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Lecture Comments (5)

1 answer

Last reply by: Andrew Cheesman
Mon Mar 2, 2015 8:47 PM

Post by Andrew Cheesman on March 2, 2015

Hello! For the X inactivation, is this seen in female humans? If so, what characteristics are shown because of this?

0 answers

Post by Kushal Patel on December 20, 2013

Mitosis will occur? Or meiosis because its sex cells(19:15

1 answer

Last reply by: Dr Carleen Eaton
Sun Dec 16, 2012 4:51 PM

Post by Hayley Wabiszewski on December 16, 2012

so could an F3 generation female have white eyes if her father had white eyes and her mother was heterozygous?

Sex-Linked Traits and Pedigree Analysis

  • Humans have 23 pairs of autosomes and one pair of sex chromosomes. Males are XY and females are XX. Genes that are located on the sex chromosomes are called sex-linked genes.
  • In females, one copy of the X-chromosome is inactivated in each cell. The inactivated copy of the X chromosome is called a Barr body.
  • A pedigree is a diagram of a family tree that indicates the phenotype of individuals for a particular trait. Pedigree analysis can help to determine the inheritance pattern of a trait or genetic disorder.
  • Pedigrees for traits with an autosomal dominant inheritance pattern show an approximately equal number of affected males and females. Autosomal dominant traits do not skip generations.
  • When a trait is inherited in an autosomal recessive pattern, males and females are approximately equally affected. The trait may skip a generation.
  • Males are more frequently affected if a disorder has an X-linked recessive inheritance pattern. Females may be carriers and these disorders can skip a generation.

Sex-Linked Traits and Pedigree Analysis

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
  • Sex-Linked Traits 0:09
    • Human Chromosomes, XY, and XX
    • Thomas Morgan's Drosophila
  • X-Inactivation and Barr Bodies 14:48
    • X-Inactivation Overview
    • Calico Cats Example
  • Pedigrees 19:24
    • Definition and Example of Pedigree
  • Autosomal Dominant Inheritance 20:51
    • Example: Huntington's Disease
  • Autosomal Recessive Inheritance 23:04
    • Example: Cystic Fibrosis, Tay-Sachs Disease, and Phenylketonuria
  • X-Linked Recessive Inheritance 27:06
    • Example: Hemophilia, Duchene Muscular Dystrohpy, and Color Blindess
  • Example 1: Colorblind 29:48
  • Example 2: Pedigree 37:07
  • Example 3: Inheritance Pattern 39:54
  • Example 4: X-inactivation 41:17

Transcription: Sex-Linked Traits and Pedigree Analysis

Welcome to

We are going to continue our discussion of heredity with the topics of sex-linked traits and pedigree analysis.0002

Remember that humans have 23 pairs of chromosomes or a total of 46 chromosomes.0012

22 pairs of the chromosomes are autosomes. One pair is the sex chromosomes.0022

Males are XY and females are XX.0041

A male has 22 pairs of autosomes- one X and one Y.0051

A female human has 22 pairs of autosomes and two Xs for a total of 46 chromosomes.0055

Previously, when we talked about traits such as eye color, we were talking about traits that are found on autosomes.0066

And the pattern of inheritance is somewhat different than what you will see for a trait found on the sex chromosomes,0074

the reason being that males have one X and one Y.0084

Whereas, with the autosomes, both males and females have two of each chromosome- two chromosome 1s, two chromosome 2s and so on.0087

We are going to talk about sex-linked traits and how their pattern of inheritance differs from autosomal traits.0097

In the previous lecture, I mentioned the studies of Thomas Hunt Morgan who used the fruit fly drosophila.0105

And during those studies, one of the things that his lab elucidated was the idea of linked genes,0112

genes that are on the same chromosome and do not assort independently from other genes.0119

A second discovery that came out of Thomas Morgan's laboratory was the idea of sex-linked inheritance.0125

First of all, sex-linked genes are genes that are located on the X chromosome or the Y chromosome. They are located on the sex chromosomes.0133

We are going to back up a little bit now and talk about drosophila and then,0146

go into some of the studies that Morgan did that demonstrated this idea of sex-linked traits and how their inheritance patterns occur.0150

Drosophila was an excellent organism to choose because they can easily be bred.0161

They produce many offspring, and it only takes about a couple of weeks to get a new generation,0170

so easy to breed, produce many offspring, only a couple of weeks to get a new generation.0177

Also, they have distinct easily observable traits.0184

We are going to talk about things like eye color and wing type that could be observed in the laboratory.0188

Males can be visually distinguished from females, which becomes important when we talk about sex-linked traits.0195

And finally, drosophila only have 4 pairs of chromosomes.0202

We are going to talk a little bit about terminology with fruit fly genetics and abbreviations that are used, so you will hear the term wild type.0208

When we talk about a wild type phenotype, we are talking about the version of a trait that is most commonly seen in the natural population.0221

An example would be eye color. Wild type eye color is red.0233

There is an alternative form, which is white eye color, so the abbreviation works like this for drosophila.0240

White eye color, the allele for that is designated as w for white.0252

The allele for the red eye color is designated as w+. This little superscript with the plus indicates the wild type trait.0260

A second example that we are going to talk about is wing type, and normal wing type is vg+. That is the wild type- regular wings.0273

Smaller wings are called the vestigial wings. They are not fully developed.0291

Vestigial wings are just vg, so when you see this plus, it means the wild type trait.0295

Morgan started out breeding fruit flies, and after a couple of years of breeding, he noticed a white-eyed fly; and remember that red eyes are the wild types.0305

Most of the flies are red, and then, finally, he noticed "wow, here is a fly that actually has white eyes", and it happened to be a male fly with white eyes.0316

He took his fly, and he bred it with another fly that had red eyes. This was a female fly with red eyes.0331

And then, here is my parental generation, the F1 generation, and then, he got his F1 generation.0346

He had both male and female flies in the F1 generation, and it was all red eyes; and it turns out that white eyes are recessive to red eyes.0354

So, it is not a surprise that he got all red eyes in this generation.0366

Then, he went on and he crossed the F1 generation- some male flies in this generation, about half male, half female, and then, he went on.0372

He got his F2 generation, and all of the females in this generation had red eyes. There were no white-eyed females.0384

Half of the males had white eyes, and half of the males had red eyes.0399

When he looked at these as a whole, he did not look at whether it is a male or a female fly and just, kind, of counted them up.0410

He said "yeah, I have the expected Mendelian ratio of 3:1 red eyes to white eyes that you would expect if you were crossing a heterozygous".0416

But this did not explain the fact that no females had white eyes.0429

Well, it turns out that the allele for eye color in fruit flies is on the X chromosome.0441

Let's talk about how this is going to affect inheritance, so let's move over here and try a couple of different things and see what we end up with.0449

Let's see what we would end up with if this eye color trait was on an autosome.0461

And then, we are going to see what we end up with if it is on a sex chromosome.0467

If eye color was on an autosome, then, I have my parental generation.0472

And let's say that they are true breeding, so we are going to end up with my male w-w and the female w+- the wild type here.0479

Get the F1 generation, and all of the F1, males, females, they are all going to be heterozygous; so this is white eyes, red eyes- all red.0493

Then, I get the F2 generation, and I would be crossing a male and a female; and there is multiple possibilities that I could get w+-w+.0509

I could get the w+ with this w, and what I am going to end up with then, I could get w and w+ and w-w, OK?0534

So, I have different possibilities. I could get w+ from dad with w+ from mom.0557

I could get w+ dad with w mom, w from dad with w+ from mom and w-w for homozygous recessive, so just your basic monohybrid cross.0562

And what I am going to end up with there is my homozygous dominant with red eyes0576

I am going to end up with some flies that are heterozygotes. They are going to have red eyes, heterozygous- red eyes, homozygous recessive- white eyes.0580

And it is going to be my 1, 2, 3, 4, so 3:1 ratio.0590

What this does not explain is why there is a difference, why there were no females with white eyes.0597

Now, let’s try this cross again with the eye color gene on the X chromosome.0608

Now, we are going to have the parental generation. We are going to have the male X.0615

And we are going to do superscript w to show that he is carrying that white eye color allele on the X chromosome.0622

And then, he has a Y. He does not have a second X.0628

The female wild type is going to have X with the w+ for red eyes: Xw+.0631

So, he is going to have white eyes. She has red eyes.0640

We crossed them, and then, we get the F1 generation, and for the F1 males, they are going to get Y from dad.0643

From mom, they are going to get the wild type allele. They will have red eyes.0657

Females: from dad, in order to be a female, you have got to get an X from each parent, so they are going to get the mutant allele for white eyes.0666

From mom who is homozygous dominant, they are going to get the wild type allele.0675

And because red is dominant to white eye color, these individuals will also have red eyes.0684

Now, the F2 generation is where things get interesting.0691

First possibility: we have a male, so we know he is getting Y from dad.0699

All the males have to get Y from dad. From mom, they get an X.0708

Half of the males will get the X with the mutant allele. The other half will get the X with the wild type allele.0712

Females: females have got to get an X from each parent, so they will get the X with the wild type allele from dad and from mom.0721

So, half of the males will get this. Half of the males will get that.0736

Half of the females will get the X carrying the mutant allele from the mom.0741

The other half will get the X carrying the wild type allele from the mom, so half of the females, half of the females.0747

What we are going to end up with, then, if you look, is this male, even though we would say "oh, well, white is recessive to red", there is no second allele.0756

All there is, is white. There is no red to cover up this white allele.0766

Therefore, even though he only has one allele for white eye color, he ends up with white eyes.0771

Half of the male flies do. The other half have red eyes.0780

The females however, have two X chromosomes, so they end up with two alleles.0786

They are not going to have white eyes because they have to get an X from their father and from their mother.0792

And the father is only carrying the red allele so when they get that from him, all it takes is one red allele to get red eyes.0800

Therefore, none of the females are going to have white eyes0808

So, what you will end up with, then, the F2 generation are red-eyed females- all red females,0812

half of the males having red eyes and half of the males having white eyes.0823

And this shows a typical inheritance pattern for an X-linked recessive trait.0827

Here, we cannot say that this male is homozygous because he does not have two of the same allele.0833

We cannot say that he is heterozygous because he does not have two different alleles.0838

What we say in this case is that the male is hemizygous.0842

For an X-linked trait in a male, since there is only one X, we say that he is hemizygous for those traits.0845

Whereas, a female has two alleles, which maybe different or they may be the same.0855

When talk about sex-linked traits, we mostly talk about the X chromosome.0862

There are Y-linked traits, but there is only about 7D genes on the Y chromosome.0866

And if you did see a case of a trait that is inherited via the Y chromosome,0871

what you are going to see is transmission from father to son because all of the sons will have the father's Y chromosome.0879

Continuing on to focus on the sex chromosomes, in females, there are two copies of the X chromosome, and males have only one.0889

The fact that males only have one tells us that we do not actually need expression0899

necessarily of all the genes on the X chromosome with both alleles. Male only need one allele.0904

And in fact, what happens in females is that one copy of the X chromosome is inactivated in each cell, and this occurs during embryonic development.0910

The inactivated copy of the X chromosome is known as a Barr body.0922

And the chromosome becomes condensed, and it moves over to the edge of the nucleus.0925

This X inactivation is different for different cells. One cell may have the paternally derived.0932

So, you have two X chromosomes if you are a female- one from your mom and one from your dad.0941

Some cells might have the maternally derived X inactivated, whereas, another cell...0946

So, this X might be inactivated on one cell, and then, there might be another cell that has the other X inactivated.0952

And this occurs during embryonic development, and then, during mitosis, the same X would be inactivated from daughter cells.0960

Mitosis occurs, then, and this offspring cell is going to have the same X inactivated as its parent cell; and that is going to form a Barr body.0971

The important point is that either X can be inactivated, either the one from the father or the one female got from her mother.0990

And this process occurs during embryonic development, and then, during mitosis, cells passed along that pattern of X inactivation -0999

so, whichever one happens to be inactivated - when that cell divides, its offspring are going to have the same X inactivated.1007

The result of this is kind of interesting.1013

Oh, one other note, both Xs are usually are active in the ovary, so somehow, the other X becomes reactivated.1016

Calico cats provide an interesting example of X inactivation.1024

In calico cats, they have patches you see here like orangish, yellowish fur, black and then, white fur.1032

And the fur color genes for black fur and yellow fur are actually...the alleles are located on the X chromosomes.1040

We have the X chromosome, and if the cat gets X with the b allele, that is black fur. If the cat has X with the y allele, that is, is yellow fur.1051

We will talk about the white fur in a minute. But for now, let's say we have a female cat, and she is heterozygous.1068

She is, therefore, carrying an allele for black fur and an allele for yellow fur.1079

However, in some of her cells, in certain cells, this X is going to get inactivated.1085

In other cells, this X is going to end up inactivated, and then, mitosis occurs; and the daughter cells will have the X superscript y active.1095

The others will have the X with the b active.1106

In areas where the cells have the y allele, the X with the y allele on it active, the other one is a Barr body, you are going to see yellow fur.1109

The cells with this pattern of inactivation, you will see yellow fur.1119

Other cells have the X with the Y allele inactivated, and the X with the black allele is active; and that is where you are going to see patches of black fur.1124

It turns out that the gene for white fur is controlled on a different allele, and that is what accounts for the white fur.1134

Anyways, females are actually genetic mosaics.1142

She is showing a different phenotype in different areas due to what we call genetic mosaicism.1148

Or we could say she is a genetic mosaic, the patchwork of different phenotype.1157

A very useful tool in genetics is the pedigree.1165

And when we are studying human genetics and particularly genetic disorders, we can study the pattern of inheritance in families using a pedigree.1170

And what a pedigree is, is just a diagram of the family tree, and on it, it indicates the phenotype of individuals for a particular trait.1181

We are going to go over three major patterns that you should recognized starting out, though, with just general facts about pedigrees.1190

A square indicates a male. A circle indicates a female.1200

This line here shows their offspring, so they had one son and one daughter. The daughter married this male, and they had 1, 2, 3, 4 offspring.1208

Darkened n means the individual is affected, so affected by a disorder or demonstrating the trait, so this is an unaffected male.1221

He could be a carrier for a disorder - we do not know - but he does not have the phenotype.1234

Here, we have an affected male, and here is an unaffected female; and then, it shows the parental F1, F2 and so on.1240

The first pattern of inheritance that we are going to discuss is autosomal dominant, and a classic example of this is Huntington’s disease.1253

Huntington's disease is a progressive neurodegenerative disorder.1262

The onset of symptoms are usually around middle age, and they include cognitive decline, dementia and jerking movements that are called chorea.1267

These movements are called chorea. This is sometimes known as Huntington's chorea.1278

We are looking here at a typical autosomal dominant pattern pedigree, and there is a couple of ways to help you identify this one.1286

What I noticed is that I have 1, 2, 3, 4 affected males and 1, 2, 3 affected females.1295

That is close enough for me to say this is likely not a sex-linked disorder.1303

When you are working with studies, the more family members, the bigger the family tree, the better.1310

But with people, it's not like we're growing fruit flies in the lab, so this is what we have.1316

And 4 and 3 is close enough for me to say that males and females are approximately equally affected.1322

It is not like I am seeing all males or 8 males and 1 female even or something, but they will make it clear on the test.1332

In reality, it is not always quite as easy to see.1341

The second thing is that autosomal dominant disorder does not skip a generation.1345

When I see an offspring, I look back, one of the parents is affected offspring, affected parent, affected offspring, affected parent.1358

I did not see this skipping any generations to where there is parents who do not have the disorder, and then, the offspring do have it.1370

So, this tells me that this is likely an autosomal dominant inheritance pattern.1378

Autosomal recessive inheritance shows a different pattern on the pedigree. Examples of this: cystic fibrosis, Tay-Sachs disease and phenylketonuria.1385

Starting out actually with Tay-Sachs disease. This is a genetic disorder caused by a decrease in an enzyme that is needed to breakdown a lipid.1398

Unfortunately, when this lipid accumulates, it causes very serious neurological damage with seizures,1408

blindness and eventually death usually at a very young age by the age of 3 or 4.1413

This disorder is more common in Ashkenazi Jews, and there is a blood test for carriers of this disease.1420

Cystic fibrosis is a disease that primarily affects the lungs and the pancreas.1429

It is the most common lethal genetic disease in the US and there is a test for some of the alleles that cause this.1438

It is more common in people of Northern European descent.1444

What ends up happening in the lungs is that they end up plugged with very thick mucus resulting in coughing, shortness of breath, respiratory infections.1449

And with the pancreas, due to malfunction of the pancreas,1458

digestive enzymes are not secreted properly, and individuals end up not being able to digest and absorb their food.1462

Finally, phenylketonuria, often called PKU, results in the inability to metabolize the amino acid phenylalanine.1471

And the results of this is that a by-product builds up that can cause brain damage.1483

This is actually screened before birth. It is a preventable cause of mental retardation.1491

Individuals who have this disorder have to be on a very strict diet of extremely low phenylalanine.1495

OK, just to give you some background about these disorders as we talk about1504

the application and looking at the pedigree for autosomal recessive inheritance.1509

When we look at this pedigree, one thing that you are going to notice again, this is autosomal, and I see 1, 2 males affected and 2 females.1513

So, males and females are approximately equally affected.1523

The second thing you will notice is that the disorder may skip a generation.1532

And you know this from earlier, talking about Mendelian genetics, that with the recessive trait,1542

in order to exhibit the trait, the individual needs to have the homozygous recessive.1549

An individual can be a carrier. They do not demonstrate the phenotype, but they can pass along that allele to their offspring.1554

Let's say here we have an affected male. Let's say he is little b-little b, and the little b causes the disorder, big B is the normal allele.1568

She is big B. We do not know what else she is.1584

Now, one thing I do know is that this individual is a carrier because they have the normal phenotype, but their offspring is affected.1587

She also is the normal phenotype, so she has got to have a big B, but her offspring is affected.1597

You say that the offspring have to be homozygous recessive. The only way that can be is if they inherit an allele from each parent.1603

The parents are heterozygous.1614

Autosomal recessive inheritance: males and females are approximately equally affected, and it may skip a generation.1620

This is different than X-linked recessive inheritance. We just talked about sex-linked genes today.1628

And examples of disorders with X-linked recessive inheritance are hemophilia, Duchenne muscular dystrophy and color blindness.1633

Hemophilia is a disorder of clotting. People with one type of hemophilia have a lack of factor VIII, which is a factor needed for clotting.1642

And when they are injured, they are at risk of very serious bleeding.1652

This can be treated, though, with the replacement of the factor VIII and transfusions if needed.1657

Muscular dystrophy results in progressive muscle weakness. Eventually, a person usually becomes wheelchair-bound, and it is even fatal.1664

That is also an X-linked recessive disorder.1676

Color blindness, most commonly, it is difficulty to distinguish between shades of red and green, and that is X-linked recessive inheritance, as well.1679

Looking at this, one thing you will notice is that there are 1, 2, 3 affected males, so males more commonly affected.1688

Females can be affected, but they are not as likely to be affected because1700

let's say we are talking about hemophilia, and big H will be the normal allele.1706

Little H will be the allele for hemophilia for not producing functional factor VIII.1713

A male, if he is hemizygous, he gets one of the mutant allele, he would have hemophilia.1721

A female could get hemophilia if she has two mutant alleles. If she has only one mutant allele, she is a carrier.1729

If a female had a father who had hemophilia and a mother who either had hemophilia or was a carrier,1741

she would get the hemophilia gene from her father and 50/50 chance of getting that from her mother, as well.1753

So, it is possible for females to have X-linked recessive disorders, but it is much rare than it is with males.1759

Males are more commonly affected, and it can skip a generation.1766

We have seen three of the more common inheritance patterns that you will need to know,1777

which are autosomal dominant, autosomal recessive and X-linked recessive inheritance.1782

Let's go ahead and do some examples.1787

Example one: color blindness is an X-linked recessive trait.1789

A man is color blind. His daughter Sarah has normal vision.1794

OK, let's use pedigrees to help us out.1800

We have a man who is color blind, and his daughter has normal vision.1802

Since this is X-linked trait, let’s say that X big C is the normal allele. The vision is normal.1809

X little c is the allele for color blindness, and we said this is X-linked recessive.1819

The man is color blind. His daughter Sarah has normal vision.1836

Sarah marries Jack, and he also has normal vision. What is the probability that they will have a child who is color blind?1840

When we do not know the sex of the child so we just use this diamond shape, so we do not know.1852

And they want to know the probability of a child who is color blind and the probability that Sarah and Jack will have a child who is a carrier.1858

Now, I need to start figuring out genotypes.1868

I know that the father of is Sarah. Here is Jack.1873

Here is the dad. He must have the genotype X little c Y because he is color blind, so he has got to have that allele.1881

Sarah received that allele from him.1894

Since she is a female, she did not get the Y allele. She got the X.1897

I do not know what Sarah's mother's genotype is.1901

And it actually does not matter because I know that Sarah has, at least, she has got to have one big C because she is not color blind.1905

I know that she has one big C because she has normal vision, and I know she got the little c from her father.1915

Sarah is heterozygous- X little c, X big C.1920

Jack has normal vision. He is a male so he has a Y.1924

He has also got an X with the big C because he has normal vision.1927

Now, I know the genotypes of Sarah and Jack. What is the probability they will have a child who is color blind?1931

I can do this using probabilities or a Punnett square. Let's try probabilities first.1940

A child who is color blind, if it is a male, would be X little c Y, so this is a color blind male.1949

A female who is color blind would have to have X little c, X little c, and this is a color blind female.1959

To have a daughter, both Jack and Sarah need to donate an X, but Jack only has an X with the big C.1970

Therefore, Jack and Sarah will never have a daughter with color blindness unless there is a new mutation or something.1979

But if Jack gives his X big C, Sarah could give either of these, it does not matter.1985

The daughter will have one X big C. She will have normal vision.1993

So, in order for their child to be color blind, it needs to be a son.1997

What this is really asking is what are the chances that they will have a son, and what are the chances that he will inherit the color blind allele from Sarah.2002

The chance that they will have a color blind child, the chance of it being a male is 1 out of 2.2014

If the offspring gets the Y chromosome, his going to be a male, so he gets the Y, there is 1/2 chance.2023

In order for him to be color blind, he needs to get the X little c from his mom. The chances of that are 1/2.2033

1/2 x 1/2 is 1/4. The probability that they will have a child who is color blind is 25%.2042

What is the probability they will have a child who is a carrier?2054

A male cannot be a carrier for an X-linked disorder. It has to be a female, and a female who is a carrier would have an X big C and an X little c.2058

Therefore, this child is going to need to be a female, so she needs to get the X big C from dad. The chances of her getting that are1/2.2072

The chance that she will, then, be a carrier depends on is she going to get the little c from her mom. The chances of that are 50/50.2084

So, in the multiplication rule, the chances of this and of this occurring multiply the probabilities of each occurring, and we get 1/4.2093

This plays out if you look at it as a Punnett square.2102

The Punnett square is going to help us just to diagram, and from the father, the gametes are going to be X big C and Y.2108

From the mother, the gametes are going to be, from Sarah, gametes are going to be X big C X little c.2129

Probabilities: X big C - this is a big C - X little c, X big C Y, X little C X - he has got X big C, let's make that clear - X little c Y.2139

What we have is 1/4 of the offspring are color blind- affected males. Then - let's make this clear - this is big C X big C X big C.2168

This individual is a female - so this is 1/4 - who is not a carrier. She is homozygous dominant for normal gene.2194

1/4 are normal males - do not have color blindness - and 1/4 are carrier females.2202

The chances of color blind offspring is 1/4. We see that right here.2212

The chance of the offspring being a carrier is 1/4. We see that right here, and here is the other two possibilities.2217

Example two: some members of the family whose pedigree is shown below have cystic fibrosis, a disease with an autosomal recessive inheritance pattern.2227

What is the genotype of the individuals marked number 1, 2 and 3?2237

Use big F to indicate the dominant allele and little f to indicate the recessive allele.2243

We can see typical autosomal recessive inheritance pattern because I see roughly equally affected males and females.2249

I have got two females affected, one male, and it does skip a generation.2259

So, they are asking me number 1, 2 and 3. What are their genotypes?2266

Well, in order to be affected, since this is autosomal recessive, an individual must be homozygous recessive.2274

Homozygous recessive, they are going to be affected by cystic fibrosis.2282

Heterozygotes are carriers, and then, we have individuals who are homozygous dominant.2290

They are neither carriers nor they are affected by the disease.2297

I really do not even need to look up here.2301

I can go straight to here and say that this couple, neither of them is affected yet, they have offspring with the disease.2304

Since they have offspring with the disease, but they are not affected, they have to be carriers.2315

This individual and her husband are heterozygotes.2321

And then, they each passed on - here is my number 2 - anyone who is affected I know is little f, little f.2326

Number 1 is a carrier. She is a heterozygote.2337

Number 2 is affected. She is homozygous recessive.2341

Number 3 is not affected, so I know she has one normal allele. However, her mother has the disease.2344

The only possibility that she can get from her mother in terms of allele is this little f.2361

See, the father has at least one normal allele. We do not know what his second allele is.2367

So, number 1 is a carrier. Number 2 is affected and number 3 - both of these actually - would be carriers.2377

Example three: Below is a pedigree for the tongue rolling trait?2396

Individuals with the ability to roll their tongues are shaded. Non-rollers are not.2399

What is the inheritance pattern for the ability to roll ones tongue?2406

When you look at a pedigree, one of the first things you are going to look at is are males and females equally affected?2411

1, 2, 3 affected males, 2 affected females.2418

I have got both males and females affected, so this is likely to be an autosomal pattern of inheritance.2422

I also see that this trait does not skip a generation.2433

These two are affected and so as their father. These two are affected and so as their mother.2441

I do not see any situation where there is offspring affected, and the parents are not affected.2445

If I saw this offspring affected and these parents were not, it is skipping over, but I do not see that.2451

This must be or is likely to be autosomal dominant, and in fact, tongue rolling is an autosomal dominant trait.2460

The ability to not roll ones tongue is autosomal recessive.2470

Example four: ectodermal dysplasia is a disorder, and this disorder may result in the lack of sweat gland production.2478

One form of ectodermal dysplasia is due to a mutation in a gene on the X chromosome.2486

Females who are heterozygous for the disorder have some areas of skin with sweat glands and other areas without sweat glands.2493

So there are patches of skin that have sweat glands and then, other patches that do not.2503

How can X-inactivation account for this finding?2506

So, this is a disorder that is found on the X chromosome.2510

Let's say that sweat glands is big S, and no sweat gland production is little s; so ectodermal dysplasia is little s.2518

If a female is heterozygous she is a big S-little s.2538

Recall, though, that X-inactivation occurs in females, and they form Barr bodies from one of their X chromosomes; and it is different in different cells.2542

In some cells, a female could have the little s X chromosome inactivated.2553

In other cells, the normal allele is going to be the one inactivated.2559

And then, her cells are going to undergo mitosis, and all the cells around that cell, that are offspring of that cell, are going to have the same X inactivated.2565

Therefore, cells that have the normal S active produce sweat glands, and those patches of skins are going to produce sweat glands.2573

Those groups of cells that have the normal allele inactivated and have the mutant allele active will not produce sweat glands.2587

And this is an example of genetic mosaicism similar to what we talked about with calico cats2598

and the tricolored pattern that you see with fur on calico cats due to genetic mosaicism.2610

That concludes this lecture of