These results suggest that most of the correlated response of PSRI to selection for NDF was due to pleiotropic effects of loci, or to very tight linkages between loci controlling the two traits, a phenomenon that cannot be distinguished from
pleiotropy without very large populations and/or several cycles of recombination.
In addition, it is necessary to determine if coincident QTL are the result of tight linkage or
pleiotropy. In the case of tight linkage, screening large numbers of recombinants may be necessary to break up the linkage.
The characteristics of the parental pairs suggest that at least partly the associated responses to selection were determined by
pleiotropy, with genes for high ABA having a negative effect on the agronomic performance.
Fine-scale mapping of QTL and ultimate identification of the genes and allelic substitutions involved is necessary to determine the relative importance of
pleiotropy versus linkage in the similarities of the LOD profiles.
Each of these can be understood through examining the process of meiosis Mendelian A method of using measured variation in genes of known randomisation function to examine the causal effect of a modifiable exposure on disease in observational studies Penetrance The probability of a gene or trait being expressed Phenotype The physical expression, or characteristics, of that trait due to a specific genotype
Pleiotropy The ability of a single gene to influence multiple phenotypic traits Table 2.
A distinctive feature of the regulatory peptides is
pleiotropy, or the ability to affect several different physiological functions.
For instance, three independent studies showed that serum uric acid was associated with subclinical atherosclerosis (1,4,19), but a Mendelian randomization analysis showed that the causal effect of uric acid could be inflated by hidden
pleiotropy (6).
Collectively, these multifactorial effects raise the possibility of gene
pleiotropy influencing the outcome measure, which could invalidate the Mendelian randomization (8).
On the other hand, NMS may manifest as symptoms of aging-associated neurodegenerative disease through an antagonistic
pleiotropy mechanism in the parental brains.
Third, genetic analysis of TFs in the nonspecific world will uncover differential
pleiotropy. Differential
pleiotropy is observed for the HD containing Drosophila, Human and yeast TFs: Ultrabithorax, Fushi tarazu, Sex combs reduced, Antennapedia, HOXA7, HOXB3, and Pho2p; therefore, in order to make a more general statement, observation of differential
pleiotropy needs to be extended to other TF families.