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The Dag defect is a rare heritable midpiece defect that carries the name of a Jersey bull who produced it in large numbers along with his full brother. The unusual and distinctive name of this defect is often highlighted in the minds of students, leading to a tendency to refer to DMRs as the Dag defect. Differing from the DMR, unique features of the Dag defect include a swollen, disrupted mitochondrial sheath and fractures of the midpiece (Figure 9.20). Oftentimes, midpiece fractures result in coiling of the tail very similar to the DMR. By studying the fractures closely, broken axonemal fibers that can be likened to broken structural cables can be seen. In addition to the Jersey breed, the Dag defect has been reported in the Hereford, Swedish Red and White, and Galloway breeds [2]. Sperm showing the true Dag defect are usually produced in very high numbers. Although very uncommon, a few sperm displaying mitochondrial disruption identical to the Dag defect may be noted in spermiograms, but in the presence of other defects. These defective sperm are called Dag‐like to differentiate their occurrence from the rare, heritable condition where high numbers of sperm express the defect in the absence of any other evidence of an insult to spermatogenesis.

Figure 9.20 Dag defect.