Horner's law. The first step in the history of the understanding of X-linked disorders.

Earlier reports on colour blindness are descriptions which almost always mention the familial occurrence. Horner's publications (1876), however, gave the first scientific analysis of the hereditary transmission of Daltonism. Since this genealogic study was published only in a local bulletin of the City of Zurich it seemed necessary to give a translation of the most important part of the article. 'Horner's law' says that colour-blind fathers have colour-normal daughters; and these colour-normal daughters are the mothers of colour-blind sons. In his first pedigree Horner demonstrates that colour-blindness is transmitted from the grandfather to the grandson. A second pedigree, however, shows the possibility that the transmission is also possible via female carriers through more than one generation. The similarity with the inheritance of haemophilia, published by Lossen (Heidelberg), was mentioned by Horner. In the further progress of genetic research the chromosomes were visualized, at first in tumour-cells 1881, in cells of human tissue. The final point in this development was the description of sex chromosomes, which made the interpretation of Horner's law possible by Wilson (1911), i.e., the localization of the pathologic gene of Daltonism on the X-chromosome.