Iowa cattle rancher Warren Gammon capitalised on the idea of breeding Polled Herefords and started the Polled Hereford registry with 11 naturally polled cattle. The American Polled Hereford Association (APHA) was formed in 1910. The American Polled Hereford and American Hereford breeds have been combined since 1995, under the same American Hereford Association name. Many strains of Hereford have used other cattle breeds to import desired characteristics, and this has led to changes in the breed as a whole. However, some strains have been kept separate, and these have retained characteristics of the earlier breed, such as hardiness and thriftiness. The Traditional Hereford is now treated as a minority breed of value for genetic conservation. Eye cancer (ocular squamous cell carcinoma) occurs in Herefords in particular in countries with continued bright sunlight and those that prefer traits of low levels of red pigmentation around the eye. Studies have been made into eye cancer in Hereford cattle in the US and Canada, and lid and corneoscleral pigment were found to be heritable and likely to decrease the risk of cancer. Vaginal prolapse is considered a heritable problem in Hereford cattle, but it may also be influenced by nutrition. Another problem is exposed skin on the udder being of light pigmentation and therefore vulnerable to sun burn. Dwarfism is known to be prevalent in Hereford cattle and has been determined to be caused by an autosomal recessive gene. Due to equal occurrence in heifers and bulls, dwarfism is not considered a sex-linked characteristic.