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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
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discontinuous
- qualitative
- few very distinct phenotypes
- tall/short
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continuous
- quantitative
- vary continuously along a scale of measurement with overlap
- weight, growth rate, blood pressure, height
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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
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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)
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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
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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)
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meristic characteristics
- measured in whole numbers&have a limited number of distinct phenotypes
- litters of mice
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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)
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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
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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
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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
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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
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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
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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
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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
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correlation****
- relationship between two characteristics-often two or more characteristics vary together
- when correlated, a change in one characteristic is likely to be associated
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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
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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
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genetic-environmental interaction variance***
- effect of a gene depends on the specific environment
- example:
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types of heritability
how much of the phenotypic variance in a characteristic is due to genetic differences
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broad sense heritability***
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narrow sense heritability***
- represents the proportion of phenotypic variance that results from additive genetic variance
- additive genetic variance primarily determines
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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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