Multiple Alleles and Incomplete Dominance – Beyond Gregor Mendel

by Georgina Cornwall, PhD

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    00:00 In another situation, we could have multiple alleles involved. Recall, Mendel choose things that had duality just two different alleles.

    00:12 Now in blood typing, A, B and O blood typing, we have three potential alleles and as you can see from the table here, there are a number of different outcomes. Each individual may only have two, but in the population, there are three and so some people are blood type A and some people are blood type AB and some people are B and some people is O, all dependent on which two alleles they get. This is an example of multiple alleles. Blood typing is interesting because it also exhibits codominance that brings us into our next variation from the phenotypic ratios that we might expect in Mendelian inheritance. With blood type A an individual's red blood cells contain the A antigen or the A name tag and that A name tag makes it specific to A individuals because the immune system is not going to attack it. Now a type B blood itself has type B antigens and a type AB blood type displays both antigens and O means simply no antigens on the surface. The AB individual in ABO blood typing scheme has codominance. This means that both phenotypes are being displayed. This is not an intermediate. For example, it is not pink. It is distinctly different in individual with type A blood cannot take type AB blood nor can B take AB because it is a distinct phenotype. It is not blending inheritance. We will look now at something that looks slightly like blending, but is also not blending inheritance.

    02:07 We call blending was the inheritance's pattern that people were subscribing to mostly before Mendel's time. Let us look at incomplete dominance in which we do get an intermediate phenotype. We will cross a red flower and the color allele in this type of flower, these are not p plants is R for red or W for white in which case no pigment is produced. The offspring of this cross will end up producing a pink flower.

    02:40 Why does it end up producing a pink color? Well that is because we have one R and so one enzyme is making red color while the other enzyme on the other chromosome is not making red color and so we get an intermediate. In this case, we have incomplete dominance because the R allele is not completely dominant as in covering up a white allele and when we cross that F1 generation to produce an F2 generation an incomplete dominance, we see a 1:2:1 genotypic as well as phenotypic ratio. So I will have to I ask you a question why is this not blending? It sure looks like blending doesn't it. Why is it not blending? Imagine that in this case, we have the red and the white flower and as I explained, we have one allele that makes red and one allele that makes white. It looks like blending because they become pink. But when we do the F2 cross, we see the reemergence of white, which is why all of these questions started anyway. Why is it that the offspring of the F2 generation might look something like one of the grandparents? Incomplete dominance is not blending and nor is codominance that we saw in the blood cells and so definitely, we were seeing an exhibition of Mendelian inheritance patterns however, the phenotypes display themselves slightly differently. We end up with different ratios than we might expect from the simple Mendelian pattern.

    About the Lecture

    The lecture Multiple Alleles and Incomplete Dominance – Beyond Gregor Mendel by Georgina Cornwall, PhD is from the course Understanding Genetics.

    Included Quiz Questions

    1. Blood type is influenced by environmental factors so that an individual can display multiple types of blood groups during one's lifetime.
    2. Three alleles IA, IB, and I control the blood types in humans.
    3. Both IA and IB alleles are dominant over the I allele.
    4. Alleles IA and IB exhibit codominance when each allele is inherited by an individual.
    5. Type A and type B blood groups will display ‘A’ and ‘B’ antigens on the cell membrane of RBCs, respectively.
    1. The heterozygotes express both alleles.
    2. The phenotypes of heterozygotes will be intermediate to those of their homozygotic parents.
    3. The phenotype of the heterozygotic individuals will resemble their recessive parent.
    4. The phenotype of the heterozygotic individuals will resemble their dominant parent.
    5. The male parent has a larger influence on the progeny than the female parent.
    1. 1 Red: 2 Pink: 1 White
    2. 2 Red: 1 Pink: 1 White
    3. 1 Red: 1 Pink: 2 White
    4. 3 Red: 1 White
    5. 3 Pink: 1 White

    Author of lecture Multiple Alleles and Incomplete Dominance – Beyond Gregor Mendel

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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