Genetics of Complex Traits

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  1. Complex Traits
    Continuous trait
    • Complex traits: are influenced by many factors, including multiple genes, interaction between alleles of different genes, variations in the environment, and interactions between genes and the environment
    • Quantitative trait loci: Genes that control the expression of continuous traits
    • Continuous trait: inherited trait that exhibits many intermediate forms; determined by alleles of many different genes whose interaction with each other and the environment produces the phenotype aka quantitative trait
  2. Many complex traits are _______ traits for which the phenotype can be measured over a range of numbers called _______ values or ______ values. Many such traits show a roughly bell-shaped "normal" distribution of phenotypic values in populations. Human height provides a good example. A few individuals are either very tall or very short, but most people have heights that are clustered around the ______ for the group
    • quantitative traits
    • phenotypic values or trait values
    • average
  3. The apparently continuous distribution of height in this bell-shaped distribution is shaped by the contributions of both ______ and the _______. The ______ genes involved in the expression of a trait, the ______ possibilities exist for phenotypic variation in any given environment and the ______ the potential phenotypes resemble a normal distribution
    • genes 
    • environment
    • more 
    • more
    • more
  4. Rather than being identical, the phenotypes exhibited by individuals of any one genotype are distributed in narrower bell-shaped curves centered around the ______ ______ for that genotype. What is the reason for this?
    • average phenotype
    • The reason is that individuals with the same genotype are subjected to slightly different microenvironments
    • *For the population as a whole, the effects of the environment superimposed on variation in just a few genes can therefore easily approximate a bell-shaped curve of phenotypic calues
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  5. To estimate heritability, scientists first need to obtain a ________ description for the curve, usually bell-shaped, of the trait's distribution in the population under study. How do researchers track the amount of variation?
    Statistically, the result of this analysis is termed the _____ _____ ______, and its calculated as the _____ ______ ______ between each individual value and the ______
    • numerical 
    • by comparing the phenotype for each individual in the population to the average phenotype for the populations as a whole.
    • total phenotype variance (VP)
    • average squared difference
    • mean
  6. As an example, let's consider the phenotype of stem length in a population of dandelions, a common weedy plant in North America. (3-story)
    What do narrower curves mean for stem length?
    • pg 696-697
    • less variance
  7. To distinguish environmental from genetic effects on phenotypic variation, you need to ________ one variable, say the environment, while _______ for the other one (that is, while holding the _______ _______ steady). This particular experiment is easy to accomplish in the case of dandelions, because most dandelion seeds arise from ______, rather than ______, divisions such that all the seeds from a single plant are genetically ________. What is the two step process?
    • quantify
    • controlling
    • genetic contribution
    • mitotic 
    • meiotic
    • identical

    • You could begin by planting genetically identical seeds on a grassy hillside and allowing them to grow undisturbed until they flower. 
    • You then measure the length of the stem of each flowering plant and determine the mean and variance of the distribution of values for this trait in this dandelion population 
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  8. Because all members of this population are genetically _______ (if we ignore rare mutations), any observed variation in stem length among individuals should be a consequence of _______ _______, such as different amounts of ______ and ______ at different locations on the hillside. When represented as a variance from the _____, these observed environmentally determined differences in stem length are called the _______ _______
    • identical
    • environmental variations
    • water and sunlight
    • mean 
    • environmental variance (VE)
  9. To examine the impact of genetic differences on stem length, you take seeds from many different dandelion plants produced in many different locations, and you plant them in a _______ ________. Because you are raising genetically ______ plants in a relatively ________ environment, the observed variation in stem length is mostly the result of genetic differences promoting _______ _______
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    • controlled greenhouse
    • diverse 
    • uniform 
    • genetic variance (VG)
  10. To illustrate the total impact on phenotype of variation in both genes and environment, you take the seeds of many _______ plants from many _______ locations (and thus with _______ genetic variants) and grow them on a ________ hillside. For the population of dandelions that grow up from these genetically diverse seeds, the total _______ variance (__) in stem length will be the sum of the ________ variance (__) and the _________ variance (__):
    • different 
    • different
    • different 
    • common 
    • phenotype variance (VP)
    • genetic variance (VG)
    • environmental variance (VE)
    • VP = VG + VE
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  11. Note that the bell-shaped curve for dandelion stem length is _______ than either taht for genetically identical individuals on a hillside or for genetically variable individuals grown in a ______ _______. For natural populations of dandelions, both ______ variation among individuals and variation in the __________ condition experienced by each plant contribute to the total phenotypic variation
    • broader
    • controlled greenhouse
    • genetic
    • environmental
  12. How do geneticists define the heritability of a phenotypic trait?
    Formally, heritability defined in such a way is called ______ ______ ______, which is abbreviated as _____ by convention because the variance is the ______ ______ deviation from the _____ for a characteristic:
    • the proportion of total phenotypic varaince (VP) ascribable to genetic variation alone (VG)
    • broad sense heritability
    • H2
    • average squared 
    • mean 
    • Broad sense H2 = VG/VP
  13. Genetic variance itself can be subdivided into three components. The first of these is ____, or variance due to ______ _____ effects VA (example on pg 698 mid left). The two other contributions to genetic variance are the variance due to ______ effects (VD) and the variance due to ______ between ______ _____ (VI)
    • VA
    • additive genetic
    • dominance
    • interactions
    • genetic loci
  14. Why does dominance add an additional component to variance?
    because, for example, a heterozygote for a dominant allele has the same phenotype as a homozygote for that allele, yet these individuals do not have the same genotype
  15. Similarly, interactions between alleles at different loci (for example, epistasis) can cause an allele of one gene to have ______ phenotypic values depending on the alleles present at the ______ gene. The total genetic variance is the sum of its three components:
    • different 
    • second
    • VG = VA + VD + VI, thus
    • VP = VA + VD + VI + VE

    • Using these identities, we can define broad sense heritability more precisely as: 
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  16. The influences of allelic dominance at individual loci and interaction among alleles at different loci mean that broad sense hertability is typically measured only when comparing ______ ______ to each other. ______ _____ share the same alleles at all loci, so all three components of ______ ______ are the same in both _____. Thus the only estimates of heritability that include all three components of genetic variance are those obtained from studies of _____ ______
    • identical twins 
    • Identical twins
    • genetic variance
    • twins
    • identical twins
  17. In contrast, comparisons of phenotypic similarities between parents and offspring cannot measure broad sense heritability (why?)
    because only one of the alleles at any individual locus is shared between any one offspring and any one parent and because the combinations of alleles among different loci also differ
  18. The allele of each locus that is shared between a parent and an offspring instead must be considered as a genetic factor that acts in a simple, ______ fashion (explain)
    • additive
    • In other words, comparisons of parents and offspring represent only the additive genetic variance (VA) component of overall genetic variance (VG)
  19. Because the ________ component to the genetic effects most precisely predicts the range of ________ expected among the progeny of crosses, plant and animal breeders often calculate a heritability estimator called the ______ ______ _______ (define and equation)
    • additive
    • phenotypes
    • narrow sense heritability (h2): the proportion of total variance due specifically to variance of the additive genetic component 
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  20. The second reason narrow sense heritability is also important to breeders
    It dictates how strongly a particular phenotypic trait will respond to selection on a phenotype
  21. Genetic relatedness
    the average fraction of common alleles at all gene loci that individuals share because they inherited them from a common ancestor
  22. The fig. illustrates the implication of _______ with respect to the _____ phenotypic values of the parents, their ______, and the _______ as a whole. Heritability can be estimated as the ______ of phenotype between ______ and ______. If the mean phenotypic value of the progeny is similar to that of the population as a whole, regardless of the particular ______ or ______ of the parents, then the heritability is 0 (why?)
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    • heritability
    • mean 
    • offspring
    • population 
    • correlation
    • progeny and parents
    • phenotypes or genotypes
    • because none of the phenotypic variance is due to additive genetic effects of the alleles inherited from the parents
  23. If the offspring deviate from the population average by 25% as much as their parents did, then the narrow sense heritability is _____. If the progeny deviate from the population mean by just as much as their parents did, then the heritability is ____ (why?)
    • 0.25
    • 1.0
    • the alleles inherited from the parents closely predict the phenotypes of the offspring
  24. The finches observed by Darwin in the Galapagos Islands (often referred to as Darwin's finches) provide an example of a population for which geneticists have measured the heritability of a trait under natural conditions in the field by comparing the phenotypes of ______ and ______
    parents and offspring
  25. How did scientists study the medium ground finch, Geospiza fortis, on the island of Daphne Major after banding many of the individual birds in the population? (2-story)
    pg 700
  26. For reasons that will become clear momentarily, when a positive correlation exists, the slope of the line relating offspring to midparent value is an estimate of the ______ _____ ______ for bill depth. In the figure, the heritability of bill depth, as represented by the slope of the line correlating midparent bill depth to offspring bill depth (that is, the line of correlation), is 0.82 (explain)
    • narrow sense heritability
    • This means that roughly 82% of the variation in bill depth in this population of Darwin's finches is attributable to additive genetic variation among individuals in the population; the other 18% results from variation in the environment and nonadditive genetic effects
  27. In the fig, we examine the extreme cases to illustrate why the slope of the line of correlation provides an estimate of the narrow sense heritability h2. Suppose first that the _______ had no influence at all on the trait. In such a case, the slope of the line representing the ________ of bill depth (that is, the line _______ bill depth in parents with bill depth in offspring) would be ____ (part c left). At the right we show, the results for four _____ _____ displayed in the same fashion previously used in Fig 21.6. You can see that both of these representations are essentially _________
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    Image Upload
    • environment
    • heritability
    • correlating 
    • 1.0
    • mating pairs
    • equivalent
  28. Now consider a population in which the bill depth for parents and their offspring is, on average, no more or less ______ than the bill depths for any pair of individuals chosen from the population at random. In such a population, no correlation exists between the _____ _____ trait in parents and in offspring, and a plot of midparent and offspring bill depths produces a cloud of points with no ______ between ________ and _______ bill depth
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    • similar 
    • bill depth
    • correlation 
    • midparent and offspring
  29. The right panel of part d shows an ______, alternative representation of this same case. You can see that analyzing data that tracks the phenotypic values of parents, their progeny, and the population as a whole by either of the two methods used in the fig allows researchers to estimate ______ values
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    • equivalent
    • heritability
  30. From these examples, you might conclude that phenotypic similarity among genetically related individuals provides evidence for the heritability of a trait. However, conversion of the phenotypic similarity among genetic relatives to a measure of heritability depends on a crucial assumption (which is?):
    • That the distribution of genetic relatives is random with respect to environmental conditions experienced by the population
    • In the finch example, we assumed taht parents and their offspring do not experience environments that are any more similar than the environments of unrelated individuals
  31. cross fostering
    random relocation of offspring to the care of other parents, typically done in animal studies to randomize the effects of environment on outcome
  32. Mating does not occur at random with respect to ______ in human populations, and researchers cannot apply techniques for controlling ________ conditions and setting up defined _______ _______ to studies of such populations. Nonetheless, in most human societies, family members share similar ______ and ______ environment. Thus, phenotypic similarity between genetic relatives may result either from ______ similarities or from similar ________ or, most often, _____
    • phenotypes
    • environmental
    • breeding crosses
    • family and cultural 
    • genetic
    • environments
    • both
  33. How can you distinguish the effects of genetic similarity from the effects of a shared environment?
    • Study monzygotic twins given up for adoption shortly after birth and raised in different families
    • Another approach is to compare the phenotypic differences between different sets of genetic relatives, particularly different types of twins
  34. In the case of monozygotic twins, any phenotypic similarity should be the result of ______ similarity. At first glance, then, the study of adopted identical twins eliminates the confounding effects of a similar _____ _____. Further scrutiny, however, shows that this is often not ______
    • genetic 
    • family environment
    • true
  35. Many pairs of twins are adopted by different genetic relatives; the adoptions often occur in the same _______ region (usually in the same state and even the same city); and families adopting a pair of twins are likely to be more ______ than a pair of families chosen at ______, and this similarity can reduce the ______ differences between the twins. A valid scientific study of separated twins must take these factors into consideration
    • geographic
    • similar 
    • random
    • phenotypic
  36. In an example of the second approach, monozygotic (MZ) twins, which are the result of a split in the zygote after _______, are genetically identical because they come from a _____ _____ and a _____ _____; they share all _______ at all _____
    • fertilization
    • single sperm 
    • single egg
    • alleles
    • loci
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  37. By contrast, dizygotic twins (DZ twins), which are the result of different _______ from a single father fertilizing ____ ______ maternal eggs, are like any pair of siblings born at separate times in that they share on average ___% of their alleles at all _____. Comparing the phenotypic differences between a pair of DZ twins (whose genetic relatedness is (0.5) can help distinguish between the effects of ______ and ______ environment
    • sperm
    • two different 
    • 50% 
    • loci
    • genes and family
  38. If twins are raised in the same family, they share a relatively common _______. Since MZ twins share ______ as many alleles as DZ twins, how is the broad-sense heritability (H2 = VG/VP) calculated?
    • environment
    • twice
    • calculated as twice the difference in the statistical correlation of the phenotype between pairs of MZ and pairs of DZ twins
  39. Because twin studies typically cannot separate the effects of _______ or gene interaction from _______ gene effects on phenotype, they allow determinations only of the _____ _____ heritability. In comparison with the more useful ______ _____ heritability measured in parent-offspring studies that consider only the additive component of the genetic vriance, the values of H2 provided by twin studies will be ______ but are useful approximations nonetheless
    • dominance 
    • additive
    • broad-sense heritability (H2)
    • narrow-sense heritability (h2)
    • higher
  40. Consider a trait in which the differences in phenotype among individuals in the population arise entirely from differences in the environment experienced by each individual, that is, a trait for which the heritability is 0.0. For this trait, you would expect the likelihood that MZ twins share the same phenotype to be the _____ as that for DZ twins. The likelihood of trait sharing would also be the same for genetically ______ ______ who had been adopted into the same family. If one child expresses a trait for which the heritability is 0.0, then the only factor that influences that chance a sibling will show the same phenotype is?
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    • same
    • unrelated siblings
    • the probability that the range of environments investigated can produce the phenotype; the degree of genetic relationship between the children has no effect
  41. Now consider a trait for which differences in phenotypes among individuals in a population arise entirely from genetic differences, that is, a trait for which the heritability is 1.0. Since MZ twins are genetically identical, they are expected to show ____% ________ in expression (explain)
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    • 100% concordance
    • meaning: if one expresses the trait, the other does as well
  42. The concordance of trait expression between unrelated individuals varies based on the ________ of the trait; the more _______ the trait in the population, the greater the chance that two unrelated people will have that phenotype. Regardless of the ________ of the trait, DZ twins, because the share _____ of their alleles, will always display greater concordance than genetically unrelated individuals, but less than the ___% concordance of MZ twins
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    • commonality
    • common 
    • commonality
    • half 
    • 100%
  43. In the highly simplified case of a dominant trait caused by an allele at a single autosmal gene, DZ twins would show a level of concordance that is ______ between the unrelated value and 100%. In reality, nearly all traits are affected by _____ ____ that may have dominant, recessive, codominant, and interacting effects, and the heritabilities of all traits lie between ____ and ____
    • halfway 
    • multiple genes
    • 0.0 and 1.0
  44. Truncation selection
    Selection differential 
    Response to selection
    • Truncation selection: a form off artificial selection used by plant breeders in which only individuals with phenotypic values above or below a particular cut-off are bred to produce the next generation
    • Selection differential (S)—Difference between value for this trait in the parents and value for this trait in the entire population (breeding and non-breeding)
    • Response to selection (R)—The amount of change in the mean value of a trait that results from selection  R = h2S
  45. The significance of heritability to the breeder comes from the fact that the response to selection (R), and thus the _________ of the breeding program, is ______ related to the selection differential (S) through what is called the _____ ______ of the trait. If the distribution of phenotypic values fit a normal bell-shaped curve, then the realized heritability is equal to the ______ _____ heritability. Derive the equation solving for R
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    • effectiveness 
    • directly
    • realized heritability
    • narrow-sense heritability (h2)
    • thus, realized h2 = R/S
    • Rearranging to solve for R: R = h2S
  46. In other words, the strength of selection (S) and the heritability of a trait (h2) directly determine the trait's _____ or _____ of evolution in each _______ (as indicated by the response to selection R). The _______ the heritablity, the ______ the likelihood that the breeding program will improve the crop
    • amount or rate
    • generation
    • greater
    • greater
Card Set:
Genetics of Complex Traits
2017-12-03 04:05:47
Ch 21.1
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