CHAPTER 22: The Genetics of Complex Traits
if all of the phenotypic variation for a trait is due to environmental effects, not to genetic variation in the pop., then the heritability of the trait will be equal to...
0
if you are studying a discrete trait and you determine that the concordance for this trait in monozygotic twins is exactly the same as in dizygotic twins, then the heritability of this trait is...
0
in a pop. of dairy cows, the phenotypic variance for daily milk production is 5.0 L^2. the genetic variance is 2.0 L^2, and the environment variance is 3.0 L^2. what is the broad-sense heritability for daily milk production in this group of cattle
0.40
if you are studying a discrete trait and you determine that the trait concordance is 0.94 in MZ twins and 0.41 in DZ twins, then the heritability of this trait is...
0.90
a student calculates his genetic relatedness to his uncle as...
1/4
for a trait in which the heritability is 1.0 monozygotic twins are expected to show _______% concordance
100
truncation selection
a form of artificial selection used by plant breeders in which only individuals with phenotypic values above or below a particular cutoff are bred to produce the next generation.
broad-sense heritability (H2)
a heritability measurement that includes all three components of genetic variance (VG): additive effects (VA), dominance effects (VD), and the effects of interactions between alleles at different loci (VI). Typically measured in studies of identical twins.
total phenotype variance (VP)
a measure of the amount of variation in a population, calculated as the average squared difference between each individual's trait value and the mean trait value for the population; the sum of the genetic variance (VG) and the environmental variance (VE).
phenotypic value
a number that describes a specific intensity of expression of a quantitative trait; also called a trait value.
trait value
a number that describes a specific phenotype of a quantitative trait; also called a phenotypic value.
normal distribution
a set of data points that scatter around a central mean (average) without left or right bias; an equal number of data points fall below and above the mean. Normally distributed data can be represented by a bell curve graph in which the x-axis is the phenotypic value and the y-axis is the number of individuals in the population who show a particular phenotypic value. The phenotypic values of quantitative traits are often normally distributed.
heritability, whether calculated in the broad or he narrow sense, is a property of...
a specific population
if the heritability value of a trait is equal to 1, it means that...
all of the phenotypic variation in the pop. is due to genetic variance
If Va is very large in comparison to Vd and VI' then narrow-sense heritability will be...
approx. the same as broad-sense heritability
unrelated individuals in a population...
are more likely to share characteristics that are common in a pop.
selective breeding of plants and animals can result in...
artificial selection
the phenotype distribution for a plant population in the wild would be...
broad, because both genetic and environmental variation would contribute to total phenotype variance
a quantitative is defined as a trait that...
can be describe numerically
concordance
describes the extent to which two different individuals share a discrete trait. In twin studies, the frequency with which the co-twin has the trait in question when one of them does.
if you calculated the heritability of a quantitative human trait in two different countries, one in South America, and another in Africa, you will likely find...
different heritability values in the two countries depending on the environmental conditions
phenotypic differences that occur when raising genetically identical plants in different conditions provides a measure of....
environmental variance
T/F: the heritability of a particular trait in a particular species will always be the same regardless of the environmental conditions in which individuals of the species are living
false
the variation in plant growth when genetically diverse plants are grown in the same environment is a result of _______ variance
genetic
monozygotic (MZ) twins
genetically identical twins that derive from the splitting of an embryo from a single zygote into two separate embryos; also called identical twins.
when narrow-sense hertiability is calculated from correlation coefficients, an assumption that is inherent in the computation is that...
genetics and the environment are independent variables
genetic studies that compare quantitative traits in identical versus fraternal twins can be used to evaluate how a quantitative trait is affected by...
genotype-environment associations
narrow-sense heritability (h2)
heritability measurement that includes only the additive component (VA) of genetic variance (VG).
a scientist studies a pop. of wild flowers and determines that the heritability of flower size is 0.56. What does that mean?
if we observe the variation in size of all flowers in the pop., we can attribute 56% of it variation to genetic difference between plants
genetic variance (VG)
in a population, the deviation of a phenotypic value from the mean attributable to inheritable factors.
environmental variance (VE)
in a population, the deviation of a phenotypic value from the mean attributed to the influence of external, noninheritable factors.
quantitative trait
inherited trait that exhibits many intermediate forms; determined by alleles of many different genes whose interactions with each other and the environment produce a phenotype. Synonymous with continuous trait.
the chromosomal locations of one or more genes that influence a quantitative trait are called quantitative trait ______
loci
the sum of all of the values of a parameter within a group divided by the number of individuals in the group is defined as the...
mean
the average phenotypic value of the mother and father for a quantitative trait is the ________ value
midparent
what is realized heritability?
narrow-sense heritability that is estimated based on the response to artificial selection
a distribution in which a trait varies in a symmetrical way around an average value is called a(n) _____ distribution and produces a(n) _____-shaped curve
normal; bell
when phenotypic variance is determined solely by genotypic and environmental variance, and genetic and environmental factors are independent of one another, then total phenotypic variance is equal to genetic variance ______ environmental variance
plus
heritability is a measurement of the...
proportion of phenotypic variance due to genetic variance
a trait that can be described numerically, such as height or weight, is defined as a(n) ______ trait
quantitative
a scientsist estimates that the heritability of a quantitative is 0.8. Based on that, if the scientist was to compare monozygotic and dizygotic twins for that trait and calculate the two respective correlation coeffcients (rMZ and rDZ) she should find that..
rMZ> rDZ and the difference between the two coefficients is about 0.4
cross-fostering is used to...
randomize environmental conditions that might influence a phenotype
to calculate realized heritability based on the results of selective breeding, it is necessary to known the value of R, which represents the _________ to selection observed in the offspring, and also the value of S, which represents the selection _________ of the parents
response; differential
when truncation selection is used, the difference between the average value of the phenotype is selected individuals and the entire parental generation is the....
selection differential
a complex trait is a trait that is influenced by...
several genes and the environment
monozygotic twins...
share 100% of the same alleles
dizygotic twins...
share an average of 50% of the same alleles
in a sample with N individuals, total phenotype variance is calculated as the....
sum of the squared between each individual value and the mean divided by N
genetic relatedness
the average fraction of common alleles at all gene loci that individuals share because they inherited them from a common ancestor.
quantitative trait loci are...
the chromosomal locations of one or more genes that affect the phenotype of a quantitative trait
to randomize the environment in which a bird is raised...
the eggs can be moved to a new nest for cross-fostering by other parents
narrow-sense heritability can be used to predict...
the expected change in the population mean for a trait that can be achieved through artificial selection
realized heritability
the heritability value measured as a response to selection.
what is the average phenotypic value of the mother and father for a quantitative trait celled?
the midparent value
genetic relatedness refers to...
the proportion of alleles that have been inherited from a common ancestor by two individuals
heritability
the proportion of total phenotype variance in a population ascribable to genetic variance.
cross-fostering
the random relocation of offspring to the care of other parents, typically done in animal studies to randomize the effects of environment on outcome.
correlation coefficient (r)
the slope of the line of correlation.
T/F: in order to calculate narrow-sense heritability from correlation coefficients, it is necessary to assume that genetics and the environment are independent variables
true
a plant breeder who chooses seeds from corn plants that are above a minimum height to plant in each generation using...
truncation selection
dizygotic (DZ) twins
two different embryos, each derived from a different zygote, developing together in one womb; also called fraternal twins.
genetic variance is a measure of...
variation in phenotype caused by genetic differences
when does genetics play a greater role than the environment in promoting phenotypic variation within a group
when Vg is high and Ve is low
