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0:00
You are listening to Professor Megan Workman. Welcome to this continuation of our discussion about inheritance. Today we will talk about complex patterns of inheritance that go beyond the simple Mendelian traits we have considered previously.
0:18
This lesson will extend our understanding of inheritance by evaluating these simultaneous other ways that complex traits are passed from parents to their children. These include incomplete dominance, co-dominance, gene by environment interaction, heliotrope polygenic inheritance and different types of linkage.
0:45
Previously, when we discussed simple Mendelian traits, we were discussing scenarios of complete dominance. Complete dominance in this simple Mendelian framework is when heterozygous displayed the exact same phenotype or traits as homozygous dominant genotypes.
1:10
Let's consider an alternative scenario which happens with some other genes where there is said to be incomplete dominance. Incomplete dominance is when heterozygote individuals look something intermediate between the two ends of the spectrum. And those two ends of the spectrum are the homozygous dominant genotypes and the homozygous recessive genotypes. So the heterozygotes look like a blending between those two extreme traits. An example of this happens in Carnation flowers,
1:47
where if you pollinated a red flower with the pollen from a white flower, the offspring produced would be pink. So pink is an intermediate phenotype between the extreme colors of red and white. Red being the homozygous dominant phenotype and white being the homozygous recessive phenotype.
2:13
In a different scenario called co-dominance. There can be multiple versions of a dominant allele. In this scenario, heterozygotes who inherit multiple dominant alleles express both of these traits fully. An example of this are the antigens expressed on the surface of human red blood cells. You've heard of blood types A, blood types B, and blood Type AB. The blood type B exists because the allele underlying the A antigen on red blood cells is dominant,
2:56
and so is the allele underlying the expression of the B antigen on red blood cells. So if you are a person who inherited the dominant A allele from mom and you also inherited the dominant B allele from dad, your red blood cells display fully the A antigen and also the B antigen. And so while your parents might be blood type A and blood type B, you, having inherited both of these dominant alleles, because the appeals are co-dominant to one another,
3:33
you are blood type AB, and you can see that in the cartoon all the way to the right. This is a type AB red blood cell with the A and B antigens fully expressed. So in this scenario, the A and B alleles are said to be co-dominant to each other.
3:59
Let's try a practice problem that considers co-dominance when multiple alleles are completely expressed at the same time, the alleles that are co-dominant to each other. So, for practice problem 1A, What are the possible blood types of children born to a couple with the following genotypes? Let's say Mom is type A and she is homozygous AA and dad is homozygous BB, so dad is type B. To solve this problem, you would first draw your Punnett square
4:33
and remembering that meiosis separates the genotypes of the parents into separate gametes. You know that mom can produce eggs that contain A or A because she's homozygous. Same with dad's sperm. All of dad's sperm contain this type B antigen. So all of their combinations of children will inherit a Type A from mom and a type B from dad. Therefore, this couple will only have children with type AB blood.
5:09
Next, consider this scenario where mom is type A still and dad is still type B, but now the parents, instead of being homozygous dominant, they're heterozygous. So you see that mom has a dominant A version of this gene, as well as a recessive little i. Similarly, dad inherited from one of his parents a dominant B version of the gene and from his other parent, he inherited the recessive little i version.
5:40
So first, draw your Punnett square, remembering that meiosis splits the genes up so that half go into... half of the gametes and half go into the other half of the gametes each parent could potentially produce. So what I mean by that, is if we look down the columns, half of mom's eggs will have the dominant A version of this gene and the other half of her eggs will have the little i recessive version of the gene.
6:11
Similar thing is going on with dad, where half of his sperm have the dominant B version of the gene and the other half of his sperm have the little i recessive version of the gene. So if we're thinking about all the potential genetic diversity that might be found in this couple's children, if they had a bunch of kids. Here is our Punnett square tool that tells us about the genetic diversity of all of their children.
6:40
Because the A and B alleles are co-dominant with each other. That means one in four of this couple's children, on average, will inherit both of these co-dominant genes and therefore have red blood cells that display both the A and B antigen. They're type AB.
7:03
Because also there is a true recessive version of this allele that is recessive to both the A and the B. Notice the kitty corner style children produced by this pairing. They each inherited one dominant allele and one recessive allele. So these kids are type A or type B, and then there is the individual in the lower right hand corner that inherited only recessive versions of the allele.
7:34
Therefore, their red blood cells do not express any antigens, neither A nor B, and they're called type O blood.
7:44
So this couple may have children with any blood type. A, B, AB, or O. Their family is a one stop shop for the Red Cross during a blood drive.
8:02
Next, let's talk about gene by environment interaction. This is common among human traits. This is when a gene is expressed differently depending on the environment in which the individual grows up or in which the individual lives. So different phenotypes can occur despite an underlying identical genetic makeup. In humans, a great example of this is height, because your adult height depends on your childhood nutrition.
8:33
And there are studies of identical twins separated at birth in two different families. And here is an example. These are identical twin brothers who were separated at birth. The brother on the left was adopted into a rural family of farmers. These guys both grew up in China. And because in his farming community, his childhood nutrition varied seasonally with the harvest, he did not grow as tall as his genetic twin brother on the right,
9:09
who was adopted into a more affluent family in an urban area, that had more stable nutrition across his childhood. So despite having the same genes, these are identical twins, their environment allowed them to grow to different adult heights. And the contrast here is stark.
9:32
Additional examples of phenotypes that result from a gene by environment interaction are things like my waistline, which can vary depending on my environment. Meaning if it's after the holidays or before. Skin tone depends both on your genotype, the types of melanin and how much your cells can produce, and also exposure to UV sunlight.
10:07
All of these types of traits that can vary with environment, are called plastic traits. I'm sure if you pause the video, you could brainstorm additional human plastic traits. Height, weight, skin tone. What else?
10:25
Our next complex inheritance topic is called Pleiotropy. Pleiotropy is when a single gene influences multiple phenotypes. In the cartoon here, you see a single gene highlighted in red, and it's leading to traits A, B, and C. So this gene encodes for a polypeptide through transcription and translation, and that polypeptide is used in multiple different traits. An example of this in humans could be the polypeptides involved in keratin production,
11:02
because keratin is a polypeptide expressed in fingernails and hair, in the skin, and so the underlying gene for your keratin
11:15
contributes to multiple of your traits. Another example in humans is the TRY gene that prevents melanin production in genetic albinos. And so a person with albinism does not produce any melanin in their hair or in their skin. And this gene also affects the structure of the eye. And so it also affects vision. So just one gene, the TRY gene, affects multiple traits. Therefore, we call it an example of pleiotropy.
11:59
Multiple of these complex inheritance patterns can co-occur. That means that the trait that a person is expressing, the phenotype that they're living with, can result from all of these things going on at once. So now I want to apply what we've learned to a human example. We're going to apply what we know about co-dominance, gene by environment, interaction and pleiotropy to better understand how sickle cell trait, which we've talked about in previous classes,
12:34
can lead to health consequences in individuals who have this genotype. So here's a blood smear, and I've circled a normal red blood cell, which is shaped like a round. Sometimes it's called a donut. This is a healthy, functioning red blood cell that can carry a lot of oxygen from your lungs throughout your body. Contrast that with the sickle red blood cell that has this crescent shape. And when it's in this shape, it cannot carry oxygen throughout your body.
13:06
So if you had many sickled red blood cells, this could be dangerous for you.
13:15
Sickle cell trait which you evaluated in a previous lesson, is co-dominant to the ancestral version of the allele. So here in the genotypes I'm showing a capital A for Ancestral and a capital S for Sickle cell trait. And you'll notice in the cartoon image of the red blood cells, a homozygous individual with only ancestral alleles has all normal shaped round red blood cells. The heterozygous is in the middle. This person has sickle cell trait and they have both versions of the red blood cells,
13:53
the round healthier version of the red blood cell and also the sickled red blood cell. And so depending on the environment, this person may not experience any symptoms because they have lots of the normal round red blood cell. Contrast that with the homozygous individual on the far right. This person has sickle cell anemia, where all of their red blood cells take on this sickle shape under certain environmental contexts. And this can be dangerous.
14:25
Because heterozygous expressed fully their ancestral allele and make normal round red blood cells, and simultaneously they express fully their sickled version of the allele, they make sickled versions of the red blood cell. These are said to be co-dominant. So heterozygous expressed both cell shapes.
14:51
But there's another layer of complexity to the story because the sickling of red blood cells, where a round red blood cell changes shape and morphs into these sickled confirmation, this happens in hypoxic environments. These are environments that are low in oxygen. So what can happen is if a person is living with sickle cell trait at sea level, they may not experience any symptoms. But if they decide to become a mountain climber and start walking up mount Everest at some altitude,
15:26
where the oxygen that they're breathing in is at a threshold low level, their normal, healthy, round, red blood cells-- I'll say deform-- they deform into the sickle shape and become less functional. So the gene underlies the shape of the red blood cells and the capacity for them to sickle. But the environment is how much oxygen is present. And so we have a complex combination of things going on. The genotype and also the oxygen level.
16:06
The oxygen level represents a gene by environment interaction where the phenotype of the red blood cell depends on the altitude and there's a danger of sickling at higher elevations.
16:23
Now, you may recall that geneticists believe that sickle cell trait persists in the human gene pool because it is actually beneficial. It's beneficial because it protects people against the malaria parasite. We learned in a previous lesson about heterozygote advantage, where a person with a heterozygous genotype, survives better on average, than individuals with either homozygous genotype. So that is showing in the cartoon in the bottom. The person is either laying down sick with malaria,
17:11
and malaria is the third leading cause of death among children under age five. So this is a very deadly illness. That natural selection-- that through natural selection has shaped the human gene pool for many thousands of generations and continues to. So we see that the person who didn't inherit any beneficial sickle traits is most susceptible to malaria and therefore is unfortunately most likely to die before reproductive age. The heterozygous individual in the middle is resistant to malaria.
17:51
So they are more likely to survive malaria than their sibling who doesn't have sickle cell trait. Unfortunately, if they become a mountain climber and go to a hypoxic environment, then they could suffer from some sickle cell symptoms. And then in this scenario, the homozygous sibling all the way to the right is also extremely resistant to malaria. But unfortunately, before modern medicine, this was often a fatal condition, full blown sickle cell anemia.
18:26
Thankfully, it's not anymore with modern medical treatment. So, what this is showing, is an example of pleiotropy where the sickle cell gene affects both the red blood cell shape that's trait one and also resistance to malaria, which is trait two. So one gene affects both traits. So to understand the complex pattern of inheritance going on with sickle cell trait, the sickle cell allele, we have to understand all of these pieces of the puzzle
19:02
to understand why this trait exists in the human gene pool.
19:09
Just a fascinating example of a very common human trait.
19:18
So pleiotropy was when one gene affects multiple phenotypes. Now, let's talk about when multiple genes affect one phenotype. This is called polygenic inheritance. Poly, meaning multiple genic, referring to genes. So multiple genes contributing to a single trait, a single phenotype. An example of this is a human blood type. You've heard of A, B, O, and AB blood types, but you'll recognize that people can also be AB negative or AB positive.
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The negative or positive refers to a second gene called the Rh factor,
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that is also expressed on the cell surface of your red blood cells. So in addition to A and or B antigens, a person might have Rh antigens.
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There are only two flavors of Rh genes in the human blood type. Remember, a flavor of a gene is called an allele. So you could have the allele that causes you to produce the antigen, that's Rh positive. Or you could have the version of the allele that does not cause you to produce the antigen-- the Rh antigen, and that's called Rh negative. So I know my blood type is A negative, and so my red blood cells display the A antigen, but not the Rh antigen.
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If you happen to know your blood type, you can look on this cartoon and imagine which combination of cell surface receptors are expressed on your red blood cells.
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And this results from multiple genes. Your i genes at your ABO gene locus and your Rh genes at your Rh gene locus.
21:17
Here's how we would write that in letters. So we have the genotype and the phenotype. The phenotype is the blood type. You would have to know the combination of ABO alleles that you inherited from your mom and dad, as well as the combination of our Rh alleles you inherited from your mom and dad. These are wheels are inherited separately from one another. They are not linked.
21:47
So you could be A negative, A positive. B negative, B positive. O negative, O positive. AB positive, AB negative.
22:00
In the previous slide, I used the word linked. So let me tell you what I mean. Genes are said to be linked if they tend to be inherited together.
22:13
And this can happen if they are located next to each other on the same chromosome. So in the cartoon at the left, we see Gene 1 and Gene 2 are very near one another. They're neighbors on this chromosome. Due to their near location recombination during prophase 1 of meiosis, only rarely separates them. I mean, it happens sometimes, but because they're so close together, it's very rare and so they tend to be inherited as a team.
22:47
Now let's look at these other scenarios. In the middle, you'll notice that the genes are located on different chromosomes. You know from our card game we played in the lab, that independent assortment separates these chromosomes into different gametes during meiosis, without considering what happened to the previous chromosomes. So this is called independent assortment. And due to independent assortment, these genes are as likely to be inherited together as not.
23:19
It's like flipping the coin. In this scenario on the right, these genes are located on the same chromosome, but because they are so distantly located from one another, recombination is very likely to separate them during prophase 1 of meiosis, when your homologous chromosomes from mom and dad get together and swap genetic information-- swap DNA. And so because of this recombination likelihood, these genes are more likely to separate than not, and make it into different gametes.
23:58
The scenario on the left is called linked. These are linked genes because they tend to be inherited together. And the other two scenarios in the middle and the right are non linked. They are not normally inherited together. I mean, they can be, but it's only by chance.
24:23
Let's think about this one more time. So here we have an example. We have Genes 1, 2, 3 and 4. If we consider the perspective of Gene 3, Gene 3 is likely to be inherited with Gene 2 more of the time than it is to be inherited with Gene 4. And this is simply because Gene 2 and 3 are closer together on the chromosome. It'd be like if they're next door neighbors on a map. Versus Gene 3 and 4, they're more distantly located.
24:58
So this would be somebody who lives five doors down from you on your street. Recombination is just more likely to happen between Gene 3 and 4 and all that space separating Gene 3 and 4.
25:13
That's more likely than recombination occurring in the limited space separating Genes 2 and 3. Now, let's consider that scenario-- let's see, Gene 3 and 4 versus Gene 3 and 1. Now, Gene 3 and 1 are more distantly located. So Gene 3 is inherited with Gene 4 more frequently than it is with Gene 1. And again, it's just because of the distance separating them on the chromosome. An example of linkage in humans,
25:47
this is called autosomal linkage, when it's linkage of genes on a chromosome other than a sex chromosome. So most of your chromosomes are autosomes. This is autosomal linkage. An example of this is red hair and freckles. These genes are located near each other on the same chromosome. And so people who have a genotype underlying red hair often also have a genotype underlying freckles. So these traits tend to appear together because the genes are located near each other on the same chromosome.
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They are linked.
26:29
That leads me to a discussion of sex linkage. When genes are located on a sex chromosome and therefore are inherited based on which of those sex chromosomes make it into their genotype. So to understand that, we have to first understand how sex is determined in humans and in many other sexually reproducing species. So the human system is that there are X chromosomes and Y chromosomes. And in particular on the Y chromosome, which is smaller, it's the little one that has been made to look blue
27:08
on this computer image of an electron microscope actually looking at these chromosomes. So the Y is the little one, the X is the big one. The Y chromosome is the smallest of all human chromosomes, and it contains very few genes. But one of them, the SRY gene, determines the sex of the individual very early in embryonic development. If the SRY gene is president present, it turns on and leads to a cascade of events, that cause that embryo to develop male traits.
27:46
So the sex of a person is determined early in pregnancy, and it's based solely on whether or not they have an SRY gene. And this gene happens to be located on the Y chromosome.
28:02
Because the X chromosome is bigger, it has a lot of genes and genetic males have an X chromosome as well. They have an X and Y, genetic females have a double dose of the X, they are XX genotype.
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As you can imagine, moms who are XX only have an X to give. So all of the mom's eggs contain the X chromosome. Dads, however, have an X and a Y, so during meiosis they get split up and 50% of his sperm bear an X and the other 50% of his sperm bare a Y. So it's the dad who decides the sex of the baby. In this Punnett square that's been tilted on its side, you have mom's Xs and dad's XYs, and you can see that it's the dad's sperm, whether it contains an X or Y that determines
28:55
whether or not they have a daughter or a son. Henry the 8th didn't know this, and he married six times trying to hope for a male heir. He's the British monarch who beheaded some of his wives and divorced some of his wives, causing great scandal because none of them gave him a son, a living son. But it wasn't his wife's fault. He just kept giving them X bearing sperm.
29:26
These images are called karyotypes. We have a normal female karyotype on the left and highlighted there you can see her two Xs. And we have a normal male karyotype on the right. You can see here it has an X and a Y. Genes that are found on the X chromosome or the Y chromosome, are called sex linked genes, because they are linked to the sex of the individual. For instance, males have only one set of X-linked genes, whereas females have a homologous pair of X-linked genes.
30:00
And this becomes important to consider if we're talking about a recessive X-linked gene that maybe causes some sort of medical disorder.
30:15
Females get two copies, so they get two chances of inheriting a non disordered allele, but males only get one copy of an X and so if they-- they have less of a chance of getting a dominant version of the allele that maybe is not related to the disorder. In other words, could you answer this question in your own words? Males are more likely to inherit sex linked recessive disorders from genes found on the X chromosome. Can you explain why? You should be able to do that before the exam.
30:54
Here's a pedigree. There's a mother who is a carrier of a recessive genetic disorder, and it's found on her X chromosome. So in tan, that's the unaffected or the dominant version of this hypothetical allele that does not lead to the disorder. And the blue version is the affected or the recessive version of the allele. And then you see that dad is not affected, so there's a normal dad having children with a carrier mom.
31:26
You can see that they're 50/50 likely to have daughters and sons, but depending on the sex of their child, that child is more or less likely to get the disorder because it's recessive and found on the X. So let's go and examine their children from left to right. On the left we have an unaffected son. He inherited an X chromosome from his mother and a Y chromosome from his father. Of course he did. He's a son. So we had to get dad's Y. His only X came from his mom.
31:56
And he happened to have inherited her unaffected tan version of the allele, so he's fine. He doesn't have the disorder and he can't pass it on to his children. Now we have next to him his sister. She inherited an X from both her father and her mother. Her dad's X, the only one he has to give to a daughter is unaffected, so she has at least one protected, unaffected X from her dad and then from her mom, she happened to have received by a 50/50 chance, her mom's unaffected X.
32:27
So this is an unaffected daughter and she cannot pass it on to her children someday. But now we go a little further to the right and we have a daughter who is said to be a carrier. She inherited. An X from both dad and mom and recall that dad only has an unaffected X to give. So she is protected by his dominant version of this allele. Her dad's X. So she's not going to have the disorder either.
32:57
But from her mom, she had a 50/50 chance and she inherited the affected recessive version of her mom's X. And so from her mom, she got the blue X, from her dad, she got the tan X, and she's called a carrier because while she doesn't have the disorder, she could through the affected X, pass it onto her children someday. In effect, she has the same genotype as her mother. They're both carriers. Now let's look at the unlucky brother all the way to the right.
33:30
He's a boy. So from dad, he got a Y. His only X chromosome came from his mom. And due to random chance, he happened to inherit her affected recessive version of that disorder on Mom's X. And so this son is affected with the disorder because he doesn't have another X to cancel out the effect of this recessive gene.
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And so he will have the disorder and he will pass it on to his daughters only, if they get that affected X from him.
34:09
Some examples of this in real life are a certain type of red-green color blindness. So if you happen to know anybody who's red-green, colorblind, they're more likely to be male. Certain types of hemophilia, including one that was famously passed through Queen Victoria to several of her children.
34:33
And a certain type of muscular dystrophy called Duchenne's muscular dystrophy. So these are all examples of X-linked recessive disorders that occur in people.
34:46
So let's try another practice problem, that has to do with sex linked inheritance when the gene is located on a sex chromosome. And in this case, we're going to say it's located on the X chromosome.
35:00
So let's consider a common form of red-green colorblindness that results from the expression of a recessive allele on the X chromosome. What is the genotype of a man who is colorblind? First of all, if he's a genetic male, he has an X and a Y. If he's a colorblind male, then the version of the X he has is the recessive colorblind version. And so it's traditional to write XY, and then in superscript above the X, we distinguish what type of X he has.
35:36
He has an X that contains, among its many genes, the recessive version of the colorblindness gene. So I just chose little b to represent colorblindness. Now, question 2b, what are the possible genotypes of a woman with normal color vision? So if she's a genetic woman and she has-- I want to say, if she's a genetic female, she has XX.
36:06
But she could have only one dominant version of the normal color vision gene, or she could be homozygous dominant. So here are that genetic female's options. Notice we indicate that she has two X's, so we know we're talking about a mom, not a dad. And in superscript, we indicate which flavor of that colorblindness or normal color vision genes she has on each of those X's. So in the first scenario, she's X big B, X big B, meaning she's homozygous dominant for normal color vision.
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Or she could be a carrier of colorblindness, meaning she's heterozygous at that gene locus on her X chromosome.
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And we would say that she's a carrier.
36:58
So now, use that information to tell me what percent of daughters this couple will produce that are colorblind, if the mom is a carrier and the dad is colorblind. Step one is always to draw your Punnett square.
37:16
Step two is to know the genotypes of the parents. So if mom is a carrier, she has an X with a big B and an X with a little b, I put all of her eggs going down the side of the Punnett square, so half of her eggs will bear the big B and the other half of her eggs will have the little b. I put Dad's sperm across the top. Half of his sperm will have a Y, and the other half of his sperm will have an X. And since he's colorblind, the type of X that he has is the recessive little b.
37:47
Then I fill in my Punnett square.
37:51
With that sperm happening to meet that egg. And here's the four types of children this couple can produce. And so by statistical chance, we see that half are boys, half are girls. These are the sons. These are the daughters. Sons are the ones who get a Y. Daughters have double Xs. Now, I could ask you all sorts of questions, like among all of the children, how many are color blind? Or what percent on average will be colorblind? Or it's called a conditional probability.
38:24
I'm asking you to limit your analysis to just the daughters and what percent of these, just daughters, will be colorblind.
38:36
And the answer is 50% of their daughters will be colorblind, the ones who happened to have inherited the homozygous recessive, little b little b, scenario on both of their Xs. So they got one little b from their mom and the other little b from their dad.
38:59
Great job. So in this lesson, you learned to extend your previous knowledge of complete dominance, which we call simple Mendelian traits to contrast with incomplete dominance. And the example of that was the carnation color. You learned about co-dominance, where both genes are fully expressed. And our example of that was A and B blood type antigens in human red blood cells. You learned about gene by environment interactions, and our example from that in humans was height.
39:30
You learned about Pleiotropy. Our example was the TRY gene that causes albinism because it affects multiple phenotypes. You learned about polygenic inheritance with the ABO gene and also the Rh gene. These two genes are inherited independently from one another and both predict something about your blood type. You learned about two types of linkage, Autosomal linkage that was like red hair and freckles being inherited together.
40:00
And you also learned about sex linkage, like those X-linked recessive disorders like colorblindness.
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