were to mate with someone else with the same genotype, they have 25% chance of passing on albinism2315

because here is that 1 out of 4 chance that those recessive alleles come together.2325

These three individuals, if that actually occurred with their offspring, not albino.2335

They might be a carrier as well.2340

These people, when you have these genotypes, not albino.2344

This one would be albino.2347

Here is the third example, you can probably see where I'm going with this.2349

Because of this genotype in the epistatic gene, all of these are not expressed.2362

If they had one of these genotypes, this person will have very dark skin.2369

They have the maximum amount of dominant alleles for these 4 skin color genes.2373

But, they are not going to have skin pigment because they are going to be albino, because of this epistatic gene.2378

There are also other factors that can impact your skin.2389

The environment, the amount of sun you get will make melanocytes more active, the food you eat, your health level.2393

There are various factors that can impact the color of your skin.2402

Definitely, there are genetics behind it and this is an example of how one gene can impact a bunch of others.2406

Multiple alleles, we talked about cases where you have a dominant and recessive, those are the only two varieties.2414

It is a lot more complicated than that.2421

With certain genes that are not just two forms of alleles, there can be more.2423

An example is human blood types.2427

There are really rare blood types like one in a million blood types, that I’m not going to discuss here.2429

I’m going to the talk about the 4 main letter types.2434

For now, we are going to ignore what is known as the Rh factor, that is whether you are positive or negative.2438

I'm O positive so I got a dominant allele that affected the surface of my red blood cells2446

and the Rh factors inherited via a different gene.2453

I’m going to ignore that for now, we are going to talk about the 4 main blood types, A, B, O, and AB.2457

There are 3 allele types here.2464

Here they are, that dominant, this other dominant, that recessive.2470

What is with the I, this is showing you that these both are dominant alleles.2480

One is the A form, one is the B form, they are equally dominant over that.2486

That is why they both get a I.2489

If instead of this, if we just wrote A and B, and wrote a, you might think that is simpler2492

but the reason why this is more confusing is you might think that since that is a dominant B and that is a different letter,2498

it does not have the ability to overshadow it like this one does.2504

To keep it straight, the geneticist that first came up with this designation,2509

they picked I as the letter representing whether it is dominant or recessive.2515

We have a superscript saying that this is the A version, this is the B version.2520

They both can mask the recessive here.2524

With blood types, let us look at, if you have phenotype A, the A blood type, whether you are Rh+ or Rh-.2528

If you are A, there are two possibilities with inheritance here.2535

You could be with your genotypes, homozygous dominant for A or heterozygous for A.2538

Both of these people have the same exact red blood cells, their phenotype is identical.2554

It is complete dominance herewith that allele.2559

B, you can probably guess what is going to happen.2563

Homozygous dominant for B, they could be heterozygous for B.2574

With AB, there is only one genotype and it is this.2584

I have heard that this phenomenon being referred to as code dominates because2593

both alleles are dominant and they are both being expressed the same time.2601

Neither one overshadows the other, the combination of these is not a blending, like you saw with incomplete dominance.2605

They are both expressed on the surface of the red blood cell.2613

And then there is one other genotype and it is the one that causes to inherit the homozygous recessive case gives you O blood.2618

Let us do some Punnett squares.2628

This is the one for me, this is how I got my blood type.2630

A lot times, when you do it a Punnett square for how you came for your parents, it is hard to know exactly what their genotypes are.2635

I know for a fact, what my parent’s genotypes are.2641

It is not because I did an extensive DNA analysis test or something.2644

I will explain why I know.2648

My dad is A, I know that he got that allele going on.2650

My mom is O, here is my mom, here is my dad, on the male side, female side.2654

I know for a fact my dad is heterozygous.2662

Why do I know that, because I got O blood.2666

If my dad is A and my mom is O, I could have inherited O blood, without me getting this allele from him.2669

Normally, I will write homozygous recessive.2676

Instead, this time around I’m just going to give you the phenotype.2679

O and O, there is 50-50 chance that you will get an O.2682

It turns out that my one sibling, my brother, he is A.2685

Him and I are like a perfect statistical example of how those 50% chance of my parents passing on A blood and a 50% chance of passing an O.2691

It is a 1 to 1 ratio or 2 to 2, if you do not reduce it.2700

If my dad was homozygous dominant for A, both me and my brother would have A blood for sure.2704

The only way that it would not be true is, I was developing in the womb,2713

there was a random mutation at that exact spot on DNA that codes for this.2716

The likelihood of that is very slim.2721

One more Punnett square for blood type.2724

I want you to think of this, if you need some extra time just pause and look at it for a second.2730

I want you tell me, why is this Punnett square really interesting?2736

When a heterozygous person with A and heterozygous person with B blood mate, what is interesting about that?2740

It is a 25% chance of getting each of the major blood types.2750

Look at that, it is a 1 to 1 to 1 to 1 ratio, of all the different blood types.2755

A question for you, can a blood type be used to prove that someone is the parent?2761

No, to prove that someone is the parent of a child,2768

you have to do a more extensive DNA fingerprinting kind of analysis which you are going to hear more with the DNA lesson in this course.2771

Can a blood type be used to prove that someone is not the parent?2780

Yes, sometimes it can.2784

For instance, if you look back at my Punnett square, if I had B blood2786

then one of my parents is not the actual parent because that cannot happen.2792

You cannot use blood type to prove that someone is the parent because there are billions of people,2800

at least hundreds of millions with that blood type, the same as yours.2806

Polygenic inheritance, the name says it all.2813

Poly meaning many, genic meaning genes.2815

It is when traits, there are a lots of traits that get this.2817

Many traits get their phenotype from inheritance of numerous genes.2821

When we are looking at a simple one gene mode of inheritance, up until this point.2826

This gene causes this trait, that other gene causes this other trait.2832

Oftentimes, especially when we look at humans, its numerous genes inherited that together give you a certain phenotype.2836

Human skin color, that is an obvious.2844

When we looked at that particular example of epistasis that,2847

you are going to have lots of varieties with the number of dominant alleles and recessive alleles.2851

Hair color, there is not just two or three hair colors, there is a lot of varieties.2856

Yes, environment can impact that.2862

But, hair color definitely caused by more than one gene.2865

Height is not just tall and short and medium sized, there is a whole rainbow of heights, sizes, and shapes with humans.2868

Let us look at height, in terms of why can someone like myself, be taller than both parents?2879

I'm about an inch taller, 1 ½ inch taller than my dad, and so with my brother.2885

We can also have cases where, the adult child from two parents ends up being shorter than both parents.2891

Besides malnutrition and health problems, sometimes your genes give you a predetermined height possibility.2897

How does that happen?2905

Let us pick R, S, T, and U.2906

These are the four genes, these are the height genes, it is completely arbitrary why I picked those letters.2916

I just did not want to do A, B, C, and D again.2926

There is T for tall, right in there.2930

Let us say that there are 4 genes that really impact your height.2932

Here I have got two parents and they are medium sized.2936

Let us say that the father is 5’9 and the mother is of 5’5.2939

We have got them both being heterozygous for all 4 genes.2950

If you are wondering, why do I say that the female is a little bit shorter, as average, than the male?2968

Testosterone, primarily male hormone, it tends to impact growth of men, a little bit differently than women.2977

The average height of women is shorter than men.2987

If they mate, chances are, it is it is more likely they are going to have somebody with a very similar phenotype to them.2991

They are probably going to inherit a lot of heterozygous possibilities with these 4 genes.3000

However, it is possible they can have a child with this.3005

If you do a Punnett square for each one of these in your head, you can see that for the R,3018

for this to be inherited, these are the parents, here is the P and here is the F1.3024

There is a 50% chance of this happening.3032

With S, 50% chance this is happening.3034

With T, 25% chance that happening.3037

With U, 25% chance.3041

This is less likely, you can see that only two dominant alleles may be shorter than dad and mom.3043

If this is a boy, it might be the same height as his adult mother.3050

If this is a girl, she might be shorter than mom as well.3054

Here is another possibility, they could actually have a son with this or daughter with this.3058

Look at that, this daughter or son has all the dominant alleles except for one, 7 of the 8 alleles dominant.3073

It is possible, less likely than some other combinations but 25% chance of that, 50% chance of the S inheritance.3082

This is 25% and that is 25% .3092

Chances are they are not going to have a lot of kids like this, but this individual will be taller than both parents.3095

This is an example of how you can get this wide variety of phenotypes happening because of polygenic inheritance.3100

With one gene causing one trait to occur, you tend to get less phenotypes.3107

With a test cross, speaking of phenotypes.3116

Let us say we have a sheep, in the case of complete dominance, let us say that sheep fur color with black or white sheep,3121

they have a case where this or this both equal black and this equals white.3133

When you look at a black sheep, you cannot tell what their genotype is.3151

Because, before I listed it I said that codes for black, the fact that you can have homozygous dominant or heterozygous.3158

You cannot visually tell.3167

How do you figure out, without doing an expensive genetics tests?3169

How do you figure out what that other allele is, are they homozygous dominant or heterozygous?3172

You do what is called a test cross.3178

You take a white sheep and you mate them together.3181

If this particular black sheep is homozygous dominant and they mate with the white one,3185

all of their offspring will also have that black wool.3193

You can have them mate 8 times, 12 times, 20 times, you are always going to get black wool offspring.3199

However, if they are heterozygous, approximately half will be black and half will be white,3205

because it is a 50-50 chance of inheriting the dominant or recessive with that heterozygous individual.3222

A test cross is a way to show what genotype is going on there that you cannot tell, by just looking at the individual.3229

Sex-linked traits, we have talked about a lot of things that have very little to do with male or female.3238

Other than the fact that you have male and female parent, epistasis impacts men and women equally.3244

Hair color, something both men and women inherit in the same way.3251

Sex-linked traits can impact things that happen in females a little bit differently than what happens in males.3255

Here are some examples.3262

Some alleles are inherited on sex chromosomes.3267

When we talk about the 46 chromosomes in humans, most of them are autosomes or autosomal chromosomes.3271

These are non sex chromosomes, that would be 22 pairs which is of course 44 chromosomes.3274

There are total of 46 chromosomes in our species.3285

Most of them have nothing to do with being male or female.3291

However, when we look at the X and Y chromosome, it is one pair and that is two total chromosomes and that equals to 46.3296

Women are XX, men are XY.3309

There are some other varieties where you can inherit too few sex chromosomes or too many,3312

that is covered in the second genetics lesson, you can check that out.3319

X and Y are the two sex chromosomes and actually with cats,3323

they have the same thing going on where XX is a female cat and XY is a male cat.3328

When we talk about sex linked, we can also say X-linked.3338

Some textbooks and sources you look up, call them X-linked.3344

I like to say sex-linked but it is the same thing.3348

X-linked is great to remember because it is only on the X, when we talk about the sex-linked alleles or genes.3351

The Y chromosome is really puny compared to the X.3358

The X looks like this, it does look like an X when it is duplicated like the other chromosomes.3362

The Y, here is the X, there is the Y, it is emasculating the fact that is it so tiny.3368

This where a male will inherit, a female would inherit two x’s.3378

When we actually say that here is the genotype for a sex-linked trait, it would look like this.3382

With cats, let us say we are talking about the color.3391

This is a calico cat, calico cats are only female.3396

It is possible to get a male calico cat but the male has to inherit an extra X chromosome which like I said,3406

if you look that up in the second genetics lesson, it is not a normal male.3413

But for normal sexual chromosome inheritance, calico cats are female.3419

Here is why, if the gene that causes this coat color, let us call it C, these are all different genotypes.3424

This is equaling the female varieties.3443

With the dominant, the large dominant C allele, that causes black.3448

The small one causes this orange.3457

In this case, this individual is female, is all black.3460

This female is all orange but this one is calico, where you see patches of black and patches of orange.3464

The reason why you see the patches is, in the average female cell, only one X chromosome used to be expressed.3478

One of them can be what it is called a bar body which is an inactivated X chromosome, named after Dr. Bar.3485

When a calico cat, wherever you see black, this chromosome is active, it is actually being expressed and this one is turned off.3496

Vice versa with the orange parts, this is the recessive allele on the other X chromosome is active, the other one is dominant.3503

If you are wondering what is with the white, why there are white parts?3512

There is this supposed piebald gene that I have read about, that can be inherited and it can turnoff,3517

have an epistatic effect on both of these alleles on the X chromosome.3530

Neither one of those X chromosome is being expressed in the white patches.3534

That explains the white along with the orange and the black.3539

With males, when you look at the male possibilities, since they are XY, there are only two possible genotypes.3543

Remember, the Y does not have that particular allele.3550

This is how you would write the male genotypes.3555

This male cat would be black, this male cat would be orange, that is it.3557

You are not going to have that patching look because they do not have the two X chromosomes.3562

That is an example for sex-linked traits.3569

Genetic disorders, the genetic disorders I’m going to talk about here are ones that are caused by inheritance of one gene.3573

There are other kinds of disorders/ illnesses that are caused by many different genes.3580

Sometimes doctors and scientists are not completely sure about all the effects, more research needs to be done.3587

These are disorders inherited genetically, we know it is due to one gene.3594

The first kind that I will talk about are autosomal recessive.3600

These are not on sex chromosomes, they are on pairs 1 through 22, one of the chromosomes there, one of the chromosome pairs.3603

It is caused by having the homozygous recessive case.3611

It is not always going to be the A gene, like for cystic fibrosis you will see F for fibrosis or C for cystic.3618

First, I would like to talk about is cystic fibrosis.3626

This disorder has to do with a dysfunctional membrane protein3628

that is supposed to shuttle particular ions back and forth across cells, and is not doing its job very well.3634

It is not doing what it is supposed to do.3640

That causes pooling of fluids because osmosis makes water go to where there is less water by concentration.3643

Since that membrane protein is not shuttling irons correctly,3649

you get pooling of mucus in lots of different organs, the lungs, the pancreas, in the nasal cavities, etc.3653

Sometimes, the organs gets so filled with mucus and damaged that they need a transplant.3661

Life expectancy for people with cystic fibrosis is lower than others.3666

If you have two people getting together, let us call it gene F.3672

If you have two getting together who are carriers for cystic fibrosis, what is the chance of them having a child with that, it is 25%.3677

The genotypic ratio like we have seen before is 1 to 2 to 1.3697

The phenotypic ratio is 3 to 1, in terms of their body, in terms of their health.3700

You might say, those two people are carriers but carriers do not have cystic fibrosis, they just have the potential to pass it on.3707

These three, they are all healthy.3715

This one, that 25% chance has cystic fibrosis.3720

If this person, if I change it up, who is homozygous dominant,3728

they have no chance of having a child with disease because you would end up having this as the Punnett square.3734

There would be no chance.3745

It turns out that cystic fibrosis is most common in Caucasians.3748

If you are wondering why is that, there are various disorders that have trends in certain racial groups.3752

Whether they are more common in people who have a descent versus others,3759

that has a lot to do with what is called the founder effect and human migration patterns.3763

If you watch the evolution lesson and human evolution lesson, you will see more about that.3769

Tay sachs disease, let us call it gene T.3776

I have to do the same exact Punnett square as I did before.3779

Tay sachs disease has to do with this lipid based molecule that cannot be broken down effectively in the person's body.3782

It is because of an enzyme that is not being made correctly due to the wrong abnormal kind of DNA.3791

That enzyme not breaking down a molecule, the buildup of it causes severe problems, especially in the brain.3798

That baby is usually is not going to live past the age of 5.3806

People born with Tay Sachs do not live long enough to reproduce3808

but you still have people being born with Tay sachs to this day because there are carriers out there.3812

This person is healthy for all intents and purposes, they do not show symptoms3818

because they have this allele on a chromosome and they can make that enzyme.3823

This person has a chance of making the homozygous recessive individual.3828

Two carriers getting together, it is unlikely it is going to happen but if they get together and have kids,3832

there is a 25% chance the child will have tay sachs disease.3839

That is named after the two doctors who discovered it.3842

Sickle cell disease or sickle cell anemia, we can do a Punnett square for that.3845

We will use A, let me move this out of the way.3850

Just like before, 25% chance of it happening.3866

This disease, I have mentioned this before, it makes the hemoglobin molecule inside of our blood cells not have the correct shape.3871

Instead of red blood cells being nice and round, they look like this.3880

They do not carry oxygen as well as this kind, they also get caught in blood vessels and in parts of the body.3886

It can cause severe complications.3892

Blood transfusion could be a temporary helper but they are going to continue to make this kind of red blood cell.3895

With PKU, this is known as phenylketonuria, we are going to use gene P for this.3906

That is a mouthful phenylketonuria but it is the inability to break down an amino acid known as phenylalanine.3913

The awesome thing about this genetic disorder is that, if there is a test done on the baby3922

confirming that indeed they have phenylketonuria, you can avoid symptoms of it,3927

as long as you do not feed the child anything with phenylalanine.3934

Do not give them phenylalanine.3939

By the way, if you watched the RNA lesson, there was a part of the chart,3944

the mrna codon chart where you saw PHE, that is phenylalanine.3949

Say no to phenylalanine, if your child has phenylketonuria.3955

I know that it is found in chewing gum, there are other foods that have phenylalanine.3960

As long as they do not consume it, they would not display symptoms of the disorder which is amazing.3964

This is one that you can actually control, if you have the right kind of environmental exposure.3970

Autosomal dominant, this means that instead of having the homozygous recessive case cause the disease, all it takes is this.3977

If a parent has it, it is a 50% chance of it getting passed on.3989

With Huntington's disease, let us use H.3994

This individual has Huntington's, this individual does not, the result of the Punnett square is as such.4005

It is a 1 to 1 genotypic and phenotypic ratio.4016

These people have Huntington, sad face, these people do not.4019

The reason why I'm assuming that this parent is not homozygous dominant is4025

because that means that both of his parents had to have Huntington's.4028

It would be very rare for two people with Huntington's disease to get together, get married, and have kids, very unlikely.4033

We are assuming it is heterozygous, the mother does not have it.4041

The shame of that Huntington's disease is that, the person typically would not show symptoms4044

until they are 40’s or 50’s, it is all downhill from there.4049

You get gradual degeneration, progressive to degenerative disease of the nervous system.4054

The nervous system starts breaking down significantly when they reach middle age.4060

It happens pretty fast, the onset, typically.4067

It is daunting, to think about it that, if one of your parents dies from it, there is a 50% chance of you inheriting that harmful allele.4071

I feel like most people would want to know, they do not want to get the genetic tests done4080

to see if they have the dominant from their parent or do they have the recessive?4084

Polydactyly, let us use P, this is having more than five fingers and five toes on hand or foot.4089

It is caused by a dominant allele.4097

If you have a mom or dad with 6 toes, 6 fingers, the other does not, there is a 50% chance.4099

There are various communities, the amish community, I have heard this in mountainous region in Spain,4109

a very isolated community where a lot of individuals have polydactyl.4116

When you have a small population with very little immigration and emigration, people coming in and leaving,4122

you tend to have more gene flow between related families.4129

You are keeping those harmful alleles within those you do not want in the population.4136

In the amish community you do have a higher incidence of polydactyl than the whole United States as a whole.4142

Polydactyl, Huntington’s disease, cause by inheriting one or more dominant alleles.4154

If we look at genetic disorders that are sex-linked, on the recessive side it there is red-green color blindness.4161

Here is a test to see if you are red-green colorblind.4167

People who can see that in green distinctly could see that there is the number 45 in here,4171

because they can tell apart the red and green dots.4178

If someone is looking at this and they just see a bunch of dots and cannot see a number at all,4180

they might be red-green colorblind.4184

There are actually other colorblindness tests that have a part of yellow and purple, orange and blue, etc.4186

But, red-green colorblindness are the more common ones.4195

The way that it is causes if you are male, you are going to get that or this,4199

because as I mentioned before with sex-linked traits, there are not these alleles on the Y chromosome.4211

This particular individual would not have red-green colorblindness, this particular individual would.4216

On the female side, she is not red-green color blind, she is a carrier and she actually has it.4222

It is unlikely that a female would be red-green colorblind, but it is possible.4242

It is just less common than males because she would have4246

to have a father whose red-green colorblind and a mother who is at least a carrier.4249

With guys, all it takes is mom passing on that particular chromosome because the Y comes from dad.4255

Here is an example of that.4264

Here is mom who is a carrier and here is dad who has normal vision as well.4271

Mom is not red –green colorblind but she can potentially pass it on.4285

Here is the Punnett square, daughter normal vision, son normal vision, carrier daughter,4289

son who is red-green colorblind, it is because mom is a carrier.4305

There is a 25% chance with this mating, with this cross, that they are going to have a child with red-green colorblindness.4310

Hemophilia, inherited the same way, different gene, different chromosome.4317

Hemophilia is the inability to clot blood.4323

You have blood proteins that are supposed to come together and kind of form a net like structure4327

that catches red blood cells and forms this clot that allows you to not continue bleed when you get cut.4333

Platelets would help seal up the blood vessel wall.4339

Hemophilia affects those blood proteins allowing it to clot.4343

Hemophilia literally means blood loving.4346

If I were hemophiliac, I would not love blood.4351

This just means that the blood keeps coming out.4354

They can carry around clotting factors or they are going to have to go to the ER if they get a smear cut.4357

How does it work, the same kind of way as this other Punnett square.4364

Here is an example.4367

Let us say that male with hemophilia, female whose father was a hemophiliac, she is a carrier.4373

Look at this, there is a 50% chance that they are going to have a child with hemophilia.4401

These particular individuals, carrier daughter, healthy male, daughter with hemophilia, son with hemophilia.4408

Those are some Punnett squares for these sex-linked recessive disorders.4417

Sex-linked dominant, one of the really rare kinds of genetic disorders.4421

This means that just having an X chromosome with a dominant allele is going to give you the disease.4426

You would not have carriers in this case.4432

Actually, I have read that with both of these, you tend to have cases where it is not actually passed on.4435

There is a mutation, a mistake that happens while the child is developing in the uterus that tends to cause these.4443

It is possible, it can be passed on by a parent.4450

Rett syndrome, the individual has twitches usually, mental processing difficulties.4454

Microcephaly is a very common trait associated with rett syndrome.4463

Microcephaly means an under developed brain, literally a small brain.4470

Fragile X syndrome, the X chromosome inherited actually looks like it is about to have a part fall off of it,4474

where it looks like it is kind of deteriorated or it is going to get detached.4483

Inheriting this chromosome results in fragile X syndrome, males and females can get it.4489

There are cases where the symptoms or the traits can vary a little bit from person to person4496

but mental difficulty having a long phase, oversized testicle is another characteristic in males.4503

These are both sex-linked dominant disorders.4512

Thank you for watching www.educator.com.4515