The "deformity" stereotype is an old one

Perhaps no opinion, upon subjects of a medical character, is more widely diffused among the public, or more tenaciously held, than that the results of the marriage of blood relations are almost uniformly unfortunate. This opinion has been so long held and so often reiterated, that by sheer force of these circumstances alone it has come to be regarded as an unquestioned and unquestionable fact.

- Dr. Bell, a New England physician in 1859