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7. Quantitative Genetics.pdf

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Biology (Sci)
BIOL 202
Tamara Western

Quantitative Genetics: In reality, the phenotypic changes observed in nature are more gradual and complex than the single, discrete changes we see with classical Mendelian genetics Most of the complex traits are controlled by many different genes, which results in their being a large variance in the types of phenotypes we observe Therefore, quantitative genetics is the study of these polygenic traits Quantitative genetic variation can be described in three ways: Traits are influenced by multiple genes, i.e. theyre polygenic They are usually influence more easily by environmental factors than simple Mendelian traits Both of the factors above usually lead to a continuous distribution of the particular trait o For example, you can see the near normal, or continuous, distribution to the right when comparing a sample population by their height There are many samples of these traits as most traits in genetics actually do involve multiple genes For example, to the right, random traits like swimming speed and general cognitive ability (IQ) both follow a relatively normal distribution in these sample populations o Even though swimming speed has been divided into men and women, pooling both genders together into one sample population will still produce a normal distribution Quantitative genetics is still driven by the basis of Mendelian genetics, where different alleles at one locus will produce different phenotypes, in this case, short and tall As we can see, if the trait is only affected one locus, then the F1 hybrid will either be short, tall or an intermediate between the two (like we have seen before) However, if the trait is affected by more than one locus, then the hybrids receive different dosages of the trait, as we saw before o As the number of loci involved increases, the distribution of hybrids will be more and more continuous Environmental factors will contribute to smoothing out the phenotypic variance to produce a smooth distribution curve Environmental factors can also produce a phenotypic variation within one specific genotype; i.e. in pea wrinkliness for example, environmental factors may vary the severity of the wrinkles in an A/A plant As can be seen in these graphs, not all of the members of the same genotype will have the same phenotype, height in this case o This could be caused by many things in the environment, like nutrition, climate, etc. There is also lots of overlap between genotypes, also caused by the environment o For example, a tall A/a may actually be taller than some short A/A individuals o Almost all traits follow patterns like this As can be seen, adding the variations of all of the genotypes will produce a height distribution of the entire population o Not only are the distributions for each genotype continuous, but the summative distribution of the entire population is also continuous The shape of a distribution curve depends on two things: the mean value and the variance of the population Changing the mean of the population will shift the curve to the left (decrease) or right (increase) on the graph The lower the variance of a population, the steeper the curve o High variance populations will have a much less steep slope Variance is a measure of the sum of the squared deviation from the mean over the size of the population; i.e. how varied the results are o Squaring the mean ensured all variance values are positive o It can be calculated using the formula to the right The source of variance in quantitative traits, called the total phenotypic variance, oP V , can be divided into a bunch of subfamilies: V can be first split into genetic variance, V , and environmental variance P G o V caG be split further into three subfamilies: Additive genetic variance, or VA, is responsible for the direct resemblance between parents and their offspring Epistatic variance, or VI, is the variance caused by interactions between loci Dominance variance, or VD, is caused by the interactions between genes at ONE locus o Environmental variance can be further divided into 2 subcategories: The variance caused by maternal effects, M is caused by, for example, nutrition in the womb Pure environmental variance, VEis a wide family encompassing all other environmental factors such as climate These variances are all additive: o V =GV + VA+ V I D o V environmental M V E o V =PV + G environmentalA+ V +IV + D + VM E Additive genetic variance, VA, is the variance among individuals due to the additive effects of alleles and genes This can be between genes in a single individual, or within individuals of a population; the principles are the same o E.g. a single locus with two alleles: A and a A gives 2 doses, a gives 1; therefore AA will have 4 units of the trait, Aa will have 3, and aa will have 2 This is assuming no interactions between the alleles, and can be used for polygenetic traits as well o E.g. in a population, two pure populations of two extreme phenotypes can mate to produce an intermediate offspring (F1 hybrids) The number of intermediates is determined by the number of loci involved In both of these cases, the effects of the various genes are additive; AA gives 2 doses + 2 doses = 4 doses o V iA important because it determines the degree to which relatives resemble each other Thus, it is the most important for evolution This can be seen, for example, in stickleback fish, who have adapted to different climates to produce two different phenotypes; lake and stream o The two phenotypes differ in color, size, and shape, but particularly in the number of gill rakers to the body depth/height ratio
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