Submitted to: Joint Abstracts of the American Dairy Science and Society of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: March 12, 2007
Publication Date: July 8, 2007
Citation: Silva, M.V., Van Tassell, C.P., Sonstegard, T.S., Malek, M., Boettcher, P., Mugambi, J., Nagda, S., Kemp, S., Hanotte, O. 2007. Genetic resistance to nematodes parasites in sheep: use of Box-Cox transformation in QTL mapping. [abstract]. Joint Abstracts of the American Dairy Science and Society of Animal Science.
Fecal egg count (FEC) is used to identify and quantify gastrointestinal parasite infestations. However, FEC values are not distributed normally, and a small percentage of the herd is responsible for a majority of parasite transmission. Non-normality is a possible source of error when (co)variance components and genetic parameters are estimated, as well as decreasing the chance to detect quantitative trait loci (QTL). Typically, the distribution of FEC is right-skewed with a long tail and a high frequency of zero values (zero-inflated distribution). Nearly all QTL studies to date have used logarithmic transformations of FEC data before QTL analysis. In our study, six first cross (F1) Red Maasai×Dorper rams were mated to both Red Maasai (R) and Dorper (D) ewes to produce 1342 double backcross progeny. These six resource families were used to identify QTL controlling resistance to gastro-intestinal (GI) nematode parasites (particularly Haemonchus contortus). FEC measures were determined for 361 animals from 1 to 6 months old. In this study, original data were transformed using an extension of the Box-Cox transformation to approach normality and to estimate the position and variance components of the QTL in 19 of the 26 ovine autosomes. QTL detection was done using QxPak. Transformation of ovine FEC data utilizing the Box-Cox were effective in reducing the coefficients of asymmetry and kurtosis for all the variables studied, which improved estimates of variance components of the QTL. These results indicate that after transforming data by the Box-Cox procedure the genetic parameters in relation to QTL were more exact.