Modelling effects of candidate genes on complex traits as variables over time.

In this study, changes in gene effects for milk production traits were analysed over time. Such changes can be expected by investigating daily milk production yields, which increase during the early phase of lactation and then decrease. Moreover, additive polygenic effects on milk production traits estimated in other studies differed throughout the 305 days of lactation, clearly indicating changes in the genetic determination of milk production throughout this period. Our study focused on particular candidate genes known to affect milk production traits and on the estimation of potential changes in the magnitude of their effects over time. With two independent data sets from Holstein-Friesian and Jersey breeds, we show that the effects of the DGAT1:p.Lys232Ala polymorphism on fat and protein content in milk change during lactation. The other candidate genes considered in this study (leptin receptor, leptin and butyrophilin, subfamily 1, member A1) exhibited effects that vary across time, but these could be observed in only one of the breeds. Longitudinal modelling of SNP effects enables more precise description of the genetic background underlying the variation of complex traits. A gene that changes the magnitude or even the sign of its effect cannot be detected by a time-averaged model. This was particularly evident when analysing the effect of butyrophilin, missed by many previous studies, which considered butyrophilin's effect as constant over time.

The effects of genotype and selected environmental factors on colostrum production and intake in cattle.

The aim of this study was to determine effects of genotype and selected environmental factors on colostrum production, intake, and efficiency in the cattle. The investigations were conducted on 67 dam-calf pairs. All cows were of Polish Holstein-Friesian breed, the Black-and-White variety (PHF-HO), whereas calves were sired by bulls of the following breeds: PHF-HO, Polish Holstein-Friesian of the Red-and-White variety (PHF-RW), Jersey (JE), Montbéliarde (MB) and Limousine (LI). The colostrum was collected from cows and offered to calves "from bucket" thrice a day. The amount of produced colostrum considerably exceeded the ability of its consumption by calves. Low share of HF genes in the cow, older cow's age and calving in the period from January to April appeared to be favorable factors for colostrum production. Calves born to cows with low HF gene share drank more colostrum than calves originating from cows with higher gene share of this breed. Crosses with JE drank the highest amount of the colostrum in relation to body weight, while MB-sired calves drank the highest amount in absolute terms. Colostrum intake was positively correlated with the dam's age. Furthermore, it was shown that in case of the first colostrum intake the most favorable period was from May to September, while in cases of total intake in the first day of life and mean intake in the first 5 days of life it was the period from January to April. Crosses with JE were characterized by extremely high use of colostrum per unit of body weight gain.

The relationship between leptin gene polymorphisms and reproductive traits in Jersey cows.

The aim of the study was to analyse the relationship between three leptin gene polymorphisms, and indices of reproductive performance--age at first calving, days open, calving interval, number of inseminations per conception, and gestation length in Jersey cattle. A total of 219 cows were tested for the R4C, A59V and C(-963)T mutations using the PCR-RFLP technique. The following allele frequencies were found: R4C--0.80 (C) and 0.20 (T), A59V--0.67 (C) and 0.33 (T), C(-963)T--0.83 (C) and 0.17 (T). The statistically significant correlation (P < or = 0.05) was found only for the A59V mutation. Cows with the TT genotype were characterized by shorter calving interval and time of days open than animals with CT and CC genotype. Also, the number of inseminations per conception was lower in A59V--TT homozygotes compared with two other genotypes. The association analysis did not reveal any significant correlations for both R4C and C(-963)T polymorphisms.