We now consider changes in the mean value of the threshold trait determining resistance.
We assume that both traits act as threshold traits (Falconer, 1989).
Estimating heritability in a threshold trait: heat shock tolerance in Drosophila buzzatii.
Because group effects for threshold traits are almost directly proportional to group size (Krebs and Loeschcke 1997), the estimate of heritability for adult thermotolerance is low ([[h.sup.2].sub.b] = 0.018), and it accounts for the reduction in interindividual variance due to using many flies to obtain proportional data (harmonic mean of 17.4 adults per replicate vial).
The existence of a threshold trait suggests that there are trade-offs associated with each morph (Roff 1984).
The analysis of the genetic basis of trade-offs associated with a threshold trait poses a more difficult problem than with the usual quantitative trait because there are two possible modes of determination.
- By considering larval survivorship a threshold trait, I estimated the selection differential each generation and calculated realized heritabilities for each replicate.
The selection differential of larval survivorship was estimated from the proportion of the population that survived using standard methods for threshold traits (Crow and Kimura 1970; Falconer 1989).
Pupal colour dimorphism in swallowtail butterflies as a threshold trait
: Selection in Eurytides marcellus (Cramer).
As diseases has threshold nature, in order to estimate heritabilities, genetic and environmental correlations, TM program (Legarra, Varona, & Lopez de Maturana, 2011) was used that is capable of analyzing threshold traits
along with continues traits using Gibbs sampling approach, simultaneously.
Meristic and threshold traits, which can be classified in discrete categories, can also be considered to vary continuously on an underlying scale.
Included here is a comprehensive treatment of experimental designs for estimating heritability for continuous and threshold traits, with details of the statistical analyses and their genetic interpretation, guidance as to optimal experimental design, and standard errors of the [h.sup.2] estimates.