Recently the American Angus Association announced the discovery of another genetic condition called developmental duplication (DD). Most affected calves that survive to term are born with multiple limbs or polymelia. Polymelia occurs spontaneously in cattle of a variety of breeds as well as other species including sheep, horses, and humans. Other than an increased occurrence of mortality associated with calving difficulty, calves born with polymelia often thrive (especially following the removal of the limb[s]).
During the last 4 years, the incidence of polymelia in purebred Australian Angus populations rose above expected sporadic levels. Drs. Laurence Denholm (NSW Department of Trade and Investment) and Jonathan Beever (Agrigenomics, Ltd. and the University of Illinois) found this condition was the result of a simple recessive mutated gene. After discovering a DNA variation that appears to directly cause the defect, Dr. Beever initially tested 1,099 high-use AI Angus bulls and found 72 carriers of the defective allele (a moderately high allele frequency of 3%). However, based on this allele frequency, the incidence of polymelia should be higher than is actually reported if all animals homozygous for the mutation demonstrated the defect (the defect would be called fully penetrant). Due to this discrepancy, Dr. Beever initially hypothesized a certain amount of embryonic loss during gestation in homozygous embryos. Upon further investigation, Dr. Beever found some homozygous animals with very minor phenotypes (e.g., a 2 inch long skin tag) and others that appear totally normal. Recently, a few dozen seemingly normal homozygous recessive animals have been discovered. When animals have the genotype and do not display the phenotype, this is referred to as incomplete penetrance. The cause of incomplete penetrance in DD is unknown but is being researched aggressively. Dr. Beever is examining the DNA of these normal homozygous recessive animals for a potential explanation (for instance, a second gene that interacts with the DD gene) and is planning breeding trials to understand the inheritance better.