On The Origin of Species by Charles Darwin Chapter 5 Page 68

been crossed only once by some other breed, the offspring occasionally show for many generations a tendency to revert in character to the foreign breed — some say, for a dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common expression, from one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed that a tendency to reversion is retained by this remnant of foreign blood. In a breed which has not been crossed, but in which BOTH parents have lost some character which their progenitor possessed, the tendency, whether strong or weak, to reproduce the lost character might, as was formerly remarked, for all that we can see to the contrary, be transmitted for almost any number of generations.

When a character which has been lost in a breed, reappears