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Genetically (un)fathomable Variation

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Lyle W. Konigsberg

This chapter was another that focused heavily on the schematics of genetics including; quantitative geneticmultivariate quantitative genetic, complex segregation, and quantitative trait locus linkage analyses all connecting back to the evolutionary models applying to these quantitative traits.

The big questions asked at the beginning of the chapter concerned human biologists and if they should be studying variation within the quantitative genetic framework?

What is mostly unknown is that a large portion of variation that is studied today is quantitative already. The example given here was anthropomorphicstudies and longevity studies and how their variability implies quantitative measure.

Below are relative ideas to keep in mind from the chapter:

  • Heritability is represented by h^2.  It is defined as the proportion of phenotypic variance due to additive genetic effects.  H^2=  Va/Va+Ve =Va/Vp
  • When there is dominance at one or more loci the concept of heritability becomes more complicated.  Pi= a1 + d1 + e1
  • If you include variances due to both additive genetic and dominance it equals            H^2= Va + Vd/ Va + Vd + Ve = Va +Vd/ Vp = Vg/Vp
  • Regression analysis is often used to estimate narrow-sense heritability.
  • Towne 1993
    • He presented an interesting quantitative genetic analysis in that they made use of serial measurements on relatives.  They modeled growth in infants within the first 2 years of life.
  • Weiss 1993
    • He compared narrow-sense heritability estimates for systolic blood pressure in different populations.  Systolic blood pressure is a quantitative trait.
  • Beall 1997
    • He studied hypoxic ventilator response.  It used the pedigree relationships to estimate the narrow-sense heritability’s.
  • Jarvik 1998-
    • Introduced a summary and description of complex segregation analysis with a welcome message, “I will describe the CSA methods with record brevity, facilitated by the absence of any equations.”
  • Drift, Migration, and Population Structure Analysis From Quantitative Traits
    • Drift and migration have the same effects on quantitative traits as they do on single locus genes

14 thoughts on “Genetically (un)fathomable Variation

  1. Meghan Steel

    This was yet another chapter that turned my eyes into ice cubes. I guess I just completely space out on equations. Which is why I appreciate you listing only the main equations mentioned in the article, especially since these are the few that I can actually understand.

    Unfortunately, and despite my lack of understanding of most of this chapter, it's highly applicable to my paper topic. I'm currently examining the heritability of chronic conditions associated with aging (specifically as it relates to evolutionary trade offs). This relates to our 23andMe project as well. SO one good topic to bring up during discussion: How do you think disease should be analyzed as a genetic "trait" when we consider human variability?

    1. Christopher Lynn

      remember to log in before you comment so the presenters don't have to wait for me to approve your comments

  2. Andrea

    You guys did a nice job of summarizing the chapter and laying out the key equations. I have never been good at understanding the equations that go along with genetics, but it's good that you showed when to use what equations. I think it'd be interesting to discuss how the technologies with genetics are evolving and also relate this to the 23andMe testing.

    1. Christopher Lynn

      I still want more narrative, less lists. You've glossed a few things from the beginning of the chapter, suggesting you didn't read very far. Granted the chapter is dense, but why did you choose to highlight only the studies of genetic parameters using pedigree data?

    2. Brittany Fuller

      If you click on his name it takes you to a site that tells about him a little. We could not find much information on him or a picture of him to put up.

  3. Jess Leonard

    Something I found interesting in this reading, if only a small aside, was the relationship between response to selection and narrow-sense heritability as described by Fisher's fundamental theorem (p.164). In short, it states that the rate of natural selection is proportional to additive genetic variance. As we understand evolution by selection to decrease the additive genetic variance within a population, it only makes sense that the rate of evolution within a population will decrease over time. This idea is readily seen in smaller human population and, as the author mentions, among animal breeders. Toward the end, the chapter also mentions the bottlenecking of human populations and specifically, "the history of bottlenecks in population size, and the resulting few contributors to a current small population, is probably typical of the history of the small isolated groups in which much of human evolution has occurred." (pp.168-9)
    Hearkening back to the last post concerning migration, I wonder, in this emerging new world of physical interconnectivity and population movement on an unprecedented scale, how will natural selection in reference to additive genetic variation change or adapt over time?

  4. lmwiggins

    I found it really helpful that the important equations were included in this summary. I’m also not very good at understanding this particular side of genetics, so it’s nice to have a guide. In the reading, what I found most interesting was the idea of “peaks” and “valleys” in Wright’s Shifting Balance Theory. In regard to that, I wonder what kind of “peaks” and “valleys” we could be seeing now and what kinds may appear in the future?

  5. rebeccaleon

    As others have already noted, this chapter was pretty heavy. So, I am going to ask about a few things that I was left wondering about after reading this chapter.

    On page 164 the author briefly mentions traits that are not heritable won't show any response to selection. This definitely makes sense, but I am curious as to what kind of traits aren't heritable. Can you think of any? If so, why would we have traits that aren't heritable (from an evolutionary perspective)?

    Sciulli and Mahaney (1991) and Christensen (1998) (pp. 167) suggest that human teeth were reduced in size due to selection and not drift, because the population sizes needed for drift to explain the reduction are too small. I am willing to take their word on it that our teeth got smaller, because of some selection pressure. However, what is the evolutionary benefit to having smaller teeth? I know we don't need big teeth anymore, because we cook our food making it easier to chew and digest. So, big teeth weren't necessary anymore, but it doesn't mean that we couldn't have kept our big teeth.

    Wright's Shifting Balance Theory incorporates mutation, migration, selection, and drift all working together. Do you think this is how evolution works with all of the forces working together? Is it possible for evolution to occur with only one force in motion? If so, would the evolution of a trait occur at the same rate as the evolution of another trait that was being pushed forward by all four forces?

  6. Jonathan Belanich

    This chapter goes very in-depth on the topic of how genes are connected to each other in the genome and several of the mathematical equations involved in this, which you nicely summarize and lay out for us. You also summarized several of the proposed ideas from other researchers, which I found very concise and informative.

    At the end of the chapter, Thompson speaks about genetic drift, and specifically, bottleneck situations. I wonder if there are any bottleneck situations in our own past that have seriously affected our genetics, possibly even still affecting us today.

  7. Taylor Burbach

    What a dense chapter! Thank you for summarizing it so nicely. My notes are just half-aware chicken scratch.
    I found the "average heritability" section of this chapter very interesting. Although the research isn't conclusive, the question is tantalizing. The difficulty behind this research lies in multiple genes coding for one trait, and it is especially hard to quantify traits highly affected by environmental variation. How do you calculate heritability with this in mind? I'm not sure I'm grasping the concept completely, but this is definitely something that interests me.

  8. Katie Coward

    I think it would be very helpful to use this chapter when thinking about the 23 & me testing. It would be interesting if two siblings were tested at the same time and compared their genetic similarities to see if they actually had 1/4 of their genes in common ( I think that's what the chapter was insinuating).

  9. Emily Barron

    To me this chapter was just a bunch of confusing equations and concepts. I haven't taken genetics yet, so I hadn't seen any of this before. The class discussion helped it make more sense, though honestly I hope I don't ever see it again in an anthropology class or at any point after next semester. This would have been a good chapter to discuss 23 & me results with, especially the benefits, drawbacks, and validity of the tests.

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