genetics 3

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julianamyduyen
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genetics 3
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2012-03-13 15:41:59
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genetics 3
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  1. Quantitative genetics***
    • genetic analysis of complex characteristics
    • oil content of corn(many genes at diff. loci and environmental factors contribute to the oil content of corn)
    • statistical procedures developed for analyzing complex characteristics must be used
    • vary continuously and are influe
  2. discontinuous
    • qualitative
    • few very distinct phenotypes
    • tall/short
  3. continuous
    • quantitative
    • vary continuously along a scale of measurement with overlap
    • weight, growth rate, blood pressure, height
  4. quantitative characteristics arise from 2 phenomena
    • polygenic-influenced by genes are many loci=many genotypes are possible, each producing a slightly different phenotype.
    • environmental factors-environmental differences result in a single phenotype producing a range of phenotypes
    • multifactorial-continuously varying characteristics are both polygenic and influenced by environmental factors
  5. Relation between genotype&phenotype***
    • for qualitative characteristics, the relation is straightforward (each genotype produces a single phenotype&most phenotypes are encoded by a single genotype)
    • if polygenic, many genotypes are possible(may produce same phenotype)
    • influence of environment can complicate the relation, may produce a range of potential phenotypes(can overlap, which makes it difficult to determine if the diff. is due to genes or the environment)
  6. statistical methods of analysis for quantitative characteristics**
    • simple relation between genotype&phenotype is absent from quantitative characteristics
    • methods used for qualitative characteristics does not work for quantitative
    • goal=make predictions about phenotypes of offspring produced from a genetic cross&to determine how much of the variation is due to genetics/environment
  7. types of quantitative characteristics
    • continuous-can theoretically assume any value between two extremes and the number of phenotypes is limited by our ability to precisely measure them(human height)
    • some characteristics are not continuous but are quantitative bc they are determined by multiple genetic&environmental factors(meristic&threshold)
  8. meristic characteristics
    • measured in whole numbers&have a limited number of distinct phenotypes
    • litters of mice
  9. threshold characteristics
    • either present/absent
    • display only two possible phenotypes-present/absent
    • are quantitative bc underlying susceptibility to the characteristic varies continuously
    • the presence of some diseases(reach threshold=have disease)
  10. polygenic inheritance
    • inheritance of quantitative characteristics can be explained by the cumulative effects of many genes, each following mendel's rules
    • nillson-ehle studied kernel color in wheat, found that the intensity of red pigmentation was determined by 3 unlinked loci(each had 2 alleles)
    • performed crosses between homozygous varieties(homozygous white&homozygous red) and studied the ratios of phenotypes in the progeny
    • the effects of the genes were additive-each gene contributed equally to color and the overall phenotype was determined by adding the effects of all genes
  11. As the number of loci affecting a character increase, the number of phenotypic classes in the F2 increases***
    • nillson-ehle's crosses shows that inheritance is dependent upon the number of loci that determine the characteristic
    • more loci&more genes cause the relation between genotype and phenotype less obvious
  12. types of statistical methods-distribution***
    • a description of the numbers and phenotypes
    • can assume different shapes=
    • normal: large # of independent factors contribute to a measurement
    • skewed/bimodal
  13. types of statistical methods-samples&populations
    • population-group of interest, can be too large to measure every thing in the group
    • sample-a smaller collection of the things in the group, measurements can be made on the sample to describe the population. a good sample must be representative of the whole population(random) and must be large enough that chance differences between sample and overall population do not distort the estimate of the measurements
  14. the mean/average
    • provides info about the center of a distribution
    • distributions can be normal, but centered at diff. heights-this difference would be indicated in their different means
  15. variance***
    • how spread out the distribution is, s2(average squared deviation from mean)
    • a statistic that provides key information about a distribution is the variance
    • indicates the variability of a group of measurements
    • distributions can have the same mean but different variances
    • provides info about the variability
  16. standard deviation****
    • the square root of the variance
    • a normal distribution is symmetrical and the mean and S.D. are sufficient enough to describe the shape
  17. correlation****
    • relationship between two characteristics-often two or more characteristics vary together
    • when correlated, a change in one characteristic is likely to be associated
  18. regression****
    • line defines the relation between variables(line of best fit)
    • a type of statistical prediction, the ability to predict the value of one variable, given the value of the other
    • allows geneticists to predict the characteristics from a given mating, even without knowledge of the genotypes that encode the characteristics
    • predicts values based on another, diff. from correlation
  19. genetic vs. environmental differences***
    • heritability is used to estimate the proportion of variation in a trait that is genetic
    • knowing how much of the variation in quantitative characteristics
    • example: cows and milk production. some cows produce more milk than others, is it due to genetics or environmental factors? we could adjust breeding/environment to favor increased milk production
  20. genetic-environmental interaction variance***
    • effect of a gene depends on the specific environment
    • example:
  21. types of heritability
    how much of the phenotypic variance in a characteristic is due to genetic differences
  22. broad sense heritability***
  23. narrow sense heritability***
    • represents the proportion of phenotypic variance that results from additive genetic variance
    • additive genetic variance primarily determines
  24. limitations of heritability****
    • heritability allows us to statistically predict the phenotypes of offspring on the basis of their parents phenotype
    • provides useful info about how characteristics will respond to selection
    • heritability does not provide info about an individuals genes/environmental factors that control the development of a characteristic or

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