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Polygenic and Pleiotropic Inheritance – Beyond Gregor Mendel

by Georgina Cornwall, PhD
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    00:01 Mendel started his assumptions with some specific rules. You will recall that he looked at some very distinct traits that exhbited certain outcomes and he did this on purpose so that he could find some general rule for how inheritance patterns happened. Now lots of things exhbit Mendelian inheritance patterns, but don't necessarily have the same sorts of phenotypes.

    00:28 Mendel had determined that each trait was specified by a single gene, whether it was flower color or seed shape or whatever. He by chance perhaps, but picked that one single gene would specify that trait and that there were only two alternatives he only saw round or wrinkled. He only saw tall or short and the gene products acted independently at each other, so there was no dependence of one gene like in a metabolic pathway. You could imagine if you have enzyme A and its broken, you don't get the product B and thus you wouldn't get C. So independence of the genes and then also he didn't choose any genes that had environmental effects or he did not chose traits because he didn't know about genes.

    01:19 Let us look at some examples where Mendelian inheritance is happening, but not getting the expected phenotypes. Skin color is a great example of a continuously variant trait, so is height. Right there isn't black or white. There isn't tall or short. There are many different varieties on a continuum. Generally when we see these sorts of things, it involves multiple genes. Here is a very simplistic example of how that might work out. Let us say there were three genes for skin color. Skin color comes about by the production of pigment and there are three genes involved in the production of that pigment. If we were to use a Punnett square to predict the outcomes of this, it would be quite complicated, but you can see that there is a continous spectrum of color that could be obtained dependent on how many alleles are displayed for pigment. This is a really simplified view of how it works, but it does demonstrate the idea of multiple genes or having a continous effect. Polygenic inheritance is meaning that we have multiple genes involved in the expression of this trait.

    02:43 Height is another example and we can see here that height in general form a continous or bell shaped curve with less very small people, lots of medium people and less very tall people.

    03:00 It is on a continuum any time you see a continuum we generally are talking about polygenic inheritance or multiple genes being involved. As we move on, we can see that other genes have more than one effect. For example, we have a single gene and it ends up in the production of one phenotype A or B or C. Examples of this can be found in all sorts of different situations, but let us look at one where one phenotype might actually be a lethal phenotype.

    03:41 Here we are looking at Agouti mice and in this case, we have an allele for yellow. The Y is dominant for yellow color, but it is lethal when there are two copies for some reason. In this case, when we cross two heterozygotes, we end up with one recessive trait, which would be the brown Agouti mouse and the two yellows because the y allele is dominant.

    04:12 But in the bottom corner, we see two dominant y alleles that are lethal and if the mouse has two copies of it, then it does not make it to live and so the ratio here is skewed.

    04:28 We have a 1 to 2 ratio in the offspring. Consider again that we have a number of offsprings not just four, but we're counting probably in the 10s and 100s to see these ratios.

    04:39 Another example where Mendel's ratio didn't play out and we may have this pleiotrophic inheritance is in albinism and sickle cell anemia. In albinism, someone is lacking the enzyme to produce pigment and that has multiple different effects not only do they not have pigment in their hair, but also in eyelashes and in skin and a number of different phenotypes are associated with one gene mutation. Sickle cell anemia is a great example of pleiotrophic inheritance. It results from our hemoglobin molecule having a mutation in the beta subunits and so when it stacks up inside the red blood cell not only does it not carry oxygen, but it causes a sickling of a cell, which is the secondary effect and that sickled cell will tend to get stuck in some of the smaller blood vessels. There are a number of circulatory phenotypes associated with just one mutation. Later we will get to understand that it is just one tiny base change in the DNA that ends up causing these multiple or pleiotrophic effects.


    About the Lecture

    The lecture Polygenic and Pleiotropic Inheritance – Beyond Gregor Mendel by Georgina Cornwall, PhD is from the course Understanding Genetics.


    Included Quiz Questions

    1. PTC tasting
    2. Albinism
    3. Phenylketonuria
    4. Sickle cell anemia
    5. Marfan syndrome
    1. Multiple alleles of a single gene are responsible for polygenic traits in humans.
    2. Height and skin color in human, being controlled by multiple genes, are categorized as polygenic traits.
    3. The polygenic trait distribution analysis in a population exhibits a bell shaped curve.
    4. Polygenic traits fall under the category of continuously varying traits.
    5. During polygenic trait expression, expression of the each dominant allele adds to the expression of the next dominant allele.

    Author of lecture Polygenic and Pleiotropic Inheritance – Beyond Gregor Mendel

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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