Ch. 22 Quantitative Genetics
The nature of Continuous Traits
-traits with only a few distinct phenotypes are called discontinuous traits (pea pod color)
-review penetrance and expressivity and pleiotropic
-norm of reaction: range of phenotypes produced by a particular genotype in different environments
-many traits (probably most) such as human birth weight and adult height, protein content in corn, and
the number of eggs laid by drosophila, exhibit a wide range of possible phenotypes. Traits such as these,
with a continuous distribution are called continuous traits.
-since the pehnotypes of continuous traits must be described by quantititaive measures, such
traits are also known as quantitative traits, and the field of quanititative genetics studies the
inheritance of these traits
Questions Studied in Quantitative Genetics
-in transmission genetics, we frequently determined the probability of inheriting a particular phenotype.
With quantitative traits, however, individuals differ in the quantity of a trait so it makes no sense to ask
about the probability of inheriting a continuous trait.
-Instead, the following are examples of questions frequently studied by quantitative genetics:
1. to what degree does the observed variation in the phenotype result from differences in
genotype, and to what degree does this variation reflect the influence of different environments?
2. How many genes determine the phenotype?
3. Are the contribution of the determining genes equal?
4. are the effects of alleles additive? To what degree do alleles at the different loci interact with
5. When selection favors a particular phenotype, how rapidly can the trait change? Do other
traits change at the same time?
6. What is the best method for selecting and mating individuals to produce desired phenotypes
in the progeny?
Quantitative Genetic Analysis
Inheritance of Ear length in corn
-for a quantitative trait, the 1 progeny of a cross between two phenotypically distinct, pure-breeding
parents usually has a phenotype intermediate between the parental phenotypes.
-the F2shows more variability than the F , 1ith a mean phenotype close to that of the F . The1extreme
phenotypes of the F e2tend well beyond the range of the F and i1to the ranges of the two parental
values. -These patters arise as a result of the heterozygosity of the F and the different combinations of parental
alleles in the F2created by ind. assortment
-heritability: is the proportion of a population’s phenotypic variation that is attributable to genetic
-as we have seen, continuous traits are influenced by multiple genes and by environmental factors.
Components of the phenotypic variance
-phenotypic variance: is a measure of all variability for a trait V P
-genetic Variance: the genetic contribution to the phenotypic variation V G
-environmental variance: any nongenetic source of variation. V E
-temp., nutrition, and parental care
- V P V +GV E
-100% of the variation among individuals is accounted for by genetic and environmental influences
-Covariance:ex. When a an above average genetic cow gts above average environement because of the
genes. (COV ) G,E
-genotype by environment interaction: there is a genetic effect, but the genetic effect depends on the
environment. (V GXE). Example: an AA plants are on avg 50 cm tall and aa plants are 35 cm tall in warm
temps. And AA plants are 40 cm and aa plants are 25 cm tall in colder. The effects of genotype and
environment cannot simply be added together.
V = V +V + 2COV + V
P G E G,E GXE
-genetic variance can be broken up into different components
-additive genetic variance (V ): Adding the contributions of all the alleles
-dominance variance (V ): wDen dominance is present, the individual effects of the alleles are not
strictly additive and as a result we must also factor in how those genotypes contribute
-Interaction Variance (V): Ihe presence of epistasis adds another source of genetic variation
V GV +A + D I
-the environmental component of variance can also be split up into components
-General environmental effects (V ). ExEgsure to varying temps or nutritional environments during
development, resulting in irreversible differences