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29 Cards in this Set
- Front
- Back
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discontinuous variation
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traits that have a small number of discrete phenotypes, such as: pea plants= tall or dwarf; squash fruit shape= spherical, disc shaped, or elongated; fruit fly eye color= red or white.
typically, a genotype will produce a single identifiable phenotype, although phenomena such as variable penetrance and expressivity, pleiotropy, and epistasis can obscure the relationship between genotype and phenotype |
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continuous variation
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more complex traits; show much more variation, with a continuous range of phenotypes that cannot be as easily classified into distinct categories.
Examples: human heigh or weight, milk or meat production in cattle, crop yield, and seed protein content Continuous variation across a range of phenotypes is measured and described in quantitative terms, so this genetic phenomenon is known as quantitative inheritance |
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quantitative inheritance
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the genetic phenomenon that measures/describes continuous variation across a range of phenotypes; quantitative traits are sometimes referred to as polygenic
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polygenic
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phenotypes resulting from the input of genes at multiple loci (quantitative traits)
polygenes= genes contributing to a quantitative trait |
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multifactorial (complex) traits
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phenotypes that result from both gene action and environmental influences (ex: height is partly genetically determined but is also affected by environmental factors such as nutrition)
Summary: Usually, a *continuous quantitative trait* is the result of *polygenic inheritance*. Furthermore, polygenic traits are frequently *multifactorial*, with environmental factors contributing to the range of phenotypes observed. |
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meristic traits
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a class of polygenic traits; those in which the phenotypes are described by whole numbers
Ex: the number of seeds in a pod or the number of eggs laid by a chicken in a year (these are quantitative traits, but do not have an infinite range of phenotypes: for ex, a pod may contain 2, 4, or 6 seeds, but not 5.75) |
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threshold traits
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are polygenic (and frequently, environmental factors affect the phenotypes, making them also multifactorial), but they are distinguished from continuous and meristic traits by having a small number of discrete phenotypic classes. An increasing number of diseases are now though to show this pattern of polygenic inheritance (one example is Type II diabetes)
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additive allele (nonadditive allele)
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additive allele= contribues a constant amount to the phenotype (ex: red grain color)
nonadditive allele= does not contribute quantitatively to the phenotype (ex: does not produce any red pigment) the greater the number of additive alleles in the genotype, the more intense the red color expressed in the phenotype, as each additive allele present contributes equally to the cumulative amnt of pigment produced in the grain (AABB= red, aabb= white; plants with 3, 2, or 1 additive alleles constitute the other categories of red color) |
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mean
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the arithmetic average of a set of measurements; provides a useful descriptive summary of the sample, but tells us nothing about the range or spread of the data
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variance
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is the average squared distance of all measurements from the mean; provides info about the spread of data around the mean
(it is possible for two sets of sample measurements for a quantitative trait to have the same mean but a different distribution of values around it) |
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standard deviation
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used to express variation around the mean in the original units of measurement (square root of the variance)
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covariance (COVxy)
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measures how much variation is common to both quantitative traits (for ex: a poultry breeder might investigate the correlation between body weight and egg production in hens: do heavier birds tend to lay more eggs?)
is calculated by taking the deviations from the mean for each trait for each individual in the sample, multiplying them together, and the sum of all these individual products is then divided by one fewer than the number in the sample |
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correlation coefficient (r)
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the standardized covariance; can range from -1 to +1. Positive r values mean that an increase in measurement for one trait tends to be associated with an increase in measurement for the other, while negative r values mean that increases in one trait are associated with decreases in the other
therefore, is heavier hens do tend to lay more eggs, a positive r value can be expected |
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heritability
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used to describe what proportion of total phenotypic variation in a population is due to genetic factors (and not the environment)
For a multifactorial trait in a given population, a high heritability estimate indicates that much of the variation can be attributed to genetic factors, with the environment having less impact on expression of the trait (and vise versa) |
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phenotypic variance (Vp)
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the extent of the spread of a phenotype around the sample mean; statistical techniques are used to partition phenotypic variance into genotypic variance (Vg) and environmental variance (Ve) components; the third component of phenotypic variation is genotype-by-environment interaction variance (Vgxe)
so, Vp= Vg + Ve + Vgxe |
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broad-sense heritability (H^2)
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measures the contribution of the gynotypic variance to the total phenotypic variance
heritability values for a trait in a population range from 0.0 to 1.0. A value approaching 1.0 indicates that the environmental conditions have little impact on phenotypic variance, which is therefore largely due to genotypic differences among individuals in the population. Low values close to 0.0 indicate that environmental factors, not genotypic differences, are largely responsible for the observed phenotypic variation within the population studied |
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narrow-sense heritability (h^2)
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the proportion of phenotypic variance due to additive genotypic variance alone
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additive variance (Va)
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is the genotypic variance due to the additive action of alleles at quantitative trait loci
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dominance variance (Vd)
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deviation from the additive components that results when phenotypic expression in heterozygotes is not precisely intermediate between the two homozygotes
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interactive variance (Vi)
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the deviation from the additive components that occurs when two or more loci behave epistatically (often negligible, so often excluded rom calculations of total genotypic variance)
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artificial selection
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the process of choosing specific individuals with preferred phenotypes from an initially heterogeneous population for future breeding purposes. Theoretically, it can be used to develop a population containing a high frequency of individuals with the desired characteristics
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selection response (R)
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the degree of response to mating the selected parents
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selection differential (S)
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the difference between the mean for the whole population and the mean for the selected population- so h^2 (narrow-sense heritability) reflects the ratio of the response observed to the total response possible. Thus,
h^2 = R/S (response observed/total response possible) |
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realized heritability
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an estimate of a narrow-sense heritability value obtained by selective breeding and measuring the response in the offspring
h^2 = R / S |
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monozygotic (MZ) twins
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identical twins; derived from the division and splitting of a single egg following fertilization; for a given trait, phenotypic differences bw pairs of identical twins will be equivalent to the environments variance (Ve)
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dizygotic (DZ) twins
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fraternal twins; originate from two separate fertilization events and are only as genetically similar as any other two siblings, with about 50% of their genes in common
phenotypic variance between the two represent both environmental variance (Ve) and approximately half of the genotypic variance (Vg) |
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concordant
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twins are said to be concordant for a given trait if both express it or neither expresses it
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discordant
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if one twin expresses the trait and the other does not, the twins are discordant
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quantitative trait loci (QTLs)
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multiple genes contributing to a quantitative trait ; identifying and studying these loci help geneticists estimate how many genes are involved in a given quantitative trait and whether they all contribute equally to the trait or whether some genes influence the phenotype more strongly than others
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