-To determine the effects of a number of non-genetic factors on milk components and estimate their indirect relationships with weaning weight (WW205), the percentages of protein, fat, lactose and solids non-fat in Charolais cows from Mexico were estimated. Lactation period (LP) and calving season (CS) had significant effects on protein and fat, whereas lactose and solids non-fat were only affected by the LP. Sire line and age of the dam had significant effects on calf birth weight, while CS affected WW205. Residual correlations between milk components and WW205 suggest a low but significant correlation with lactose. These results demonstrate the importance of several non-genetic factors on the composition of milk from Charolais cows managed under extensive cow-calf production systems and the indirect relationships between these factors and the weaning weight of a calf.

RESUMENSe determinaron las frecuencias alélicas y genotípicas de ocho marcadores localizados en los genes calpaína (CAPN, 4751 y 316), calpastatina (CASTT1) y tiroglobulina (TG5), asociados a calidad de carne, y en los genes, miostatina (MSTN, Q204X), arginino succinato sintasa (ASS), monofosfato sintasa (UMPS) y miofosforilasa (PYGM), asociados a enfermedades genéticas de ganado bovino. Se muestrearon 493 animales Charolais de registro de dos hatos ubicados en Sonora (n=157) y tres en Nuevo León (n=336). No se encontraron portadores de los alelos T-ASS y T-UMPS, pero sí portadores del alelo Q204X del gen MSTN en frecuencias de  1 % en las poblaciones de Sonora y de 8.6 a 14.4 % en las de Nuevo León. Además, se identificaron portadores del marcador del gen PYGM, en frecuencias del 6.5 y de 1.0 % para un hato de Sonora y otro de Nuevo León, respectivamente. El análisis de diferenciación génica pareado entre las poblaciones y con los cuatro loci mostró que hay diferencias altamente significativas dentro de poblaciones del noroeste (P<0.0001) y entre éstas y las del noreste (P<0.001), la cual es explicada principalmente por los loci CAPN-316 y TG5. De acuerdo a los resultados obtenidos se recomienda el monitoreo del marcador del gen PYGM y del alelo Q204X del gen MSTN, así como también implementar estrategias para confirmar la utilidad de los marcadores asociados a calidad y productividad como herramienta para complementar los programas de mejoramiento genético. PALABRAS CLAVE: Defectos genéticos, Marmoleo, Suavidad de la carne, ADN. ABSTRACTAllelic and genotypic frequencies of eight markers previously associated to genetic disorders and meat quality and located in the calpain (CAPN1-4751 and CAPN1-316) , calpastatin (CAST-T1), thyroglobulin (TG5), myostatin (MSTN, Q204X), argininosuccinate synthase (ASS), monophosphate synthase (UMPS) and myophosphorylase (PYGM) genes, were determined from 493 registered Charolais animals sampled from two herds located at Sonora (n= 157) and three at Nuevo León (n= 336), Mexico. No carriers of mutated alleles of ASS and UMPS genes were found, but carriers of the MSTN Q204X allele were identified at frequencies of  1 % in Sonora populations and 8.6 to 14.4 % in Nuevo Leon. In addition, carriers of PYGM marker were identified at frequencies of 6.49 and 1% in a herd from Sonora and other from Nuevo León, respectively. Analysis of gene differentiation among herds and with four loci showed that there are highly significant differences within Northwest populations (P<0.0001) and between them and the Northeast (P<0.001), differentiation is mainly explained by the loci CAPN-316 and TG5. According to the obtained results, the periodic monitoring of the PYGM marker gene and of the allele Q204X the MSTN gene is propose; also it is important to implement strategies to confirm the usefulness of those markers associated with quality and productivity as a tool to complement breeding programs.