The allelic frequency of the CSN2 gene in Gyr and Guzerat pure breed animals was studied in an experiment conducted at the Rio Grande do Norte State, in Brazil, with 88 Guzerat and 68 Gyr animals of different categories. Hair samples from the cow tail tassel were collected and the DNA extraction was performed from the hair follicles, following the precipitation method with salt. Nucleotide sequence readings of the amplified fragment for the β-casein gene (A1 and A2) were aligned and edited. Allele frequencies (Xi) for β-casein alleles (1) and genotypic frequencies for the genotype (2) were obtained using the equations: Xi = 2n_ii + ∑n_ij / 2n;x_ij = n_ij / n, in which n_ii and n_ij correspond to the number of homozygotes and heterozygotes observed in the i allele, respectively; and n corresponds to the number of individuals analyzed. Using the Hardy-Weinberg theorem, expected genotypic frequencies at equilibrium were estimated from expanding the binomial: (x_i + x_j) = x_i² + 2x_ix_j + x_j², where x_i² is the expected frequency of homozygous for allele i; 2x_ix_j is the expected frequency for heterozygotes ij; and x_j² is the expected frequency of homozygous for allele j. It was found that Guzerat animals had a higher amount of heterozygous animals compared with Gyr of the population (Table 1). None of the evaluated animals presented homozygosity for A1. The allelic frequency of A2 allele and the genotypic frequency of A2A2 genotypes for β-casein gene in the assessed Zebu breeds indicate that these breeds may produce less allergenic milk for individuals who are sensitive to β-casein protein. Table 1. Allelic and genotypic frequencies for the A1 and A2 alleles of the CSN2 gene in Gyr and Guzerat breeds

Breed	Allelic frequency			Genotypic frequency			HWE1	P-value2
	A1	A2		A1A1	A1A2	A2A2
Gyr	0.02	0.98		0	0.04	0.96	0.07	0.99
Guzerat	0.03	0.97		0	0.07	0.93	0.23	0.97

¹HWE = Hardy-Weinberg equilibrium. ²Chi-squared test.