它们是一代又一代选择性繁殖和杂交的产物。
They are the product of generations of selective breeding and hybridisation.
科学家们研究过一些品种后表示牛的基因组显示出这种有选择性繁殖的迹象。
Scientists examined several breeds and say the cattle genome appears to show evidence of this selective reproduction.
其原因之一可能因为牛是通过选择性繁殖的方式来提高牛奶的蛋白质含量（乳品生产者所期望的品质）的。
One reason this may be so frequently found is because cows have been selectively bred this way to increase the protein content of the cow-milk (a quality desired by dairy producers). 9
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