The Casein Gene and Milk Quality in Goats

Liv Lønne Dille

Knowledge about the relationship between genetic casein variants and milk quality parameters could enable us to utilize DNA information about individual genes in addition to traditional breeding schemes, with the aim of adapting the quality of goat’s milk to human needs. It should be possible to alter the milk’s fat and protein contents, or to select genetic variants that give easily digestible products.

Casein (milk protein) in milk is a transcription of the DNA code of four casein genes in goats. Many other genes are involved in the production of casein, but the quality of this casein is determined by the four genes because the DNA code is transcribed as casein. Each casein gene has several gene variants. Different casein variants give different cheese-making qualities – e.g., when making white goat’s cheese, and may also be associated with the digestibility of goat’s milk. (Its digestibility is one of the arguments for the increased use of goat’s milk instead of cow’s milk.)

In a previous joint project between Tine Norwegian Dairies and the Department of Animal and Aquacultural Sciences at UMB, we found there was 86 % of what we believed to be a zero allele in the first casein gene (CSN1S1) on two goat farms in northern Norway. This gene variant was significantly correlated with important milk quality parameters. Goats with a double dose of the zero allele (from both parents) had lower fat and casein percentages and inferior cheese-making properties.

We then mapped the genetic variants in all four casein genes from 450 breeding bucks that had been used in recent years. This is done by genotyping each animal at 39 positions (SNP) in the gene area coding for casein. One of these positions characterizes the assumed zero allele, the others give information about other genetic variants. The frequency of the assumed zero allele was 75 %.

The milk yields of the daughters of the surveyed bucks are registered in the Goat Recording Scheme. We analyzed the data to see if the bucks’ genotypes and the yields of their daughters were correlated. For the SNP of the zero allele, no significant correlation was found, even though the tendency was the same as for the goats from the two northern Norwegian farms. But if one takes all alleles of the CSN1S1-gene into consideration, there is a significant effect on both protein and lactose percentage. We also found a significant effect of gene variants of the last casein gene (CSN3) on the protein and lactose percentage in the goats’ milk. No other parameters were analyzed in the milk than those regularly monitored by the Goat Recording Scheme, so we have no direct data on the milk’s cheese-making qualities. Furthermore, we lack data on the relationship between casein variants and digestibility, since we only have data about the sire’s casein variants, but no information about the genotypes of the milking goats.

The four casein genes are located on a single chromosome. They are thus usually inherited from the sire or the dam to the offspring as a unit. This combination is called a haplotype. Based on the genotyping of the bucks and their mutual kinship, about 20 probable casein haplotypes have been established for the Norwegian dairy goat, although the 39 SNPs can be combined into more than 4000 haplotypes.

In a new experiment, the casein gene of about 500 goats from eight different herds, and of 500 new breeding bucks is being analyzed. This will enable us to compare the performance of goats with a known casein genotype, and to verify the previous results from the breeding buck survey.

Updated: 14.05.09
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