Mendel's Discoveries and Chromosomal Inheritance

Chapter 15 is going to focus on the chromosomal basis of inheritance. Sorry about that. 15.1 is going to connect what we learned with Mendel, keeping in mind that when Mendel did his work, he did not know about DNA. He did not know about chromosomes. So we're going to take what Mendel was able to describe with his pea plants and see how that connects to what we know now is present in cells with chromosomes. Mitosis and meiosis were first described back in the late 1800s, and the idea behind chromosomal inheritance, the chromosome theory of inheritance, tells us that Mendelian genes have specific positions on our chromosomes, specific placements, and that these chromosomes are able to undergo both segregation and independent assortment, which would account for this behavior of the chromosomes, how they move during meiosis, would provide support for Mendel's laws of these, of segregation and independent assortment. So again, this is just providing, connecting the dots between Mendel's characters and what we now know takes place with our chromosomes. And we've talked about both of these previously, especially in our meiosis lab. how the alleles for a particular gene are going to separate during gamete formation. You're going to end up with one of the homologs from each parent in a particular gamete. And since there are multiple chromosomes for many organisms present in their nuclei, when they undergo meiosis with this independent assortment, the chromosomes do not have to be passed together as a unit. aka mother's chromosomes don't have to go all together. They can be mixed with dad's chromosomes. And that is what's going to result in having physical representations that phenotypic presentations that do not match either parent. And this is not, there we go. Sorry, I didn't think I threw any questions in this one. I will be skipping through the questions. We're not answering those in class. So Morgan was able to provide evidence for the idea that genes were located on specific chromosomes and that actually was done using the sex chromosomes. He did experiments with fruit flies. Fruit flies really work very well for experimentation because they make lots of offspring. They can be bred fairly quickly and they have a limited number of chromosomes that can be examined. When we talk about phenotypes... from Morgan's work, we're going to be dealing with wild type and mutant. Wild type would be what we would normally expect to see if you had one of your dominant alleles, and mutant would be what we would expect to see if you only had recessive alleles. So the red eyes are wild type, the white eyes are mutant for these fruit flies. And so how he was able to correlate the alleles behavior with chromosomes was he mated male flies that had white eyes with female flies with red eyes that were wild-type for both alleles and the white eyes obviously are mutant for both and when they produced offspring whether they were male or female they only had red eyes when those offspring were mated they produced the three to one ratio that we would expect to see but it was only the males that had the white eyes. And so if only the males were able to have white eyes, this eye color gene had to be present on the X chromosome, and the females weren't getting both copies of that mutant allele. So this was just some support for this theory of inheritance with chromosomes. And there you can kind of see it. From the male and female perspective, you see a physical representation of F1, F2, and what that had to tell us about the alleles and where they were found on the chromosomes. So 15.2 is going to focus more on sex-linked genes and that humans and other animals, there are chromosomal differences that are able to determine what sex an individual will be. humans we have a larger X chromosome and a smaller Y chromosome. There are only small sections at the end of the Y chromosome that are homologous with regions on the X chromosome. The majority of the alleles that are coded for, the majority of the genes that are coded for on the Y chromosome. are going to be dealing with male anatomical features. One example would be the SRY gene. Females get two copies of the X chromosome. Males get both an X and a Y. When we look at our gametes, females can only pass on an X chromosome, which is why the males are guaranteed to get an X chromosome, as long as the chromosomes do not have any alterations. or malfunctions in the meiosis process, while sperm can either pass on an X or a Y chromosome since males have both chromosomes available. Depending on what animal you're talking about, you can see different systems used to represent those genders. And so there's just some examples right there. Again, we're not doing questions. Sex-linked genes. If you have genes that are located specifically on one of your sex chromosomes, they're known as sex-linked genes. If they're on the Y chromosome, they're called Y-linked genes. We already talked about the SRY one. If they're on the X chromosome, they are X-linked genes. And the X chromosomes are the ones that we typically are going to be talking about when it comes to disorders because the X chromosomes have genes that don't just code. for sex while the Y chromosome tends to focus specifically on that. When you get your X-linked genes, they're going to follow, sorry, when X-linked genes are inherited, they follow certain patterns. Females have to have two copies of a recessive allele to show that trait. They have to be homozygous for it, but because males only have one X chromosome, they only have to get one copy of that recessive allele. to show its phenotypic expression. So they are hemizygous for that. So any sort of X-linked recessive disorder is more likely to be found in males than females since they only have to get one copy as opposed to two. Some examples of X-chromosome disorders that are due to recessive alleles present there include color blindness, Duchenne muscular dystrophy, hemophilia, baldness is another one that comes to mind. So in this particular slide, we have three different combinations of parents with alleles that are on the X chromosome. Here in the first picture in A, we have a mother who is homozygous dominant for those alleles, paired with a father who is showing that trait phenotypically. They have a recessive allele on the X chromosome, and when they pass those alleles on to their offspring. Because the mother can only pass on a dominant allele, there's no way for any of the offspring to have that recessive allele present itself. You can't get both recessive alleles for the female, and the male definitely can't get a recessive allele since the female can only pass on, or sorry, the male offspring can't get a recessive allele since their mother can only pass on a homozygous dominant allele. The females, however, would be heterozygous for this particular trait because the father can only give them that recessive allele. If you take a heterozygous female and mate it with a homozygous dominant male, you do have the possibility of having some offspring have the recessive trait present itself because you could have a female. provide a male offspring with that recessive allele. And since there's nothing else to balance it, because they can only get the Y allele from their father in order to be male, you have a 50-50 chance of showing that trait or having that trait be physically represented. You can have a female that is homozygous dominant or is a carrier for that trait. because you have a one in two chance of getting the dominant or the recessive allele from the mother if you have a heterozygous mother mate with a male that is recessive and is expressing that recessive trait your odds definitely are going to increase again you have a one in two chance as a female of getting that dominant allele from your mother and that means you have one in two chance of getting the recessive allele. If you are female, you can only get that recessive allele from your father. So because you that's the only little you can get you have it's 50-50 odds as to whether you're going to show the recessive trait or not. With males, it would also be 50-50 because although the male father had that trait, it would be the mother that would provide the allele for that trait to her offspring, and she can give either the dominant or the recessive allele. So X inactivation, males have an X chromosome, females have two X chromosomes. We do not need as females twice the amount of genes produced that you would have present on your X chromosomes if they were both being transcribed and translated. So one of those X chromosomes gets randomly inactivated as the embryo is growing and it gets condensed into what we call a bar body. And if a female is heterozygous for genes found on that X chromosome, because it is randomly inactivated in one cell it could be the X chromosome she got from the mother and The other cell it could be the X chromosome she got for her father She would become a mosaic for that character because she would have gene products that are produced with both the homozygous and the heterozygous alleles and so here You see that there are alleles on the X chromosome for fur color orange and black And so in cats here, we see that you can have splotches of black and splotches of orange, depending on which X chromosome was inactivated and which gene products are being produced by the active X chromosome. Linked genes are genes that tend to be found and are passed on to offspring as a unit because they are close together on a chromosome. There are many, many, many genes present on chromosomes with the exception of your really small Y chromosome. The closer they are to one another on a chromosome, the more likely they will be passed on as a unit and considered to be genetically linked. And Morgan was able to provide some justification for this by looking at two characters on the same chromosome, body color and wing size. to see how they should be passed on to their offspring. So here we have a wild type for both traits, gray body, normal wings, that is crossed with a double mutant, black body, vestigial wings. And we see that when they got crossed, they were able to produce an offspring that was heterozygous for both traits. They then did another test cross. where they took that hybrid, dihybrid offspring from the part of the F1 generation and crossed it with another double mutant. And we see what possibilities of chromosomal alleles can be passed on in the eggs. For a female, you would expect it to have either both of the wild type alleles, both of the mutant alleles. or a combination of a mutant and a wild-type allele. And so if we don't know, in particular for these two alleles, what we were trying to determine is, are they on different chromosomes? What would happen if they are on the same chromosome and they are close together? What would happen? So are they linked or are they not? Well, with those different combinations of alleles present in the eggs, And if we take what the sperm could provide, which was only the mutant alleles, if they were physically unlinked, if they were on different chromosomes, or if, what we'll see soon, if they were on the same chromosome, but far apart from one another, we would expect to see all four combinations of alleles. We would expect to see them basically in a one-to-one-to-one-to-one ratio. If they happen to be linked, genetically speaking, we would only expect to see the parental phenotypes, all wild-type or all mutant. And when the data was collected for this particular cross with fruit flies, we do see that it was majority wild-type phenotypes expressed, but there was a small portion of the non-parental phenotype expressed, where you had a combination of wild-type and mutant alleles. So here you see the different combinations that you could get from that F1 dihybrid female along with the recessive male. You could have your the B plus and the VG plus, if these are truly genetically linked, go off with the mutant BVG from the male, or you could have both mutant, both chromosomes carry mutant alleles passed on to future offspring. And so when the alleles were passed on these specific combinations, they were not assorting in theory independently from one another. So they most likely are on the same chromosome. But because you're having these non-parental phenotypes also show up, something else had to be at play. And this was support for crossing over with meiosis because we were having genetic recombination. We were having offspring produced that had traits that did not get past the same or were not the same as what we're seeing in the parents of those offspring. So when the phenotypes match the parental phenotypes, they are called parental types. When the offspring have phenotypes that do not match their parents'different combinations of traits, they are known as recombinants. We can calculate recombination frequency by adding together the number of individuals that do not have those parental phenotypes and dividing it by the total number of offspring that were produced. to calculate a recombination frequency and the frequencies less than 50 percent are indicative of the genes being linked but that there was some crossing over taking place if you have 50 recombination or higher those genes are either not physically linked they are on different chromosomes or they are far apart on the same chromosome and not genetically linked. And so here, if we look at our pea plants, you can have your parental type offspring and you can have recombinant offspring. So this is all connecting together with what we saw with Mendel's work, just looking at it from a chromosomal perspective. So there you have your two fruit flies, your F1 dihybrid, your double mutant. As you go through meiosis, We see that the sperm for the double mutant, because that was the male, can only produce that one chromosome no matter how much crossing over is taking place. Those are the only combination of alleles you can have because your homozygous recessive. Whereas with the F1 dihybrid, because we have different alleles on both chromosomes and crossing over can take place in prophase I, we can have the parental chromosomes passed on intact. or we can have recombinant chromosomes passed on to future offspring. So since the linkage is not complete in terms of having just truly a one-to-one ratio with the parental phenotypes, there had to be something taking place causing the alleles to be combined differently. Again, that's that crossing over the homologs in prophase I. So here's an example of how you can take that data and calculate it to determine what that recombination frequency is. So if you have alleles, again, that are close together on the same homologous chromosomes with the crossing over, you will get different combinations of alleles in the gametes that are produced from that particular cell. If they are on separate... chromosomes or if they are far apart on the same chromosome, they are able to assort independently from the other alleles. And so you would have no genetic links. If they were on the same chromosome, you could have a physical linkage. Having these different combination of alleles is going to provide a lot of variation is going to give us different combinations of gametes. It's going to, along with random fertilization, which is also going to increase those combinations. And having all of this genetic variation allows natural selection to kind of be at work in terms of determining which combination of alleles within a population are going to provide the greatest likelihood of success for that particular population to be able to survive and reproduce. We can look at these alleles along with these recombination frequencies to determine how the alleles are kind of located in comparison to one another. Genetic maps are just going to give us a list of those loci that are found along a chromosome. And again, the linkage maps. are going to be allowed to show us how we can look at chromosomes based on for the alleles based on their recombination frequencies these units that are used to describe the distance between these alleles gives us an idea of their relative distance and the order in which these alleles are found on the chromosomes but that does not mean that we have an exact location it just kind of helps us to order things So here with these three recombination frequencies, by knowing the relationship between B and CN and between CN and VG, we can kind of ascertain how B should fall, whether B should fall between CN and VG, whether B should fall on the opposite side of VG. I've already talked a lot about this physical versus genetic linkage. Cytogenetic maps. are able to identify where genes are located at in respect to the features that we physically see on the chromosomes with the centromeres and chromosomal banding. And you've seen one of those in my classroom. There's an example with fruit flies. And we have two sections left. This one here you have covered a lot previously in your first biology class. that when you have significant alterations in chromosomes, if the results in chromosomes not being split properly in meiosis, metaphase 1, anaphase 1, and in meiosis 2, or sorry, in part 2 of meiosis, which would be your metaphase 2 and anaphase 2, so the splitting of the homologous chromosomes, and the first part of meiosis, the splitting of the non-identical sister chromatids, and the second portion of meiosis, that could result in non-disjunction. Or if you just have physical alterations to your chromosomes, that too can lead to some pretty significant issues. Plants are able to tolerate this a lot better than we are. And so when that takes place, place with plants, they often will end up, and there's examples of your non-disjunctions, they will often be able to survive. And actually, they definitely do a lot better with increased numbers of gene products, but they typically are just going to get multiple copies of their chromosomes. They're genetically engineered to do that. Aneuploidy is when gametes are fertilized, or it's going to result when gametes that are fertilized that had a non-disjunction event occur, you will typically have either a monosomic zygote or a trisomic zygote, one with a single copy of a chromosome or one with three copies of a chromosome. Because when that non-disjunction event takes place and the chromosomes don't split, one of the gametes will get an extra copy of a chromosome and another one will be missing one. So polyploidy again is when you're going to get multiple sets of chromosomes you can have triploidy and tetraploidy and by having those extra set of chromosomes with your plants that allows them to make multiple amounts of gene products and that typically is going to especially like when you buy fruits and vegetables in stores that's just going to help make them more hardy So these different alterations of chromosomal structure, you've talked about these before, deletion, removing chromosomal segments, duplication, repeating them, inversion, reversing them, and translocation, physically moving them. All of those different events can have very substantial impacts on what gene products are produced when the DNA is transcribed and translated. that can cause a cascade of issues oftentimes again it can result in stillbirths or miscarriages if you have individuals that are able to survive birth or survive to birth, they will have symptoms that are indicative of their particular disorder. In this case with Down syndrome, you get a third copy of chromosome 21. There's some pretty typical physical features that we see with individuals that have Down syndrome. There are some intellectual deficits, physical benchmarks that are not meant. One of every 700 children in the U.S. has Down syndrome. There has been seen to be an increase in frequency of Down syndrome with the age of the mother. It's not truly known what is the cause behind that. One possibility has to do with the age of the eggs, because those have been present in a female since they... reached the point where they were able to produce offspring. But there's a lot more work to be done in that regard. We also have aneuploidy with sex chromosomes. And there's many different types of aneuploidy you can have with sex chromosomes. You can have Klinefelter, where you end up with three sex chromosomes, two X chromosomes and one Y. You can have Turner syndrome, where you only end up with a single X chromosome. This is the only monosomy that is known that humans are able to live and be able to go on with life. If you have monosomy for any of the other chromosomes, most likely you will not make it. You will not be able to be born and still be alive. Credo shot is when you have a deletion with chromosome 5. Individuals with this have some intellectual deficits. They have a cat-like cry. They do not live very long. Other cancers are known, or sorry, not other, but some cancers are known to be present when you have the translocation. CML would be one of those. And there you can see the piece of chromosome 9 being translocated to chromosome 22. And then the final thing that we're going to talk about in this are some inheritance patterns that don't follow Mendel. There's two major exceptions we're going to touch on. And one is due to imprinting, and that's going to involve genes in your nucleus. And the other is going to involve DNA that is found outside of the nucleus. Both of them are going to have inheritance patterns based primarily on the gender. the sex of the parent that's contributing the alleles. With genomic imprinting, that could be either the male or the female. It depends on which parent passed along the alleles that were not imprinted on. When alleles have imprinting, you will have genes that basically have methyl groups attached to cysteine residues. And when they have these cysteine or the methyl groups attached, it silences the genes. They cannot, those pieces of DNA cannot be transcribed. This often is going to happen during gamete production. So when they're first made and the genes that are going to be imprinted on are going to play pretty significant roles in embryonic development. You would not have both the mother and the father's alleles imprinted on. then you would not get the gene products so you would only have one of the two and any imprinting that is done during the scammy production gets erased when an offspring are formed any imprinting that was present from the mother or father's alleles that gets erased and new imprinting would get done this is only going to affect a very small fraction of mammalian genes And there's kind of a picture of one example of this with a mouse. If you have the normal allele for this size gene, it won't matter which one gets imprinted on, the mouse would still be a normal size. If the mother or the father has the mutant allele gene and the mutant allele is imprinted on, then... the mass would still be normal sized. If the normal allele is imprinted on and only the mutant allele is expressed, the mass would be significantly smaller. And again, it does not matter which parent has it. It's not that one parent's alleles are going to be imprinted on more than another, but only one of the two will be imprinted. You only want one of the two gene products being generated. The final piece we're going to talk about are organelle genes. There are organelles that contain circular pieces of DNA, such as mitochondria, chloroplasts, and some plant plastids. And because those organelles initially came from the mother this one is going to be sex dependent because remember that it was the ovum that was fertilized the eggs that are fertilized and that's part of why through meiosis you have one egg and three polar bodies that are formed from those four cells with chromosomes you have one with an uneven distribution of cytoplasm because you need to make sure there's enough there are enough nutrients and enough materials for that egg to be able to divide and grow into a fetus so if there are defects in the alleles that are passed on by the mother and those pieces of dna they would be due solely to the mother's alleles this one was seen first with plants We have seen some issues with mitochondrial genes in terms of how it impacts ATP and how it has some impacts on disorders with both your muscular and your nervous systems.

He did not know about chromosomes. So we're going to take what Mendel was able to describe with his pea plants and see how that connects to what we know now is present in cells with chromosomes. Mitosis and meiosis were first described back in the late 1800s, and the idea behind chromosomal inheritance, the chromosome theory of inheritance, tells us that Mendelian genes have specific positions on our chromosomes, specific placements, and that these chromosomes are able to undergo both segregation and independent assortment, which would account for this behavior of the chromosomes, how they move during meiosis, would provide support for Mendel's laws of these, of segregation and independent assortment. So again, this is just providing, connecting the dots between Mendel's characters and what we now know takes place with our chromosomes. And we've talked about both of these previously, especially in our meiosis lab.

how the alleles for a particular gene are going to separate during gamete formation. You're going to end up with one of the homologs from each parent in a particular gamete. And since there are multiple chromosomes for many organisms present in their nuclei, when they undergo meiosis with this independent assortment, the chromosomes do not have to be passed together as a unit. aka mother's chromosomes don't have to go all together. They can be mixed with dad's chromosomes.

And that is what's going to result in having physical representations that phenotypic presentations that do not match either parent. And this is not, there we go. Sorry, I didn't think I threw any questions in this one. I will be skipping through the questions.

We're not answering those in class. So Morgan was able to provide evidence for the idea that genes were located on specific chromosomes and that actually was done using the sex chromosomes. He did experiments with fruit flies. Fruit flies really work very well for experimentation because they make lots of offspring.

They can be bred fairly quickly and they have a limited number of chromosomes that can be examined. When we talk about phenotypes... from Morgan's work, we're going to be dealing with wild type and mutant. Wild type would be what we would normally expect to see if you had one of your dominant alleles, and mutant would be what we would expect to see if you only had recessive alleles. So the red eyes are wild type, the white eyes are mutant for these fruit flies.

And so how he was able to correlate the alleles behavior with chromosomes was he mated male flies that had white eyes with female flies with red eyes that were wild-type for both alleles and the white eyes obviously are mutant for both and when they produced offspring whether they were male or female they only had red eyes when those offspring were mated they produced the three to one ratio that we would expect to see but it was only the males that had the white eyes. And so if only the males were able to have white eyes, this eye color gene had to be present on the X chromosome, and the females weren't getting both copies of that mutant allele. So this was just some support for this theory of inheritance with chromosomes.

And there you can kind of see it. From the male and female perspective, you see a physical representation of F1, F2, and what that had to tell us about the alleles and where they were found on the chromosomes. So 15.2 is going to focus more on sex-linked genes and that humans and other animals, there are chromosomal differences that are able to determine what sex an individual will be. humans we have a larger X chromosome and a smaller Y chromosome. There are only small sections at the end of the Y chromosome that are homologous with regions on the X chromosome.

The majority of the alleles that are coded for, the majority of the genes that are coded for on the Y chromosome. are going to be dealing with male anatomical features. One example would be the SRY gene. Females get two copies of the X chromosome.

Males get both an X and a Y. When we look at our gametes, females can only pass on an X chromosome, which is why the males are guaranteed to get an X chromosome, as long as the chromosomes do not have any alterations. or malfunctions in the meiosis process, while sperm can either pass on an X or a Y chromosome since males have both chromosomes available.

Depending on what animal you're talking about, you can see different systems used to represent those genders. And so there's just some examples right there. Again, we're not doing questions.

Sex-linked genes. If you have genes that are located specifically on one of your sex chromosomes, they're known as sex-linked genes. If they're on the Y chromosome, they're called Y-linked genes. We already talked about the SRY one. If they're on the X chromosome, they are X-linked genes.

And the X chromosomes are the ones that we typically are going to be talking about when it comes to disorders because the X chromosomes have genes that don't just code. for sex while the Y chromosome tends to focus specifically on that. When you get your X-linked genes, they're going to follow, sorry, when X-linked genes are inherited, they follow certain patterns.

Females have to have two copies of a recessive allele to show that trait. They have to be homozygous for it, but because males only have one X chromosome, they only have to get one copy of that recessive allele. to show its phenotypic expression. So they are hemizygous for that.

So any sort of X-linked recessive disorder is more likely to be found in males than females since they only have to get one copy as opposed to two. Some examples of X-chromosome disorders that are due to recessive alleles present there include color blindness, Duchenne muscular dystrophy, hemophilia, baldness is another one that comes to mind. So in this particular slide, we have three different combinations of parents with alleles that are on the X chromosome.

Here in the first picture in A, we have a mother who is homozygous dominant for those alleles, paired with a father who is showing that trait phenotypically. They have a recessive allele on the X chromosome, and when they pass those alleles on to their offspring. Because the mother can only pass on a dominant allele, there's no way for any of the offspring to have that recessive allele present itself. You can't get both recessive alleles for the female, and the male definitely can't get a recessive allele since the female can only pass on, or sorry, the male offspring can't get a recessive allele since their mother can only pass on a homozygous dominant allele.

The females, however, would be heterozygous for this particular trait because the father can only give them that recessive allele. If you take a heterozygous female and mate it with a homozygous dominant male, you do have the possibility of having some offspring have the recessive trait present itself because you could have a female. provide a male offspring with that recessive allele.

And since there's nothing else to balance it, because they can only get the Y allele from their father in order to be male, you have a 50-50 chance of showing that trait or having that trait be physically represented. You can have a female that is homozygous dominant or is a carrier for that trait. because you have a one in two chance of getting the dominant or the recessive allele from the mother if you have a heterozygous mother mate with a male that is recessive and is expressing that recessive trait your odds definitely are going to increase again you have a one in two chance as a female of getting that dominant allele from your mother and that means you have one in two chance of getting the recessive allele. If you are female, you can only get that recessive allele from your father.

So because you that's the only little you can get you have it's 50-50 odds as to whether you're going to show the recessive trait or not. With males, it would also be 50-50 because although the male father had that trait, it would be the mother that would provide the allele for that trait to her offspring, and she can give either the dominant or the recessive allele. So X inactivation, males have an X chromosome, females have two X chromosomes.

We do not need as females twice the amount of genes produced that you would have present on your X chromosomes if they were both being transcribed and translated. So one of those X chromosomes gets randomly inactivated as the embryo is growing and it gets condensed into what we call a bar body. And if a female is heterozygous for genes found on that X chromosome, because it is randomly inactivated in one cell it could be the X chromosome she got from the mother and The other cell it could be the X chromosome she got for her father She would become a mosaic for that character because she would have gene products that are produced with both the homozygous and the heterozygous alleles and so here You see that there are alleles on the X chromosome for fur color orange and black And so in cats here, we see that you can have splotches of black and splotches of orange, depending on which X chromosome was inactivated and which gene products are being produced by the active X chromosome.

Linked genes are genes that tend to be found and are passed on to offspring as a unit because they are close together on a chromosome. There are many, many, many genes present on chromosomes with the exception of your really small Y chromosome. The closer they are to one another on a chromosome, the more likely they will be passed on as a unit and considered to be genetically linked.

And Morgan was able to provide some justification for this by looking at two characters on the same chromosome, body color and wing size. to see how they should be passed on to their offspring. So here we have a wild type for both traits, gray body, normal wings, that is crossed with a double mutant, black body, vestigial wings.

And we see that when they got crossed, they were able to produce an offspring that was heterozygous for both traits. They then did another test cross. where they took that hybrid, dihybrid offspring from the part of the F1 generation and crossed it with another double mutant. And we see what possibilities of chromosomal alleles can be passed on in the eggs. For a female, you would expect it to have either both of the wild type alleles, both of the mutant alleles.

or a combination of a mutant and a wild-type allele. And so if we don't know, in particular for these two alleles, what we were trying to determine is, are they on different chromosomes? What would happen if they are on the same chromosome and they are close together? What would happen? So are they linked or are they not?

Well, with those different combinations of alleles present in the eggs, And if we take what the sperm could provide, which was only the mutant alleles, if they were physically unlinked, if they were on different chromosomes, or if, what we'll see soon, if they were on the same chromosome, but far apart from one another, we would expect to see all four combinations of alleles. We would expect to see them basically in a one-to-one-to-one-to-one ratio. If they happen to be linked, genetically speaking, we would only expect to see the parental phenotypes, all wild-type or all mutant.

And when the data was collected for this particular cross with fruit flies, we do see that it was majority wild-type phenotypes expressed, but there was a small portion of the non-parental phenotype expressed, where you had a combination of wild-type and mutant alleles. So here you see the different combinations that you could get from that F1 dihybrid female along with the recessive male. You could have your the B plus and the VG plus, if these are truly genetically linked, go off with the mutant BVG from the male, or you could have both mutant, both chromosomes carry mutant alleles passed on to future offspring.

And so when the alleles were passed on these specific combinations, they were not assorting in theory independently from one another. So they most likely are on the same chromosome. But because you're having these non-parental phenotypes also show up, something else had to be at play. And this was support for crossing over with meiosis because we were having genetic recombination. We were having offspring produced that had traits that did not get past the same or were not the same as what we're seeing in the parents of those offspring.

So when the phenotypes match the parental phenotypes, they are called parental types. When the offspring have phenotypes that do not match their parents'different combinations of traits, they are known as recombinants. We can calculate recombination frequency by adding together the number of individuals that do not have those parental phenotypes and dividing it by the total number of offspring that were produced. to calculate a recombination frequency and the frequencies less than 50 percent are indicative of the genes being linked but that there was some crossing over taking place if you have 50 recombination or higher those genes are either not physically linked they are on different chromosomes or they are far apart on the same chromosome and not genetically linked.

And so here, if we look at our pea plants, you can have your parental type offspring and you can have recombinant offspring. So this is all connecting together with what we saw with Mendel's work, just looking at it from a chromosomal perspective. So there you have your two fruit flies, your F1 dihybrid, your double mutant. As you go through meiosis, We see that the sperm for the double mutant, because that was the male, can only produce that one chromosome no matter how much crossing over is taking place.

Those are the only combination of alleles you can have because your homozygous recessive. Whereas with the F1 dihybrid, because we have different alleles on both chromosomes and crossing over can take place in prophase I, we can have the parental chromosomes passed on intact. or we can have recombinant chromosomes passed on to future offspring.

So since the linkage is not complete in terms of having just truly a one-to-one ratio with the parental phenotypes, there had to be something taking place causing the alleles to be combined differently. Again, that's that crossing over the homologs in prophase I. So here's an example of how you can take that data and calculate it to determine what that recombination frequency is. So if you have alleles, again, that are close together on the same homologous chromosomes with the crossing over, you will get different combinations of alleles in the gametes that are produced from that particular cell. If they are on separate...

chromosomes or if they are far apart on the same chromosome, they are able to assort independently from the other alleles. And so you would have no genetic links. If they were on the same chromosome, you could have a physical linkage. Having these different combination of alleles is going to provide a lot of variation is going to give us different combinations of gametes. It's going to, along with random fertilization, which is also going to increase those combinations.

And having all of this genetic variation allows natural selection to kind of be at work in terms of determining which combination of alleles within a population are going to provide the greatest likelihood of success for that particular population to be able to survive and reproduce. We can look at these alleles along with these recombination frequencies to determine how the alleles are kind of located in comparison to one another. Genetic maps are just going to give us a list of those loci that are found along a chromosome. And again, the linkage maps.

are going to be allowed to show us how we can look at chromosomes based on for the alleles based on their recombination frequencies these units that are used to describe the distance between these alleles gives us an idea of their relative distance and the order in which these alleles are found on the chromosomes but that does not mean that we have an exact location it just kind of helps us to order things So here with these three recombination frequencies, by knowing the relationship between B and CN and between CN and VG, we can kind of ascertain how B should fall, whether B should fall between CN and VG, whether B should fall on the opposite side of VG. I've already talked a lot about this physical versus genetic linkage. Cytogenetic maps. are able to identify where genes are located at in respect to the features that we physically see on the chromosomes with the centromeres and chromosomal banding. And you've seen one of those in my classroom.

There's an example with fruit flies. And we have two sections left. This one here you have covered a lot previously in your first biology class. that when you have significant alterations in chromosomes, if the results in chromosomes not being split properly in meiosis, metaphase 1, anaphase 1, and in meiosis 2, or sorry, in part 2 of meiosis, which would be your metaphase 2 and anaphase 2, so the splitting of the homologous chromosomes, and the first part of meiosis, the splitting of the non-identical sister chromatids, and the second portion of meiosis, that could result in non-disjunction. Or if you just have physical alterations to your chromosomes, that too can lead to some pretty significant issues.

Plants are able to tolerate this a lot better than we are. And so when that takes place, place with plants, they often will end up, and there's examples of your non-disjunctions, they will often be able to survive. And actually, they definitely do a lot better with increased numbers of gene products, but they typically are just going to get multiple copies of their chromosomes. They're genetically engineered to do that.

Aneuploidy is when gametes are fertilized, or it's going to result when gametes that are fertilized that had a non-disjunction event occur, you will typically have either a monosomic zygote or a trisomic zygote, one with a single copy of a chromosome or one with three copies of a chromosome. Because when that non-disjunction event takes place and the chromosomes don't split, one of the gametes will get an extra copy of a chromosome and another one will be missing one. So polyploidy again is when you're going to get multiple sets of chromosomes you can have triploidy and tetraploidy and by having those extra set of chromosomes with your plants that allows them to make multiple amounts of gene products and that typically is going to especially like when you buy fruits and vegetables in stores that's just going to help make them more hardy So these different alterations of chromosomal structure, you've talked about these before, deletion, removing chromosomal segments, duplication, repeating them, inversion, reversing them, and translocation, physically moving them.

All of those different events can have very substantial impacts on what gene products are produced when the DNA is transcribed and translated. that can cause a cascade of issues oftentimes again it can result in stillbirths or miscarriages if you have individuals that are able to survive birth or survive to birth, they will have symptoms that are indicative of their particular disorder. In this case with Down syndrome, you get a third copy of chromosome 21. There's some pretty typical physical features that we see with individuals that have Down syndrome. There are some intellectual deficits, physical benchmarks that are not meant.

One of every 700 children in the U.S. has Down syndrome. There has been seen to be an increase in frequency of Down syndrome with the age of the mother. It's not truly known what is the cause behind that. One possibility has to do with the age of the eggs, because those have been present in a female since they... reached the point where they were able to produce offspring.

But there's a lot more work to be done in that regard. We also have aneuploidy with sex chromosomes. And there's many different types of aneuploidy you can have with sex chromosomes. You can have Klinefelter, where you end up with three sex chromosomes, two X chromosomes and one Y. You can have Turner syndrome, where you only end up with a single X chromosome.

This is the only monosomy that is known that humans are able to live and be able to go on with life. If you have monosomy for any of the other chromosomes, most likely you will not make it. You will not be able to be born and still be alive.

Credo shot is when you have a deletion with chromosome 5. Individuals with this have some intellectual deficits. They have a cat-like cry. They do not live very long. Other cancers are known, or sorry, not other, but some cancers are known to be present when you have the translocation.

CML would be one of those. And there you can see the piece of chromosome 9 being translocated to chromosome 22. And then the final thing that we're going to talk about in this are some inheritance patterns that don't follow Mendel. There's two major exceptions we're going to touch on.

And one is due to imprinting, and that's going to involve genes in your nucleus. And the other is going to involve DNA that is found outside of the nucleus. Both of them are going to have inheritance patterns based primarily on the gender.

the sex of the parent that's contributing the alleles. With genomic imprinting, that could be either the male or the female. It depends on which parent passed along the alleles that were not imprinted on. When alleles have imprinting, you will have genes that basically have methyl groups attached to cysteine residues.

And when they have these cysteine or the methyl groups attached, it silences the genes. They cannot, those pieces of DNA cannot be transcribed. This often is going to happen during gamete production.

So when they're first made and the genes that are going to be imprinted on are going to play pretty significant roles in embryonic development. You would not have both the mother and the father's alleles imprinted on. then you would not get the gene products so you would only have one of the two and any imprinting that is done during the scammy production gets erased when an offspring are formed any imprinting that was present from the mother or father's alleles that gets erased and new imprinting would get done this is only going to affect a very small fraction of mammalian genes And there's kind of a picture of one example of this with a mouse.

If you have the normal allele for this size gene, it won't matter which one gets imprinted on, the mouse would still be a normal size. If the mother or the father has the mutant allele gene and the mutant allele is imprinted on, then... the mass would still be normal sized. If the normal allele is imprinted on and only the mutant allele is expressed, the mass would be significantly smaller. And again, it does not matter which parent has it.

It's not that one parent's alleles are going to be imprinted on more than another, but only one of the two will be imprinted. You only want one of the two gene products being generated. The final piece we're going to talk about are organelle genes. There are organelles that contain circular pieces of DNA, such as mitochondria, chloroplasts, and some plant plastids. And because those organelles initially came from the mother this one is going to be sex dependent because remember that it was the ovum that was fertilized the eggs that are fertilized and that's part of why through meiosis you have one egg and three polar bodies that are formed from those four cells with chromosomes you have one with an uneven distribution of cytoplasm because you need to make sure there's enough there are enough nutrients and enough materials for that egg to be able to divide and grow into a fetus so if there are defects in the alleles that are passed on by the mother and those pieces of dna they would be due solely to the mother's alleles this one was seen first with plants We have seen some issues with mitochondrial genes in terms of how it impacts ATP and how it has some impacts on disorders with both your muscular and your nervous systems.

Transcript for:Mendel's Discoveries and Chromosomal Inheritance

Transcript for:
Mendel's Discoveries and Chromosomal Inheritance