genetics 3
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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

discontinuous
 qualitative
 few very distinct phenotypes
 tall/short

continuous
 quantitative
 vary continuously along a scale of measurement with overlap
 weight, growth rate, blood pressure, height

quantitative characteristics arise from 2 phenomena
 polygenicinfluenced by genes are many loci=many genotypes are possible, each producing a slightly different phenotype.
 environmental factorsenvironmental differences result in a single phenotype producing a range of phenotypes
 multifactorialcontinuously varying characteristics are both polygenic and influenced by environmental factors

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)

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

types of quantitative characteristics
 continuouscan 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)

meristic characteristics
 measured in whole numbers&have a limited number of distinct phenotypes
 litters of mice

threshold characteristics
 either present/absent
 display only two possible phenotypespresent/absent
 are quantitative bc underlying susceptibility to the characteristic varies continuously
 the presence of some diseases(reach threshold=have disease)

polygenic inheritance
 inheritance of quantitative characteristics can be explained by the cumulative effects of many genes, each following mendel's rules
 nillsonehle 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 additiveeach gene contributed equally to color and the overall phenotype was determined by adding the effects of all genes

As the number of loci affecting a character increase, the number of phenotypic classes in the F2 increases***
 nillsonehle'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

types of statistical methodsdistribution***
 a description of the numbers and phenotypes
 can assume different shapes=
 normal: large # of independent factors contribute to a measurement
 skewed/bimodal

types of statistical methodssamples&populations
 populationgroup of interest, can be too large to measure every thing in the group
 samplea 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

the mean/average
 provides info about the center of a distribution
 distributions can be normal, but centered at diff. heightsthis difference would be indicated in their different means

variance***
 how spread out the distribution is, s^{2}(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

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

correlation****
 relationship between two characteristicsoften two or more characteristics vary together
 when correlated, a change in one characteristic is likely to be associated

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

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

geneticenvironmental interaction variance***
 effect of a gene depends on the specific environment
 example:

types of heritability
how much of the phenotypic variance in a characteristic is due to genetic differences

broad sense heritability***

narrow sense heritability***
 represents the proportion of phenotypic variance that results from additive genetic variance
 additive genetic variance primarily determines

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