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.

    538/578 page 516
    516 ANIMAL GENETICS case. It is true that some genetic weakness, such as impaired abiUty to form antibodies, can make animals susceptible to any of several different kinds of infection, hence the normal individuals not homo¬ zygous for that defect are more resistant to those same infections. It has also been shown that animals genetically resistant to one pathogen may in some cases be similarly resistant to another that is closely related to the first. Thus, White Leghorns are comparatively resistant both to Salmonella pullorum and to S. gallinarum (which causes fowl typhoid). Apart from such exceptions, the more general rule seems to be that genetic resistance to one disease is specific for that disease, and does not often extend to others that are entirely different. For ex¬ ample, the remarkable ability of zebu cattle to withstand extreme heat, and to repel ticks, is matched by their equally remarkable re¬ sistance to bovine tuberculosis ( Carmichael ), but most of the evi¬ dence available indicates that in Africa they are more susceptible than some other breeds to trypanosomiasis. Similarly, when a dis¬ ease commonly called blue comb struck Leghorns at Cornell one year, mortality from that disease ( at about three or four months of age) and from leucosis (to 500 days of age) ran as follows in three different strains : Blue comb Leucosis Strain ( per cent ) ( per cent ) К Resistant (to leucosis) 0.99 6.5 С Resistant (to leucosis) 7.40 8.7 Susceptible (to leucosis) 0.00 65.0 Clearly the strain genetically most susceptible to leucosis was most resistant to blue comb. Although the two resistant strains are about equally resistant to leucosis, they differ significantly in sus¬ ceptibility to blue comb and to some other diseases as well. Simi¬ larly Gowen's mice most resistant to typhoid disease proved highly susceptible to a virus causing pseudorabies. From these examples it is evident that the physiological basis for genetic resistance to one disease can be quite different from that for resistance to some other. To breed domestic animals that can better cope with adverse environments of different kinds, which include pathogenic organisms of assorted sizes and species, it would seem desirable to learn more about the divers physiological mechanisms to which genetically resistant animals owe their biological superiority over those genetically susceptible. Examples of such mechanisms are considered in the next few paragraphs.
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