-A project of QTL detection was carried out in the French Holstein, Normande, and Montbéliarde dairy cattle breeds. This granddaughter design included 1 548 artificial insemination bulls distributed in 14 sire families and evaluated after a progeny-test for 24 traits (production, milk composition, persistency, type, fertility, mastitis resistance, and milking ease). These bulls were also genotyped for 169 genetic markers, mostly microsatellites. The QTL were analysed by within-sire linear regression of daughter yield deviations or deregressed proofs on the probability that the son receives one or the other paternal QTL allele, given the marker information. QTL were detected for all traits, including those with a low heritability. One hundred and twenty QTL with a chromosome-wise significance lower than 3% were tabulated. This threshold corresponded to a 15% false discovery rate. Amongst them, 32 were genomewise significant. Estimates of their contribution to genetic variance ranged from 6 to 40%. Most substitution effects ranged from 0.6 to 1.0 genetic standard deviation. For a given QTL, only 1 to 5 families out of 14 were informative. The confidence intervals of the QTL locations were large and always greater than 20 cM. This experiment confirmed several already published QTL but most of them were original, particularly for non-production traits. dairy cattle / QTL detection / genetic marker / granddaughter design

-Genetic analysis for mastitis resistance was studied from two data sets. Firstly, risk factors for different mastitis traits, i.e. culling due to clinical or chronic mastitis and subclinical mastitis predicted from somatic cell count (SCC), were explored using data from 957 first lactation Lacaune ewes of an experimental INRA flock composed of two divergent lines for milk yield. Secondly, genetic parameters for SCC were estimated from 5 272 first lactation Lacaune ewes recorded among 38 flocks, using an animal model. In the experimental flock, the frequency of culling due to clinical mastitis (5%) was lower than that of subclinical mastitis (10%) predicted from SCC. Predicted subclinical mastitis was unfavourably associated with the milk yield level. Such an antagonism was not detected for clinical mastitis, which could result, to some extent, from its low frequency or from the limited amount of data. In practice, however, selection for mastitis resistance could be limited in a first approach to selection against subclinical mastitis using SCC. The heritability estimate of SCC was 0.15 for the lactation mean trait and varied from 0.04 to 0.12 from the first to the fifth test-day. The genetic correlation between lactation SCC and milk yield was slightly positive (0.15) but showed a strong evolution during lactation, i.e. from favourable ( 0.48) to antagonistic (0.27). On a lactation basis, our results suggest that selection for mastitis resistance based on SCC is feasible. Patterns for genetic parameters within first lactation, however, require further confirmation and investigation.dairy sheep / somatic cell count / mastitis / genetic parameters / risk factors

Un programme de détection de QTL est conduit dans les races bovines Prim’Holstein, Normande et Montbéliarde. Ce dispositif dit ‘petites-filles’ implique 1554 taureaux d’insémination artificielle, fils de 14 pères et indexés sur descendance pour 24 caractères de production laitière, conformation, fertilité, résistance aux mammites et facilité de traite. Ces taureaux sont génotypés pour 169 marqueurs répartis sur le génome. La recherche de QTL est réalisée par régression linéaire intra père, en prenant en compte toute l’information marqueurs par groupe de liaison. Des QTL sont détectés pour chaque caractère, y compris pour les caractères peu héritables. L’effet de substitution allélique est souvent compris entre 0,6 et 1 écart type génétique. Une minorité de pères (entre 1 et 5 seulement sur 14) apparaissent hétérozygotes pour chaque QTL identifié. La localisation des QTL est peu précise, l’intervalle de confiance étant toujours supérieur à 20 cM. Ce travail se poursuit d’une part avec la mise en place d’une sélection assistée par marqueurs, d’autre part avec la caractérisation de certains QTL dans le cadre du programme européen Euribdis.