AP Biology Mendelian Genetics

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We are going to start our discussion of heredity with the topic of Mendelian genetics.0002

Let's start out with an exercise. Try to quickly clasp your hands together, and then, look down and see which thumb is on top.0007

Most of you will actually have clasped your hands so that your left thumb is on top. That is actually the dominant trait.0018

Some of you will have your right hand on top. That is the recessive trait.0026

And what we are going to be talking about in today's lesson is going to explain the inheritance of dominant versus recessive traits, and this is just one example.0029

Let's go ahead and get started with Mendelian genetics.0038

To give you some background, Mendelian genetics is named after Gregor Mendel.0044

Mendel was an Austrian monk who performed studies on pea plants in the mid-19th century.0047

And using his observations, he described fundamental principles of inheritance that came to be known as Mendel's laws.0055

Prior to Mendel's discoveries, people believed that the offspring of plants, animals and humans had traits that were a blend of the traits of both their parents.0065

And this was known as the blending hypothesis, so this predated Mendel.0077

For example, if one parent had the blue eyes, and the other had brown eyes, according to the blending hypothesis, the offspring, the children of that couple0086

would have eyes that were a color between the two, maybe light brown or very, very dark blue, so a blend of the two, but it does not always work this way.0099

As you know, the father and mother who both have dark brown eyes can actually have a child with blue eyes.0109

Or parents who the father is very tall and the mother is very short, their kids do not all end up with medium height.0116

So, this theory did not really explain how inheritance worked.0124

Through careful studies of pea plants, Mendel was able to determine that inheritance is through a particulate mechanism, so particulate inheritance.0129

Particulate inheritance describes traits as being passed along via discreet units.0149

Inheritance is through discreet units, and we, now, know the modern term for this is genes.0158

Back in Mendel's time, they did not have the concept of genes, so he called them units; but it is the same idea.0168

Before we go on to Mendel's experiments and Mendelian inheritance, we are going to go through some terminology, just an overview.0175

And then, we are going to go into detail about these concepts throughout the lesson.0182

Starting out with gene, a gene is a section of a chromosome that contains the information about a trait.0186

And in lectures earlier on in the molecular genetics, molecular biology lectures, we talked about DNA, RNA and protein, and we talked about the concept of genes.0194

But just to review, it is a section of a chromosome that contains the information about a trait.0206

And on the molecular level, when we say "contains the information", what we actually mean is the DNA sequence.0226

The DNA sequence - I will put that up here - for a trait.0232

Locus: the locus is the location of a gene on a chromosome.0238

We might say the locus for height. If we have a chromosome, we mean right here.0243

If the gene for height is located right there, we would say this is the locus for height.0252

This is the location of a gene on a chromosome. The plural form is loci- L-O-C-I, this plural.0260

Allele: alleles are alternative forms of a gene.0278

For example, we would say there is a gene for eye color, and two alleles would be a brown allele for brown eyes or blue allele for blue eyes.0289

We could say there is a gene for height. Example: the gene for eye color has two alleles- blue and brown.0300

The gene for height that determines height, there could be a tall allele. There could be a short allele.0314

When we talk about just simple Mendelian inheritance, there are only two alleles per trait.0325

What we are going to be talking about today with pea plants, there will be a tall allele, a short allele, for flower color, purple allele, white allele.0330

In reality, it is often more complex.0339

But a lot of the problems you will see in the AP test, and the problems we will work on today, we will just assume that there is two alleles per trait.0341

The dominant allele: if an allele is dominant, only one allele is needed for the genotype to be expressed, so one allele needed for expression.0348

What I mean by that is recall that humans and many, many organisms are diploid, so if an organism is diploid,0365

they are going to have two copies of each chromosome- one maternally derived, one paternally derived.0376

Let's say that we have a cell, and there is, for human, 46 chromosomes, 23 sets, and on chromosome 1, there is the allele for eye color.0386

And there would be a chromosome 1 from your father and a chromosome 1 from your mother.0399

Maybe your father gave you the blue-eyed gene, the blue-eyed allele for the eye gene, the trait eye color.0403

And maybe from your mother you got the brown-eyed allele.0415

Well, only one allele is needed for expression of the dominant trait, so it turns out that brown eyes are dominant.0421

So, if you have one blue eye allele and one brown eye allele, your eyes would be brown according to the type of expression.0428

The terminology or the symbolism used in Mendelian genetics for a dominant allele is a capital letter.0443

If we are talking about eye color, brown eyes would be big B. That is the dominant allele.0452

The recessive allele, two alleles are needed for expression of that form of the trait.0459

An example would be blue eyes. Little b is blue.0470

So, this individual has a big B, brown and a little b, blue. This individual has brown eyes.0475

There may be another individual who has received from both their mother and father blue-eyed allele- blue from mom and blue from dad.0484

This individual would have little b-little b and, therefore, blue eyes.0496

You can think of it as the brown takes over. It covers up the blue allele, so therefore, it is dominant.0501

Just another example is height.0510

If tall is dominant, then, we would say that the tall allele is big T, and if short is recessive, then, we would have a little t and say that is short.0513

An individual who is big T-big T would be tall. An individual who is little t-little t would be short.0524

And an individual who is one big T-one little t would be tall because when there is a dominant allele, it does not matter what the second allele is.0533

Any individual with the big T, no matter what the second allele is, will be tall.0543

An individual with a big B, no matter what the second allele is, will be brown-eyed according to Mendelian genetics.0547

And again, we are going to go through all of this step by step, but just to give you a vocabulary to be able to understand what we are going over.0555

Genotype: I already mentioned the word genotype, but formally, that is the alleles an individual has for a trait.0563

This person has the genotype big B-little b, brown-eyed allele and a blue-eyed allele.0579

This individual has the genotype little b-little b. Here, we might have an individual big T-big T.0588

So, it is the alleles that they inherited, brown and blue; blue and blue; tall and tall.0596

Phenotype is the physical manifestation of the genotype, so it is the physical manifestation of the genotype.0602

Here, the genotype is big B-big B, so example would be big B-big B. I would say that is the genotype; the phenotype- brown eyes.0619

Here, the genotype is little b-little b; phenotype- blue eyes.0634

Here, genotype- big T-big T; phenotype- tall height.0639

Homozygous means that both alleles are the same.0646

If an individual is homozygous dominant, they would have two dominant alleles.0664

This individual is homozygous dominant, so example would be big T-big T- homozygous dominant.0670

If an individual is, say, little t-little t, they have both alleles for short height,0681

then, we say that individual is homozygous recessive concerning their genotypes.0687

So, the genotype here is homozygous recessive. The genotype here is homozygous dominant.0694

Heterozygous means the alleles are different.0700

This individual has an allele for brown eyes and the allele for blue eyes, so they are heterozygous for the trait of eye color.0706

Penetrance refers to the proportion of people with the particular allele who express the phenotype,0719

so proportion of individuals with an allele who express the phenotype. Now, what does this mean?0728

Well, when we talk later today about Mendelian inheritance,0747

we are going to assume full penetrance meaning if an individual gets an allele for brown eyes, she will have brown eyes.0751

If an individual is heterozygous for height, big T-little t, we expect an individual to be tall. He will be tall, so that is full penetrance.0761

A lot of times when we talk about penetrance, we are talking about diseases, so let's talk about disorders that have a genetic basis as an example.0773

One example is a mutation that can occur in - or mutations - in the BRCA1 and BRCA2 genes.0784

Mutations in these genes have been associated with a greatly increased risk of developing breast cancer. However, there is not complete penetrance.0794

Complete penetrance means if somebody has the allele, they show the phenotype.0803

Complete penetrance with eye color would mean 100% of the time that somebody has the brown genotype,0809

big B-little b or big B-big B, they have brown eyes. That is complete penetrance.0817

Looking at mutations in these genes, if somebody gets the mutated allele of BRCA1 and BRCA2,0823

they have an increased chance of breast cancer, but it is not 100%.0830

Let's say that it is 80%. Let's say they have an 80% chance of developing breast cancer.0833

That means that the penetrance is 80%.0839

Complete penetrance is 100%. That means an individual has the allele.0843

They will express the trait. When we talk about Mendelian genetics, we are going to assume full penetrance.0849

Expressivity is a little bit different. This refers to the range of symptoms a person could have for a disease.0856

If penetrance means they have the disease, they do not have the disease or the trait, but within that, there could be variable expression of it.0866

You could have influenza. You could have the flu.0875

You might have just a little bit of coughing and a low-grade fever.0879

Somebody else gets the flu. They might be very, very ill, have respiratory distress, very high fever, get extremely sick.0883

Those are not genetic disorders, but just to give you an example of range of symptoms.0894

When we are talking about a genetic disorder, same idea with the range of symptoms, so a range of symptoms.0899

Let's say the penetrance was 70% if somebody gets an allele for disease, and the 70% of individuals with the allele get the disease.0910

But within that, there is variable level of expression.0921

Some of those individuals, lot of symptoms. Some only have a few symptoms.0925

With the terminology down, we are going to go on and talk about the experiments that Mendel performed.0933

And Mendel used pea plants as a method of studying inheritance.0937

And pea plants were an excellent organism to use, and the reason is, they had advantages such as the ability to both self-pollinate and to cross-pollinate.0942

They had traits that are easily distinguishable. You could just look and see, there is flower color that is different or the height or the seeds.0956

They also produced a large number of offspring. To have a statistically significant result, you need a lot of offspring to tally up.0964

I will list the traits that he looked at. You do not have to memorize all these, and we are just going to focus on a couple of them.0976

But just so you know, he studied the length of the stem, which gives the height; pod shape; seed shape;0981

seed color; the position of the flower; the color of the flower; and the color of the pods.0988

These are the traits that he studied, and each of these traits came in two forms.0993

Looking first here at height, height in pea plants, they could be either tall or short.0999

And we are going to have, just as I mentioned, big T is going to be the allele for tall, and little t is going to be the allele for short.1006

And at this point, of course, when he started out, Mendel did not know about alleles and all that.1016

So, what he started out doing first is establishing that the plants he started out with were true breeding.1021

And by true breeding, that means that when he self-pollinated a tall plant, all of the offspring were tall.1029

When he self-pollinated a short plant, all of the offspring were short- over and over and over.1038

If he took one tall plant, that when it was self-pollinated, always a tall offspring, and he took a tall plant like that and crossed it.1043

By crossing, we mean he mated it with another tall plant. All of those offspring were tall, so they were true breeding for a trait.1052

The offspring had that trait as long as they were bred with another true breeding tall plant1059

or the short with the short or crossed with another true breeding plant.1065

Looking at some more terminology when we talk about these crosses or these matings, you will see this frequently used.1074

So, this capital P stands for the parental generation.1083

The next generation is F1, the offspring from the parental generation, and this F means filial, so F1 and then, F2, so filial 1, filial 2.1094

Once Mendel had determined that he had true breeding tall plants and true breeding short plants, then, he went ahead, and he crossed them.1109

He mated them, and he was able to do this cross-pollination by cutting this stamen.1120

The stamen is the male reproductive part in the plant. It produces the pollen, so it is the male reproductive organ on the plant.1126

And he cut those off before they were mature so that the plant could not self-pollinate.1139

He, then, took that pollen from the stamen and dusted it on the carpal. The carpal is the female reproductive organ in a plant.1143

He did these cross-pollinations with a tall plant and a short plant.1153

And he did many of these, and what he found was that all of the offspring turned out to be tall.1159

So, here, we have a tall plant, and then, here, we have a short cross in the parental generation- the F1 generation, all tall.1167

According to the blending hypothesis, what you would have expected crossing a tall and short plant is a bunch of medium height plants.1180

That did not happen, and for now, the short trait seemed to be gone.1188

This finding right here, first of all, did support the particular model that it is not just blend between the two.1194

But he went further and really showed what happened.1204

What Mendel, then, did was self-pollinated these F1 plants and then, looked at their offspring.1207

And what happened is the short height reappeared. The short phenotype reappeared.1216

And he found that there was a 3:1 ratio of tall to short, so it was 3:1 ratio of tall to short.1222

The allele or the form of the trait for height that is the short trait stayed intact.1242

It did not blend together with the tall and get changed in some way. It stayed intact, but it just remained hidden for one generation.1251

And now, we are going to talk a little bit about the observations he made and the laws that he derived from this,1258

and talk about how chromosomes and what we know about organisms being diploid can account for these findings.1268

Mendel made some observations and some conclusions, and then, based on the conclusions, we get Mendel's laws; and one of them is the law of segregation.1279

Before we go into that, let's look at Mendel's conclusions. One of his conclusions was that inheritance of each trait is determined by units.1287

Again, we call these units genes now. That is what we realized that they are and that genes are located on chromosomes.1314

The second conclusion that he drew is that an individual inherits one unit from each parent.1325

And you know this already from the molecular biology that we are diploid and that we inherit one of each type of chromosome from each parent.1341

The third conclusion that Mendel drew is that a trait may not show up in an individual, but it can still be passed on to the next generation.1352

So, just abbreviating that as a trait may skip a generation.1367

Let's look at what is happening. The parental plants were true breeding.1378

If they were true breeding, it turns out that all the alleles that they carry, the only alleles that they had for the tall plants, the tall allele.1385

So, they were homozygous dominant. They were homozygous dominant.1395

The short plants carried only the short alleles, the short height alleles, so they were homozygous recessive.1406

And what the law of segregation is telling us is that for any given trait, the pair of alleles will separate, and each gamete receives only one of the alleles.1420

In the P generation, we have got a tall parent who is diploid, and we have the short parent who is also diploid. They form gametes.1428

Let's say that this is the mother. This is the father.1438

Then, gametes are going to form for the female parent, and during meiosis, homologous chromosomes separate.1442

The egg is going to end up with just one of each chromosomes, so here, we are going to end up with gametes carrying just one big T.1457

That is the only choice because there is no other type of allele in there.1466

Here, the alleles also separate to form male gametes, and each of those will carry a little T.1470

Now, when these are crossed, fertilization occurs. Pollination occurs, and the gametes will join to form a zygote, and that is in the F1 generation.1481

The female parent is going to donate a gamete containing a big T. The male parent is going to donate a gamete containing a little t.1498

And so here, we have the F1 generation, and they are all heterozygous.1507

There is no other choice. There is no other combination because they are going to get one allele from each parent, so they are heterozygous.1518

Because tall is dominant, the heterozygotes are going to be tall because a dominant trait only requires one allele for phenotypic expression.1528

In order to get expression of the recessive trait, there needs to be two alleles. There needs to be two alleles.1539

The only way you are going to get short height is if the individual is homozygous recessive.1545

Now, how did short height reappear?1551

Well, in the F1 generation, once again, gametes are formed.1554

And this time, the pair of alleles separate as usual, and gametes form one allele in each gamete.1561

Some gametes get the big T. Some gametes get the little t.1570

You have all these gametes, and now, self-pollination occurs, and you could get a variety of combinations.1577

The big T from the mother could pair up with the little t. It is self-pollination, so it is the same plant, but still, there would be male and female gametes.1591

Big T with the little t- that is one possibility for the F2 generation.1603

The mother might donate a big T. The father might donate a little t.1609

The mother might donate a big T, and the father donates a big T.1616

The other possibility is that the mother donates the big T. Father donates the little t, or the mother donates the little t; and the father donates the little t.1626

These gametes can mix and match, and we are going to lay this all out in a couple minutes in a Punnett square.1637

But the idea is that the gametes, some are carrying big T. Some are carrying little t from the mother.1643

The male gametes, some are carrying big T. Some are carrying little t, and these can mix all different ways to give you four possibilities.1649

Big T-little t, the heterozygote, that plant will be tall. Big T-Big T, homozygous dominant, that plant will be tall.1660

Big T-little t, that is just another heterozygote I wrote it the opposite way. We usually write it this way, but I was trying to show each match, again, tall.1668

Little t-little t, homozygous recessive, that plant will be short, and here is where we get the 3:1 phenotypic ratio of tall to short.1678

The genotypic ratio is actually 1:2:1, one homozygous dominant to two heterozygotes to one homozygous recessive.1689

And the easiest way to understand this F1 cross is by organizing information as a Punnett square.1704

Punnett squares are a way of diagraming out information into a table and allowing you to analyze it and predict the outcome of a particular cross.1711

Let's start with our parental generation.1720

In the parental cross, we had true breeding tall plants and true breeding short plants.1725

And in a Punnett square, what you are going to do at the top is put the gametes from one parent, so let's say that this is the female, and this is the male.1733

It does not matter, but let's just say. One parent will create gametes, and her gametes are all going to be...she is going to give T or T.1743

That is the only choices, so the gametes are going to be right here.1757

Then, we have the male parent. His gametes are all going to carry little t alleles, so little t-little t.1763

So, we have two parents. We showed the possibilities for the alleles that will be carried in their gametes, and now, we do the cross.1769

The father could donate a big T, and the mother or excuse me, the father could donate a little t, the mother, a big T.1777

And we usually write the dominant allele first.1785

Here, the second box, we are going to cross the little t and big T. We are going to combine to show what the offspring could be genotypically.1788

Next, little t-big T, next, little t again, big T again.1800

So, this shows the result that you are going to see in the F1 generation and why they are all heterozygotes.1809

Now, let's go on and look at when we take these F1 parents and either self-pollinate or cross-pollinate, what would we get?1816

Well, we are going to have one parent who is big T-little t heterozygote, and the other parent is big T-little t heterozygote.1829

We are going to put one parent up here just to be consistent.1836

This parent when it forms gametes, and the alleles will segregate, and in some gametes, half the gametes will have a big T. Half will have a little t.1842

Same for this parent, it forms gametes.1852

Half the gametes will have bit T. Half will have little t.1854

The cross is performed.1858

The chromosomes come together, and diploidy is restored in the offspring.1861

And we end up with, in this offspring, a big T from one parent, a big T from the other.1869

This cross, a big T from dad, little t from mom. Here, little t from dad, big T from mom, and here, little t and also a little t from mom.1875

Here, we get our 3:1 ratio. This plant will be tall.1888

This plant will be tall. This plant will be tall.1894

The homozygous recessive plant will be short, and we get 3:1 1, 2, 3 tall plants to one short plant.1898

The genotypic ratio is going to be one big T for every big T-big T homozygous dominant,1907

for every two heterozygotes and then, to every one homozygous recessive.1916

Punnett square can be very useful in organizing information about a cross and predicting offspring1926

at what the genotypes and phenotypes of the offspring will be, so this is the F2 offspring.1932

This first law that we talked about, Mendel's law, was the law of segregation,1941

the idea that the alleles segregate during gamete formation, and each gamete receives one allele.1946

The second law is the law of independent assortment.1955

The cross we talked about before, we looked at only one trait. We looked at height.1959

And when you cross individuals who are heterozygous, they differ by one trait, and they are heterozygous for that trait.1964

That is what you are studying. That is called the monohybrid cross.1971

The first cross that we looked at with the height was monohybrid.1974

You cross individuals who are heterozygous for one trait.1979

We did a monohybrid cross that was studying height.1994

And in the F1 generation, that was a monohybrid cross because in the F1 generation, we were crossing big T-little t with big T-little t.1997

Another of Mendel's very important laws is the law of independent assortment, and to determine this, it required a dihybrid cross.2012

In a dihybrid cross, so when we say hybrid, we are talking about heterozygotes and di is two.2019

We crossed individuals who are heterozygous, so we crossed heterozygotes for two traits, so individuals who are heterozygous for two traits.2026

What the law of independent assortment says is that alleles for a particular trait assort independently of alleles for other traits.2043

So, here, Mendel was studying tall versus short, so the two traits here are height.2051

And the second trait is flower color- height, tall or short and flower color.2058

And for flower color, purple is dominant. Big P is purple.2068

It is dominant. Little p is white, and it is recessive.2073

And from these crosses, what it turned out is that the assortment of alleles for height is not affected by the assortment of the alleles for flower color.2083

Those two separate out and mix and match independently, so let's look at what happened in this cross.2097

First, Mendel had to determine that to determine that he had true breeding plants.2103

So, he started out with plants that are true breeding for height and flower color.2106

One plant here is true breeding for tall height, so it is big T-big T.2112

It is also true breeding for purple flower color, so it is big P-big P.2116

The second plant is true breeding for short height, little t-little t and for white flower color, so little p-little p.2122

He crossed these plants and what he came up with is a bunch of plants that were tall with purple flowers.2134

And this is not surprising because if we look at what gametes are going to form, the gametes that are the only possibility is a gamete for big T, big P,2145

homozygous dominant and homozygous recessive because each gamete has to get an allele for height and an allele for flower color.2158

And they are all going to get tall and purple. These gametes are all going to get short and white.2165

So, the result is a bunch of heterozygotes who all are carrying a big T from one parent,2172

a little t from the other, a big P from one parent, a little p from the other.2178

All the plants are heterozygotes, and they are tall with purple flowers.2183

And here is the dihybrid cross self-pollinating the F1 plants so that you are crossing2191

heterozygotes for two traits, big T-little t, big P-little p, little t-little t, big T again.2196

This is parental. This is F1, OK, crossing these heterozygotes.2206

And what happened is Mendel found that he got in the F2 generation some plants that were tall and purple, others that were short with purple flowers.2214

And then, he also got tall with white flowers, short with white flowers.2231

So, we see all different mixes represented, and we are going to talk in a second about why this occurs.2236

But for right now, just know that it was a 9:3:3:1 phenotypic ratio.2244

The 9 is the dominant-dominant, so tall-purple, so both dominant traits, phenotypes, then, 3 and 3 with one dominant trait and one recessive trait.2255

Here, we have height was recessive. Flower color, we got the dominant purple.2274

Here, we have the dominant height but the recessive flower color, and here, recessive for both, so phenotypic ratio 9:3:3:1.2281

What happened? Well, again, we are going to look at Punnett squares and talk about how alleles assort independently.2293

Allele for one trait does not affect where the allele for another trait goes.2302

Let's look at our Punnett square. Let's look at the F1 generation.2311

In the F1 generation, we have big T-little t, big P-little p being crossed with big T-little T, big P-little p.2316

When gametes form, let's look at this parent first, gametes are going to form.2327

One gamete could have a big T and a big P in it, so that is one possibility; or a gamete could have a big T and a little p.2333

A gamete could have a big T with a big P. A gamete could have a little t with a little p.2351

So, you see, this is not just sticking big T with big P. They are mixing and matching.2361

That is one parent. Now, let's look at this second parent and put the gametes over on this side.2372

Again, we could have big T and big P end up in a gamete dominant-dominant.2377

We could have little t with big P. We could have big T, actually, let me start that over, stay consistent, big T with big P.2383

We could have big T with little p, little t-big P, little t-little p.2399

Then, we do our crosses, big T-big T, big P-big P, big T for both, big P, and this one has a little p.2408

Here, we have a gamete with big T and little t, both homozygous dominant for flower color.2421

Here, we have tall with short allele purple flower with white allele.2429

And I am just going to go ahead and write the rest of these in, but definitely try this on your own.2435

Here, we have big T with a little t and then, homozygous dominant for flower color.2454

And then, make sure you double check your work and not go too fast on this.2461

Here, we have homozygous recessive for height and homozygous recessive for height,2466

heterozygous for flower color, big T-little t and then, homozygous for flower color.2477

OK, let's look at what we have.2491

Mendel observed 9:3:3:1. He saw 9 tall purple that says dominant, two 3 tall with white flower, one dominant trait, one recessive phenotype.2494

And then, there were 3 purple short again, one recessive trait, one dominant, and then, the 1 with both recessive phenotypes 9:3:3:1.2514

That would be a plant that is short with white flowers, and let's see what we did get: tall purple.2531

Let's have this be blue, so there is one tall purple, two tall purples, three tall purples2539

because heterozygotes are going to have the dominant phenotype, 4, 5, 6, 7, 8 that is short-short, 9 right here, so 9 tall purples.2546

Now, how about tall-white? Tall-white, that is one.2564

Tall-white, that is two, short-short, tall-white, that is three, then, short-purple.2571

So, we have short and purple one, short and purple two, short and purple three, and finally, short and white with this circle that is only one of those.2583

OK, the genotypic ratio is complicated, but you cannot look through it the Punnett square and figure that out, as well.2595

And this Punnett square demonstrates this 9:3:3:1 ratio from an F1 dihybrid cross that we did.2610

Now, let's look at how the events in meiosis can account for the findings that Mendel had when he bred pea plants.2618

Looking at this on a chromosome level, on a molecular level, the laws of segregation and independent assortment, so let's review meiosis.2627

Here, we have an individual, and let's say that this is a human, and they have their usual 46 chromosomes, 23 pairs.2638

And let's say that the purple chromosome, the large chromosomes are chromosome 1.2648

This individual is diploid, so they have one chromosome 1 from their mother.2654

And we are going to say that purple is the chromosome that is maternally derived.2659

This individual inherited the purple chromosomes from his mother.2666

And he inherited the green chromosomes from his father- maternally derived and paternally derived are inherited.2670

They get a chromosome 1 from each parent, and they received a chromosome 2 from each parent-2683

chromosome 1 and 2 from mom, a chromosome 1 and 2 from dad.2692

Now, let's say that on chromosome 1, let's just say that eye color is on chromosome 1, and let's say that height is on chromosome 2.2695

Maybe this individual received a brown-eyed allele from his mother and a blue-eyed allele from his father.2714

And let's say he received a tall allele from the mother and a short allele from the father2722

because we are saying that eye color is in chromosome 1 and height is in chromosome 2.2728

Now, let's first look at the idea of segregation.2732

According to Mendel's law of segregation, the gametes right here, and if this individual is a male, these are going to be sperm.2736

During spermatogenesis, the formation of sperm, recall that homologous chromosomes will line up on the metaphase plate.2745

And they will separate to opposite poles of the cell.2753

An individual will receive only one chromosome 1 in each cell, and then, one chromosome 2 in each cell.2756

Here is chromosome 1, one over here, chromosome 1, one, one over there, and then, the sister chromatids separate as you will recall.2775

Chromosome 2, they received one here and one there.2784

This is the law of segregation, whereby the alleles separate during gamete formation, and then, gametes each only get one allele.2787

The second law is the law of independent assortment.2804

Now, in independent assortment, it says that alleles from one trait assort or separate out independently from the alleles of another trait.2807

What that means is that the alleles for eye color are not going to affect the assortment of the alleles for height.2821

Even though brown eye color came from the mom, and tall came from the mom, it does not matter.2835

Brown could end up with short. Brown does not have to stay with tall.2844

These two do not stay together. They assort independently.2855

It might so happen that the maternally derived chromosome goes into one cell for chromosome 1.2859

And the paternally derived chromosome 2 goes into that cell.2867

Or it could have so happened that these two did stay together, and then, the gamete could have been tall with brown eyes alleles.2870

The alleles segregate independently because they are in different chromosome.2884

Now, we are going to talk in the next lecture about linkage, what happens if two traits are on the same chromosome.2888

If it just so happened that eye color and height were both on chromosome 1, then, they are going to stay together more often.2895

But for simple Mendelian genetics, we assume that traits are on different chromosomes. They assort independently.2904

And what happens with an eye color allele has nothing to do with what happens to a height allele or allele for flower color or allele for another trait.2911

You can see here the genetic, the chromosomal basis of Mendelian inheritance that each gamete does get one allele for each trait because of segregation,2920

and that the assortment of the alleles for different traits is independent of one another.2938

Alright, we are going to talk about another kind of cross called the test cross.2948

A test cross is sometimes also called a backcross.2953

It can be used to determine the genotype of an individual who exhibits the dominant phenotype for a trait.2955

Again, we are going to talk about height.2962

If a plant is short, that is the phenotype. I know the genotype.2964

It has got to be homozygous recessive because the only way you are going to get a short individual is if they have two alleles for short height.2971

Dominant traits are different. Tall, all you need is one allele to get a tall plant.2979

So, I could look at a plant, see that is tall, and I do not know what its genotype is. I know it is big T something else.2987

But the question is, is it big T-big T, homozygous dominant, or is it big T-little t, homozygous recessive, heterozygous, heterozygous?2993

I can determine the genotype of this individual by crossing it with an individual who has the recessive phenotype.3010

So, a test cross is crossing the unknown individual with a dominant phenotype.3020

And then, I am going to cross that individual with an individual who has the recessive phenotype, again, illustrating it with this example.3033

I have tall question mark. For my test cross, I am going to take that plant, and I am going to cross it with another plant that is short- tall, short.3046

And then, I am going to look at the offspring and see what the offspring are, and that will tell me what this plant's genotype is; so there is two possibilities.3067

The first possibility is it could turn out...so, I know that this plant, this parent is little t-little t, and I know that this parent is big T.3077

Now, what if the second allele is also tall? What am I going to end up getting for these offspring?3086

Well, I am going to get big T-little t here, big T-little t here. Down here, I am going to get big T-big T, little t-little t.3102

If this plant is homozygous dominant, this offspring is tall. This offspring is tall.3116

This offspring is tall, and this offspring is tall; and I have to get enough offspring for this to be statistically significant.3123

So, if I get many, many offspring, and I see they are all tall, I know that this parent plant is actually homozygous dominant.3132

Now, let's say that this plant turns out to be heterozygous. Then, what I will all have is this parent little t-little t, this parent big T-little t right here.3142

Actually, let's make that blue.3163

Big T-little t, big T-little t, here, we have in blue, little t.3170

So, this is if the parent is actually heterozygous, or the unknown genotype plant is heterozygous.3182

What I am going to have here is tall, tall, short, short.3187

If I see that half the plants are tall and half are short, I know that the genotype was big T-little t.3193

If they are all tall, the offspring, then, I know it was homozygous dominant, the unknown genotype.3203

So, this is a test cross, and it is used to determine the genotype for an individual with the dominant phenotype.3210

OK, we have talked about Punnett squares, and we have been using probabilities.3221

And now, we are going to cover some of the laws of probability more formally.3224

You can use a Punnett square, but there are times when it is much faster and simpler to just go straight for the math to do the probabilities.3228

The probability that an event will occur ranges between 0 and 1.3238

0: there is no chance that an event will occur. With 1 there is 100% chance.3243

If you are looking at probability, then, the probabilities of all the possible outcomes need to add up to 1.3266

Probability of all outcomes must add up to 1.3279

For example, if I am rolling a die, the chances that I will roll one are 1 out of 6. The chances that I am going to roll a two are 1 out of 6.3290

The chances I will roll a three, 1 out 6, four, then, I will roll a five; and then, I will roll a six.3303

Now, when I roll the dice, I have to roll one of these numbers. There is 100% chance I will roll one of these numbers.3311

So, the chance that I will roll a one, a two, a three, a four, a five and a six, when I add that up, that equals 1.3317

Thus, I am going to roll one of these, so that is the first thing.3323

Probability, 0 has no chance. A probability of even bet occurring can be all the way up to one.3327

If every side of the dice had a one on it or a four on it, my chances of rolling a four would be 1. It would be 100%.3333

For independent events, the outcome of one event does not affect subsequent outcomes.3343

So, if I roll a two, and then, I roll the dice again...3356

OK, so if I roll the dice and I get a two, and then, I roll it again, and I say 'hmm, what are the chances I am going to get a two?". It is still 1/6.3362

If I roll the dice again, I have gotten 2 twos, what are the chances I am getting a two again? 1/6.3373

Now, let's say, though, before I roll the dice at all, I want to know what are the chances I am going to get 2 or 3 twos.3380

That is where the multiplication rule comes in. The multiplication rule will give you the probability of two independent events occurring.3390

The probability of two independent events occurring is equal to the product of the probabilities of each event occurring.3423

So, the probability of two independent events occurring is equal to the product of the probabilities of each of those events occurring.3448

What does that mean?3454

Let's say that I have two balls, and one is red. One is blue, and I put the balls in a bag; and I cannot see what is in there, and I pull out a ball.3455

And then, I look at what color it is. I throw it back in.3468

I, kind of, shake it up again. I pull out another ball.3470

Those are independent events.3474

What I pulled out the first time has no effect what I pick the second time, and I ask, what is the probability of choosing red two times?3475

If I grab a ball, look at it, throw it back, grab another ball, what are the probability that I am going to choose red twice?3493

Well, the probability that I will choose red the first time is 1/2, 1 out of 2.3499

When I grab a ball, there is red one in there. There is blue one in there.3507

The chances that I am going to get a red ball are, it is 1 out of 2, so that is first time.3512

The second time, same thing. There is still a red ball in there and a blue ball.3519

The chances of choosing the red one, the probability is 1/2.3522

So, the probability of choosing a red ball the first time is 1/2 times the probability of choosing a ball that is red the second time is 1/2.3529

The probability of choosing red both times is 1/4.3538

So, the probability of two events occurring that are independent, this and this occurring, when I hear "and", I am going to multiply.3542

The addition rule has to do with mutually exclusive events, so that is slightly different.3550

Now, before we go on to that, just to quickly apply this probability rule, this multiplication rule, to genetics.3558

Let's say I am doing a monohybrid cross, and I have height, big T-little t. It is heterozygous, and I do the cross.3568

What are the chances that the offspring will be homozygous dominant?3577

Well, in order for it to be homozygous dominant, this parent needs to donate a big T.3585

The chances of that, the probability is 1/2, and so I multiply.3591

What are the chances, the probability, that the second parent will donate a big T? 1/2.3596

Therefore, the probability of the offspring being a homozygous dominant is 1/4.3603

Sometimes, you will be asked to use decimals.3610

It is the same thing. You can convert after, or you can just say "OK, 0.5 x 0.5 is a 0.25", so sometimes, it is easier to work with decimals.3613

Now, the addition rule, the multiplication rule here had to do with independent events. The addition rule has to do with mutually exclusive events.3627

When you are talking about mutually exclusive events, you are talking about a situation where both events cannot occur at the same time.3639

Mutually exclusive, so both events cannot occur.3647

Let's say that I am rolling a die one time, and I want to know the probability that I will roll a one or a six on that single roll.3653

I cannot roll a one and a six. They are mutually exclusive.3665

It has the word "or".3668

If I roll the die one time, what is the probability that I am going to get one or a six?3670

If I get the one, it excludes getting a six. If I get the six, it excludes getting a one.3674

Well, we use the addition rule. The addition rule involves adding the probability of each.3680

The probability that if two events are mutually exclusive, the probability of one of the events occurring3690

and this is for mutually exclusive events, is equal to the sum of the probability of each.3709

For example, if I roll the die one time, what are the chances I will roll a one or a six?3731

When using the addition rule, what is the probability that I am going to roll a one? 1 out of 6.3746

What is the probability that I am going to roll a six? 1 out of 6.3753

So, I add those up. I get 2/6 or 1/3.3757

The chances that I am going to get a one or a six are 1/3. Now, if I am dealing with "and", the chances that two things will happen.3761

They are not mutually exclusive then, the chances that both will happen, then, I need to use the multiplication rule.3770

Alright, so far we have talked about the very straightforward situation where with Mendelian genetics,3777

each trait is controlled by one gene, like a gene for eye color.3784

Each gene only has two possible alleles, like brown or blue, and one allele shows complete dominance over the other.3789

If the individual gets brown-eyed gene, eyes are brown. If they get the tall or brown-eyed allele, eyes are brown.3797

If they get the tall allele, the height is tall.3804

That is when we just talked about simple Mendelian genetics.3810

And on the AP test, they will often give you problems and tell you to assume that there is just these two alleles.3814

One shows complete dominance. They are just 100% penetrance.3821

But, there is a lot of more complex situations in nature, for example, incomplete dominance.3825

A classic example of incomplete dominance is Snap Dragons. Snap Dragons are flowers, and they can be red.3833

Another possibility is that they can be white, and there is third possibility.3845

Let's let red be big R. This would be for a red flower- red-flowered Snap Dragons.3849

Little r is white, so it is for Snap Dragons that have white flowers.3860

With complete dominance, you would expect that if we had big R-little r, heterozygote, that the flowers would be red.3868

It turns out, that is not how it works with Snap Dragons.3876

If we take a true breeding red Snap Dragon and cross it - the P generation - with a true breeding white-flowered Snap Dragon,3880

we get an F1 generation that is much different than expected.3892

They are heterozygotes, big R-little r, so we get a bunch of heterozygous offspring, but they are actually pink.3900

Now, this would seem to support the blending hypothesis. It is a colored part way.3913

It is like a combined phenotype, but it does not because these alleles do not change.3920

They maintain their nature, and we can see that in the F2 generation.3927

If you, then, cross some of these heterozygotes or self-pollinate them, what you will find is interesting.3933

White comes back. Red comes back, and there is some pinks.3943

Now, using our Punnett square, remember that what this cross is going to be for the F1 is the gametes will look like this.3949

And we will end up with genotypes 1 to 2 to 1, one big R-big R for every two heterozygotes, for every one homozygous recessive.3960

So, we end up with one white, which is the little r-little r, two pinks, so let's switch this order around just for consistency.3975

Those are my heterozygotes to a red.3991

This is an example of the pink demonstrates that this is incomplete dominance. Red is not completely dominant over white.4008

The white is still, somewhat, expressed, so incomplete dominance.4014

Codominance and multiple alleles can be illustrated together through one example, and that is using the ABO blood groups.4027

Before, we just talked about having two alleles per trait: tall-short; purple or white-flowered alleles; seeds could be round or wrinkled;4038

ABO blood groups; ABO blood groups; so there turns out to be three alleles: A, B and for O.4053

And the way we show these are I superscripted A, I superscripted B and then, little i.4065

So, this is the allele for A. This is the allele for B, and this is the allele for O.4074

I stands for immunoglobulin. This is an example of multiple alleles, more than two alleles for a trait.4080

It is also an example of codominance. Here is why.4087

If an individual has this A allele, on her blood cells, she will make this A antigen, so I will just draw it like this- an A antigen.4090

If she has a B allele, the B allele will code for a B antigen. The O allele does not code for either one.4108

What about the second allele? Well, if the second allele is the same, this is what you are going to see, right, for the homozygotes.4125

What about for heterozygotes? If the second allele is the O allele, you are going to see the same thing because the O allele does not code for antigens.4134

So, you are just going to see the A antigen on the surface.4147

If the individual is heterozygous, but the second allele has the O which is recessive, then, again, this is going to be blood type A.4152

This individual will be a type A blood, type B blood, type A blood, type B blood, type O blood.4165

Now, here is where the co-dominance comes in.4171

If an individual has both the A allele and the B allele, they are blood type AB because the A antigen will be expressed, and the B antigen will be expressed.4174

It is not incomplete dominance. It is not a type halfway between A and B.4191

They are expressing A, so they are type A; but they are expressing B, so they are type B, so they are actually type AB.4195

This is an example of codominance as well as multiple alleles since there are three alleles for the ABO blood group.4203

Another extension of Mendel's laws, another level of complexity, is the idea of polygenic inheritance and the idea of pleiotropy.4217

Before, what we have been talking about is just one gene per trait. There is a gene for height.4227

It could have two alleles. We talked about how it could have three alleles but just one gene per trait.4232

In reality though, there are many traits that are controlled by multiple genes. In humans, height is an example so is skin pigmentation.4238

And that is why you see these two traits. They go along a continuum.4252

It is not just that a tall parent who is 6-foot and a short parent who is 5-feet can only have tall kids or short kids. There is a continuum.4260

We see all kinds of heights, and the same with skin color from very, very light to very dark; so both of these are polygenic in inheritance.4269

Just one note as well to keep in mind when you think about genetics,4279

we are just talking about genetics and how it affects expression, but the environment can affect expression, as well.4283

A child could inherit plenty of alleles for tall height that they have the nucleotide sequence that says they should be 6-feet tall.4291

But if they do not get adequate nutrition, then, they could end up not achieving the height that is being coded for by those alleles.4300

So, just keep in mind that it is much more complex than just the genetics.4309

OK, so, polygenic means that traits are controlled by multiple genes. Pleiotropy is pretty much the opposite.4315

Here, we have several genes affecting one trait. Here, we have one gene affecting multiple.4322

So, this is an example here of polygenic with multiple genes controlling it.4329

In pleiotropy, we could have one gene that affects several traits. An example is the phenotype of red hair, light skin pigmentation and freckles.4334

This is due to a variant of a gene that encodes the melanocortin 1 receptor, and a variant of that gene will not just change one trait.4350

It will not just give red hair or light skin. It actually controls multiple traits, so polygenic inheritance and pleiotropy.4364

Epistasis is another concept you should be familiar with, and this is when a gene at one locust affects the expression of a gene at another locust.4374

This is not the same as just dominant and recessive alleles.4381

When we talk about dominant and recessive alleles, we are talking about alleles at the same genes.4384

So, brown is dominant over blue eye color, but we are saying those are at the same loci.4389

Here, we are talking about one gene totally in a different place on the chromosome affecting another gene.4395

And what can happen is one of these genes can mask the expression of another gene.4402

We say that the expressed gene is epistatic. We say that this gene is the one that is expressed.4407

It is epistatic to another gene. Hypostatic is the one that gets masked.4414

A classic example is coat color in Labrador retrievers.4424

Labradors can have black coat color, black labs, yellow labs and chocolate labs, so chocolate or brown, and they could also have yellow coat color.4431

Coat color is controlled by two loci, and one is epistatic to the other.4444

The first locus determines whether or not the color will be black or brown depending on what happens at the second loci.4450

So, first just looking at this first locus. This first locus, what we have is the dominant allele, big B,4461

which would code for black coat color and the recessive allele, little b, that would code for chocolate coat color.4469

If an individual has one big B, it does not matter what the second is, coat color will be black.4480

They could be big B-big B or big B-little b, heterozygous- black coat color.4487

They may be, I say may because it depends on the second locus what actually happens.4494

If the first locus has two of the recessive alleles, the individual, the Labrador, will be chocolate if they have a certain genotype at the second allele.4501

The second locus has to do with...it controls the deposition of the pigment.4514

This first allele says what the pigment will be. Will it be pigment that will make black coat color or chocolate coat color?4526

The second locus controls whether the pigment actually ends up in the fur.4533

There are two possibilities: dominant allele, which the pigment is deposited.4539

So, if an individual is homozygous dominant or heterozygous, the pigment can be deposited.4549

If the individual is homozygous recessive, the pigment is not deposited.4554

OK, so, let's see the possibilities here. Let's say an individual is big B-big B.4565

If they have an E, it does not matter what else if they have a little e or big E.4574

But if they have one E, their coat color will be black because it is black pigment, and it is being deposited in the fur.4580

If they are big B-little b, E anything, black coat color will occur.4587

If the individual is little b-little b, that is chocolate coat color. If they have E and anything else, chocolate.4595

So, it does not matter. This second one could be E.4604

It could be big E or a little e. It does not matter what the second is.4608

They just need the one allele because it is dominant.4610

Now, let's say that the individual at the second locus is homozygous recessive. Then, no matter what is over here, it will be a yellow lab.4612

It does not matter if it is big B-big B, big B-little b. No matter what, if they are not depositing this color from this first locus, they are yellow.4624

Therefore, what is happening here is masking what is happening at the first loci.4634

So, we say that this locus no. 1 is hypostatic, and this second locus is epistatic.4640

This is an example of epistasis. Again, this is going beyond just the basic Mendelian rules of inheritance.4650

What we are going to do now is practice some problems based on what learned starting with example one.4658

Assume that eye color and height are inherited in a simple Mendelian manner.4664

The alleles for eye color are represented by big B and little b with brown-eyed color dominant, so I am going to start doing some notes.4670

Big B is brown- dominant, and blue eye color is recessive, so little b is blue.4679

The alleles for height are represented by big T and little t with tall height dominant and short height recessive.4688

If a man and a woman are heterozygous for both traits, and they have a child, so we have a man who is heterozygous for both traits,4699

and he and a woman, who is heterozygous for both traits, have a child, what is the probability that here, she will be short and have blue eyes?4711

And what is the probability the child will be tall and have brown eyes?4729

Well, short and blue eyes, since these were assuming simple Mendelian inheritance, these traits assort independently.4733

They are independent events, and I can use my multiplication rule.4743

First, I am just going to address short, chances of being short. Well, height is T-T times T-T.4747

Dad is a heterozygote. Mom is a heterozygote, and in order to be short, the child would have to have both little t-little t.4760

What are the chances that the father will donate a little t? Well, 1 out of 2.4774

What are the chances that the mother will donate a little t? 1 out of 2.4783

What are the chances that the father and the mother will donate little t and the chance of both these events occurring? 1/4- we multiply.4789

Alright, that is the chances of being short. The chances of being blue-eyed, same, by the same logic.4801

So, for the blue eyes, little b-little b is blue. The chances of inheriting the blue-eyed allele from the father? 1/2.4809

The chance of inheriting the blue-eyed allele from the mother? 1/2. The chance of inheriting both, this and this are 1/4.4820

Now, I want to know the chance that this individual will be short and have blue eyes. The chances of being short are 1/4.4829

The chances of having blue eyes are 1/4, so the chances of being short and the chance of being blue-eyed multiplied is 1/16.4837

That is the chance of having the genotype little t-little t, little b-little-b.4850

And if you look back at your Punnett square for a dihybrid cross, you will see that this is indeed correct.4855

What is the probability that the child will be tall and have brown eyes?4861

Well, actually, we are going to do this one another way. There is quicker way to do this.4865

Recall the phenotypic ratio 9:3:3:1 for a dihybrid cross. This would be dominant-dominant, so tall-brown.4868

That is the 9, and here, we have what we just saw, we could have done it this way, as well.4882

Short with blue eyes, I see, is 1 out of a total of 9, 10, 11, 12, 13, 14, 15, 16, so 1 out of 16, we said short and blue.4887

Tall and brown, the chances are 9 out of 16 because they are asking me phenotype, not genotype, so I can just do it this way.4899

Chances of tall with brown eyes are 9 out of 16. Chances of short with blue eyes are 1 out of 16.4908

Example two: a man has type A blood. His mother has type O blood, and his father has type A blood.4916

The man's wife has type AB blood.4926

What are the possible genotypes of their offspring, and what phenotype is associated with each of the genotypes?4928

We take this one step at a time.4935

We know that the man has type A blood, and we eventually want to figure out the possible genotypes of the offspring and phenotypes.4938

What is this man's genotype? Well, if he has got type A, so there is two possibilities for type A blood.4946

He could be homozygous for the A allele, or he could be a heterozygote; and the second allele is an O allele.4956

To figure out what he is, we have to figure out what his parents are.4966

His mother has type O blood, so the man right here, and here are his parents, his mother, she is O, and his father is A.4970

So, I know that his father also have this A allele.4986

O, recall that type O blood is recessive, so the mother has to be homozygous recessive. Therefore, the man has to be heterozygous.4990

He got the A from his father. From his mother, the only thing he can get is the O allele.5003

So, now, I have determined this man's genotype, and the wife has type AB blood.5009

He is a type A. The wife is type AB.5018

Recall that type AB means that she has an A allele and a B allele. Now, we are finally down to the genotypes of the man and the wife.5022

What are the possible genotypes of their offspring?5033

In a simple cross like this, go for a Punnett square.5037

Here, we have the man. I am going to put him right here.5044

He has gametes that will either be the A allele or the O allele. The woman, the mother of the offspring, she is right here.5046

Her gametes will be either of the A allele or the B allele.5058

So, the possibility for their offspring could be homozygous for the A. It could be heterozygous with an A and an O.5062

It could have an A allele and a B allele, or it could have a B allele and an O, so these are the genotypes.5073

What phenotype is associated with each? Well, here, type A blood.5082

Here, A is dominant over O- type A blood. Here, we have codominance, so the blood type is type AB blood.5089

And here, we have B dominant over O, so blood type would be type B.5100

Example three: a scientist is studying pea plants, and one of the plants has round seeds.5110

In pea plants, round seeds are dominant, and wrinkled seeds are recessive.5116

I am going to have round, since this dominant will be big R. Wrinkled is going to be little r because wrinkled seeds are recessive.5121

The scientist wants to determine the genotype of the pea plant.5132

So, one plant has round seeds, and she wants to determine the genotype of that pea plant so round-seeded plant.5138

Since round is dominant, I know that the plant has one dominant allele, but I do not know what the second is.5152

This plant could be homozygous dominant or heterozygous.5161

What type of cross can he perform to determine this, so the scientist wants to determine the genotype of this plant.5170

How can he do it through a cross? What are the possible outcomes of this cross, and what would they indicate the genotype to be?5177

Well, recall that a test cross will tell me the genotype of a dominant plant, a plant with a dominant phenotype.5187

This plant has the dominant phenotype, round, and an unknown genotype.5197

So, what I am going to do is I am going to cross this plant that I want to study with the plant that is wrinkled-seeded.5203

And I know that the wrinkled-seeded plant is homozygous recessive because that is the only way you will end up with the recessive phenotype.5215

What are the possible outcomes of this cross? Well, there is two possibilities.5225

Possibility one is that the plant I am trying to figure out is actually homozygous dominant, and then, here is my homozygous recessive plant.5228

I do the cross, and then, what I would see are offspring that have all round seeds.5246

The second possibility is that the plant I am testing, trying to figure out, actually has a big R and a little r.5256

So it is a heterozygote and then, the homozygous recessive I crossed it with, and what I would see in this case is that these two plants would be round.5269

These two would be wrinkled, so it is ratio. It is probability.5281

So, I would see half the plants would have round seeds, and half would have wrinkled because half would be heterozygotes. Half would be homozygotes.5285

If I have got plenty of offspring and looked at them, and they all had round seeds, I would know that the genotype is homozygous dominant.5298

If I looked at the plants, and about half had wrinkled and half had round, I would know that I was working with a heterozygous plant.5306

Example four: coat color in mice may be black, brown or white. Coat color is controlled by two loci.5315

The first locus has two possible alleles with black dominant to brown, and these may be represented as big B and little b.5323

Alright, so we have two loci, and then, this is going to be the first loci; and it has big B and little b, and black is dominant to brown.5333

Big B is the allele for black coat color, and little b is the allele for brown coat color.5347

There is a second locus, and this locus controls the deposition of melanin; and it has two alleles possible.5354

There is a dominant allele, big C, and this allows for deposited melanin.5363

Little c is recessive so not deposited, and mice with little c-little c, their recessive homozygous genotype are albino. They have white coats.5374

OK, this is similar to the situation with the Labradors. We are talking about epistasis that little c-little c, it does not matter what you have at the first locus.5390

If you have little c-little c, no matter what you have here, these are going to end up being albino mice.5404

Determine the probability of two mice that are heterozygous for both loci producing an offspring that is albino, OK, heterozygous for both loci.5418

So, we have a mouse that is big B-little b, big C-little c, so this is a dihybrid cross, and big B-little b, big C-little c- heterozygous for both loci.5429

And I want to know the chance that the offspring will be albino.5445

Well, in order for the offspring to be albino, all of that matters is that they inherit this.5451

So, this problem is not as complex as it looked at first because I do not even have to pay attention to what is happening here.5458

It does not matter what is happening in there.5463

All I have to do is say "OK, what are the chances of this offspring receiving a little c from one parent and from the other parent?".5465

So, I am just going to treat this as a monohybrid cross: little c-little c times little c-little c.5478

The chances of inheriting the little c from this parent are 1 out of 2, and so I multiply.5486

The chances of inheriting the little c from the second parent are 1/2, 1/4.5493

So, the chance that offspring will be albino is 1 out of 4, and I could have just also done a Punnett square for this monohybrid cross or use probabilities.5500

And I see that 2 out of 4 or 1/2 are going to be albino.5514

OK, that concludes this lecture on Educator.com on Mendelian genetics.5521