Hereditary diseases exhibit certain eminently characteristic phenomena, which a late writer[2] enumerates as follows:
1. "They are transmitted by the male as well as by the female parent, and are doubly severe in the offspring of parents both of which are affected by them.
2. They develop themselves not only in the immediate progeny of one affected by them, but also in many subsequent generations.
3. They do not, however, always appear in each generation in the same form; one disease is sometimes substituted for another, analogous to it, and this again after some generations becomes changed into that to which the breed was originally liable—as phthisis (consumption) and dysentery. Thus, a stock of cattle previously subject to phthisis, sometimes become affected for several generations with dysentery to the exclusion of phthisis, but by and by, dysentery disappears to give place to phthisis.
4. Hereditary diseases occur to a certain extent independently of external circumstances; appearing under all sorts of management, and being little affected by changes of locality, separation from diseased stock, or such causes as modify the production of non-hereditary diseases.
5. They are, however, most certainly and speedily developed in circumstances inimical to general good health, and often occur at certain, so called, critical periods of life, when unusual demands on the vital powers take place.
6. They show a striking tendency to modify and absorb into themselves all extraneous diseases; for example, in an animal of consumptive constitution, pneumonia seldom runs its ordinary course, and when arrested, often passes into consumption.
7. Hereditary diseases are less effectually treated by ordinary remedies than other diseases. Thus, although an attack of phthisis, rheumatism or opthalmia may be subdued, and the patient put out of pain and danger, the tendency to the disease will still remain and be greatly aggravated by each attack.
In horses and neat cattle, hereditary diseases do not usually show themselves at birth, and sometimes the tendency remains latent for many years, perhaps through one or two generations and afterwards breaks out with all its former severity."
The diseases which are found to be hereditary in horses are scrofula, rheumatism, rickets, chronic cough, roaring, ophthalmia or inflammation of the eye,—grease or scratches, bone spavin, curb, &c. Indeed, Youatt says, "there is scarcely a malady to which the horse is subject, that is not hereditary. Contracted feet, curb, spavin, roaring, thick wind, blindness, notoriously descend from the sire or dam to the foal."