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Complement component 4 (C4A) is a candidate gene that reflects complement activity. The primary role of this gene in the classical and lectin-activation pathways is to provide protection against bacterial pathogens. In the current study, the bovine complement C4A gene was screened for polymorphisms, and the associations of these polymorphisms with the hemolytic activity of the classical pathway (CH50), C4 serum levels, and milk performance traits were examined. Three novel single-nucleotide polymorphisms (rs 132741478: g.2994 AG, rs 134006517: g.3508 AG, and rs 137485678: g.3649 GC) were detected by DNA sequencing and PCR-RFLP in 1182 Chinese Holstein cows. The rs 132741478: g.2994 AG mutation in exon 10 led to methionine and valine exchange at position 362, whereas rs 134006517: g.3508 AG and rs 137485678: g.3649 GC were synonymous substitutions. The statistical analyses revealed that cows with rs 132741478: g.2994 AG-AG and rs 137485678: g.3649 GC-CC have significantly lower somatic cell scores (SCS, P0.01). Homozygote cows with GAC haplotypes have the lowest SCS, whereas AAG/AAC cows have the highest. The serum concentration of C4 by ELISA and the hemolytic and antibacterial activity of CH50 were also evaluated in the current study. The results confirmed that rs 132741478: g.2994 AG in the coding sequence of the β-chain of the bovine C4A gene is related to mastitis resistance. This polymorphism may be very important in marker-assisted selections in dairy cattle breeding programs.

The nucleotide sequence of the melanocortin-1-receptor (MC1R) gene was studied with the help of the polymerase chain reaction (PCR), in which the protein structure in Chinese Holstein was predicted, and the molecular mechanism of the red coat color was investigated. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was performed to genotype the individuals. The bioinformatics and biotechnology softwares were used to predict the secondary structure of MC1R. The results showed that the EE genotype was the dominant genotype in Chinese Holstein Black and White herd, whereas, it was ee in Chinese Holstein Red and White herd. The secondary structure of the mutational MC1R protein was changed and the deletion mutation caused an earlier termination in translation, which led to the formation of the red coat color. The allele E was mainly associated with the black coat color, whereas, e was associated with red.