Animal genetics / Frederick B. Hutt.

  • Frederick Hutt
Date:
[1964]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Animal genetics / Frederick B. Hutt. Source: Wellcome Collection.

    527/578 page 505
    GENETIC RESISTANCE TO DISEASE 505 strain, but among 25 Adalból sheep in the same flock only one died. On one large farm where careful pedigrees were kept, there were significant differences among rams in susceptibility of their progeny, as the following figures of Dungal et al. show: On recognition of such genetic differences, farmers were urged to breed from resistant families and to kill all rams from susceptible ones. It became evident that, when jaagsiekte appeared in a flock, the mortality rate remained about the same during the first two years but thereafter it decreased gradually (Fig. 22-1). Jaagsiekte is apparently not a serious disease in other parts of the world where it has long been known. Presumably in such areas nat¬ ural selection over many years has produced stocks which are geneti¬ cally highly resistant. The extreme susceptibility of the Icelandic sheep can be attributed to the fact that they had not previously been exposed to jaagsiekte and had therefore not been able to raise genetic resistance to it, as sheep elsewhere have done, by natural selection. Similarly, the Indians of North America proved to be unusually susceptible to measles and chicken pox, which, as a result of unknown centuries of natural selection, were considered by the invaders who brought them from Europe as no more than normal, inevitable, and usually mild illnesses. It seems likely also, that, as with most dis¬ eases, susceptibility to jaagsiekte is influenced greatly by the environ- merit, and that confinement in close winter quarters may have aggravated the innate susceptibility of many of the Icelandic sheep. Later reports indicate that improved conditions of husbandry, along with continued selection, have greatly reduced the losses. The remarkable variation in resistance found among strains and families from different rams is both interesting and significant. It shows that genetic heterogeneity—the one thing indispensable if selection to enhance resistance to disease is to be effective—can lie latent in an animal population, unrecognized until revealed by some situation like that forced upon the Icelandic sheep, or, perhaps, by some perspicacious geneticist who realizes its economic value.
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