Human Pedigrees – Classic Mendelian Genetics

by Georgina Cornwall, PhD

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    00:01 Now humans also have dominant and recessive traits. However, we have a much longer generation time and so it is very hard to grow a few humans and watch what flower color is and then have them self with each other. It doesn't really work out. When humans we really have to look at family histories or pedigrees. So I wanted to briefly introduce you to a couple of pedigree showing both dominant and then recessive inheritance patterns. Dominant traits will generally show up in every generation because if the parent has the allele as in the purple flower, then they will produce purple offspring, at least some. Here to introduce the pedigree, we separated out by generations. Here are three generations and males are generally exhibited as squares, females as circles, the lines between them indicate reproduction and the next generation down we can see that there are a certain number of pairings and a certain number of offspring. In the case of the dominant allele, you have an affected individual. Anyone that has one of those dominant alleles is affected.

    01:26 So if the father is affected, then some of the offsprings at least will be also affected, and if they do not get one of the alleles, we consider them as unaffected. But again generally in a pedigree of a dominant trait, you will see the trait show up in every generation.

    01:48 The opposite is true when we consider a recessive form as we saw with Mendel's crosses, you would have the purple phenotype in the beginning and then it would may disappear in the F1 generation and reappear in F2. If we look at a pedigree with a recessive trait, you may see them skip generations. At the top here, we have unaffected parents, but somehow one of them must be a carrier because we see affected individuals show up. In this case, we have unaffected in green, affected in red and then we tried to mark the carriers as half a square or a circle. We tried to assess whether some of them was a carrier. Now we can tell in the second generation, the individual that is second over that has the half circle, we can tell that they must have been a carrier because there are offspring in generation three that are affected. You have to have the homozygous recessive in order for the individual to be affected. Generally, that will lead a trait to skip generations or at least much more prevalent in one generation and less prevalent and more prevalent. An example of this would be twining. You can see that every other generation generally you will have a mother give birth to twins and so there is some hereditary pattern in there. Pedigrees are a way that we detect inheritance patterns by looking at family histories.

    About the Lecture

    The lecture Human Pedigrees – Classic Mendelian Genetics by Georgina Cornwall, PhD is from the course Understanding Genetics.

    Included Quiz Questions

    1. The trait should show up in every generation.
    2. The trait should skip generations.
    3. The trait should show up in every generation but only in female offspring.
    4. The trait should show up in every generation but only in male offspring.
    5. The trait should skip generations and only affect male offspring.
    1. They are both carriers of the trait.
    2. One is a carrier of the trait and the other is not.
    3. Neither are carriers of the trait
    4. Only the mother could carry this trait.
    5. Only the father could carry this trait.

    Author of lecture Human Pedigrees – Classic Mendelian Genetics

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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