Statistical Analysis
Associations of the animal genotypes with growth traits and IGF-I concentrations were determined by analysis of variance of quantitative traits, which included birth weight, weaning weight, preweaning gain, on-test weight, weight at day 28 and 56 of the 140-day postweaning test, off-test weight, weight gain during the 20-day period between weaning and the beginning of the postweaning test, postweaning gain, and serum IGF-I concentration on days 28, 42, and 56, and the mean serum IGF-I concentration, using GLM procedures in SAS. Fixed effects of Pit-1 genotypes, year of birth, season of birth (spring vs fall), age of dam, sex, and IGF-I selection line (high vs low) were included as independent variables in the linear model. Age of calf was treated as a covariate in the model. To test for possible differences in the association between genotypes and growth traits in the high and low selection lines, data were also analyzed separately within each selection line using the same model, except that selection line was deleted from the model.
Results
Three polymorphisms-a new SSCP site detected in Pit-1 intron 5, a new MspI polymorphism in Pit-1 intron 2, and a PCR-RFLP HinfI polymorphism previously reported (Woollard et al., 1994) in Pit-1 exon 6- were observed in Angus beef cattle. For the intron 5 polymorphism, we found genotypes AB and BB with frequencies of 0.09 and 0.91, respectively. No AA individuals were found in this sample. Genotypic frequencies were not different between the high and low IGF-I selection lines (P value for X² test was 0.48). For the exon 6 polymorphism, genotypic frequencies of 0.11, 0.45, and 0.44 were observed for CC, CD, and DD, respectively. Again the genotypic freqencies did not differ between the high and low IGF-I selection lines (P value for X² test was 0.91). For the MspI site, a fragment of 3,000 bp starting in exon 2, including intron 2, and ending at exon 3, was amplified. The PCR product was digested with MspI and revealed three genotypes, EE, EF, and FF, with frequencies of 0.16, 0.48, and 0.36, respectively. The genotypic frequencies were significantly different between the high and low IGF-I selection lines (P value for X² test was 0.03). In the high line, the genotypic frequencies were 0.22, 0.25, and 0.53 for EE, EF, and FF, respectively. In the low line, the genotypic frequencies were 0.09, 0.51, and 0.40 for EE, EF, and FF, respectively. In these 80 animals, the frequency of EE was significantly higher in the high IGF-I selection line than in low line (Table 1).
Relationships of these genotypes with growth traits and IGF-I
concentration were also analyzed using the GLM procedure of SAS. For the
intron 5 polymorphism, no significant associations between the genotypes
and the growth traits were found. However, animals with genotype AB
tended to have higher values than animals with genotype BB for every
trait except birth weight. A moderate association of genotypes with
weight at day 56 (P = 0.08) and with IGF-I concentration at day 56 (P =
0.09) was found for this polymorphism ().
Significant associations were found between HinfI genotypes and birth
weight (P = 0.03) and preweaning gain (P = 0.01). Animals with genotype
DD had higher birth weight but lower preweaning gain. A moderate
association of genotypes with weight gain during the 20-day period
between weaning and the beginning of the postweaning test (P = 0.07) was
found for this polymorphism and animals with genotype CC had higher
value (). For the MspI polymorphism, no significant association
was found between the genotypes and growth traits ().
However, when the high and low IGF-I selection lines were analyzed
separately, a significant relationship was found between MspI genotypes
and IGF-I concentration at day 56 (P = 0.02) in the high IGF-I selection
line. The EE genotype was associated with higher d 56 IGF-I
concentration.
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