The DD Gene

DNA samples from calves born with a rare condition where extra limbs originate from the neck or shoulder area, have led to the identification of an inherited, simple recessive condition, known as Developmental Duplication or DD. A common term for animals or humans born with additional limbs is polymelia.

Phil Trowbridge, president of the American Angus Association, said in a letter to association members that he expects they will use DNA testing to deal with the condition in a strategic manner, but fell short of calling for mandatory testing. He said this policy "represents an evolution in the association's approach to genetic conditions."

The American Angus Association has dealt with genetic challenges before, going back to 2008, when policies were created to first address curly calf syndrome (Arthrogryposis Multiplex or AM), then Neuropathic Hydrocephalus (NH) and most recently Contractural Arachnodactyly (CA). In the case of DD, Trowbridge wrote the following:

"With the onset of DD, it became clear that the discovery of genetic conditions will be a part of the future for all breeds of cattle. The association's early 'DNA era' began in 2008, with the development of policies related to first AM and then NH. Those policies were based on the perception that each represented catastrophic, once-in-a-lifetime events. Both conditions were lethal. These early policies were premised on the good faith belief that the best way to eliminate the condition and, at the same time, protect the interests of our commercial customers, was to impose some form of testing as a precondition for registration. This mandatory testing approach has undoubtedly resulted in a measurable decrease in the frequency of AM, NH and CA, but has come with a significant price tag for the membership."

Investigation into DD goes back to 2011, when Jonathan Beever, a genetics specialist with the University of Illinois, was asked to investigate polymelia in Australian Angus cattle. Over a 4-year period about 20 calves were reported to have been found with this condition. Beever's report on DD noted that since early 2013, four calves with these features have been reported in the U.S. He wrote "we believe these data indicate that calves presenting with polymelia at birth are rare events that survive embryonic death."

To date, 1,099 Angus bulls have been tested by Beever to see if they are carriers of DD. He said "the allele frequency among U.S. sires is moderately high at approximately 3%; this corresponds to a carrier frequency of approximately 6%." He said no homozygous individuals were in the population.

A list of bulls tested is available at www.angus.org/Pub/DD/DD_Update08122013.pdf. Those bulls that are carriers of the genetic condition are listed as DDC. Of the 1,099 in this round of testing, some 72 bulls were noted as DDC.

The American Angus Association's policy regarding the registration status of potential and known carriers of DD states the following:

-- Status of Currently Registered Females and Bulls. Notwithstanding any subsequent test results, all registered females and bulls with the impacted genetics in their pedigrees as of August 14, 2013, shall remain registered.

-- Resulting Progeny of Carrier Females and Bulls. All resulting progeny of currently registered carrier females or carrier bulls may be registered without submitting to testing. (The policy adds that a notation will be placed by the Association saying the animal has ancestors known to carry the mutation.)

-- Currently Registered Animals Determined to be Affected by the Mutation. Any animals identified as being homozygous for the mutation, shall therefore be considered to be affected by the condition, and are not eligible for registration ... in the event that a registered animal is discovered to be affected by the condition, its registration shall be considered null and void and the certificate of registration most be returned to the Association for cancellation.

The Association will list on its website (www.angus.org) the name, registration number and test results of animals tested. DDF will mean Developmental Duplication Free; DDC will mean Developmental Duplication Carrier; and DDA will mean Developmental Duplication Affected.

Genetic tests for DD are available from Angus Genetics Inc. and GeneSeek. Links to these companies and additional information on DD can be found on the American Angus Association website. Results of testing are provided to the association and the submitting member.

(VM/CZ)

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