Dairy Genetics: Coding system identifies carriers of undesirable recessive traits

Widespread use of AI around the world and selection for similar traits across dairy populations has reduced diversity of pedigrees of influential cows and bulls. The frequency of carrier to carrier matings for undesirable recessive characteristics has increased as relationships between cows in dairy herds and bulls in AI service has increased in recent years.

The dairy breed societies in the United States have implemented good systems for identifying carriers of undesirable recessive genes. When an animal is established as a carrier of an undesirable recessive, the information is published as part of an animal's identification.

New genetic technologies have enabled scientists to establish "non-carrier" status for several genetic conditions, and codes have been developed for these as well. Following are the codes currently applied to Holstein animals. Codes referring to coat color hardly qualify as "undesirable", but are informative, nonetheless.

Recessive gene codes

BD - Bulldog                              HL - Hairless
MF - Mule-foot (syndactylism)  DF - Dwarfism
PG - Prolonged gestation          IS - Imperfect skin
BL - BLAD                                  B/R - Black-red animal
PT - Pink tooth (Porphyria)       DP - DUMPS
RC - Red hair color

Recessive tested codes

TM - Free of mulefoot               TL - Free of BLAD
TD - Free of DUMPS                  TR - Free of red hair color

The Jersey breed reports carriers of limberleg (LL) and rectovaginal constriction (RVC). The Brown Swiss breed identifies carriers of Weavers condition (W) as well as animals tested free of Weavers (TW). This listing is not necessarily exhaustive, and additional undesirable recessive genes will be added to these lists as they are discovered in years to come. All the breed societies and AI organizations deserve the support and praise of dairy producers for their commitment to the control of undesirable recessives through testing and open disclosure of carrier status.

By Bennet Cassell, Extension Dairy Scientist, Genetics and Management