Certain genetics can be tested for and managed rather than be completely removed from the breeding population
There are many undesirable genetic conditions in cattle, including dwarfism, hairlessness and edema. Some problems are inherited as dominant traits passed by one parent and some are recessive but received from both parents.
A recessive trait can hide in each carrier parent, only appearing in calves if doubled up. Risk of genetic problems rises with inbreeding or line breeding with the same individual on both sides of the pedigree.
Kajal Devani, director of science and technology at the Canadian Angus Association, says recessives are easy to predict. A carrier animal bred to a non-carrier animal will never produce affected offspring, but those offspring have a 50 percent chance of being carriers.
Breeding a carrier to a carrier, however, carries risk for defects. There is one chance in four that a calf will not inherit the recessive gene, two chances out of four that it will be a carrier, and one chance in four that it will inherit both recessives and be affected.
“Mutations and altered genes occur in all animals. We are fortunate today to have DNA technology and tests that help us identify animals that are carriers and can avoid mating carriers to carriers,” said Devani.
“We don’t have to completely remove certain genetics from the breeding population. We can work around the undesirable recessive trait and never produce an affected animal.
“In my job I help Angus breeders manage and test for unwanted genetic conditions. Some mutations are specific to certain populations. People say such-and-such condition happens in Angus, or in a certain other breed but we need to realize that even if a certain gene occurs in Angus, there can be another mutation of that gene in other breeds, or that a mutation in Angus can also appear in Sim-Angus or any other cross with Angus.”
Mutations happen all the time. Some are never noticed but some are visible and damaging.
Devani explains mutation as nature’s way of creating variations in animals to more readily adapt to changing environments or conditions. It ensures survival of a species but not every change is a good one.
“We use the word deleterious to describe the unwanted outcome. We try to avoid anything that negatively impacts profitability when raising cattle,” said Devani.
“If you end up with several calves born dead or defective, we want to make sure it never happens again in a future animal.”
Fiona Buchanan at the University of Saskatchewan said several companies now test DNA samples using hair or blood. There are tests for conditions such as alpha-mannosidosis, a recessive lethal disease in which affected animals have head tremors, inco-ordination and aggression.
“When the affected calf is born it gets weaker and dies. This was a problem in Angus cattle in New Zealand and Canada until the breed associations offered a DNA test. Then a similar condition was found called beta-mannnosidosis, in Saler cattle, producing newborn calves that never get up,” said Buchanan.
Arthrogryposis multiplex, commonly called curly calf, was identified in Angus and caused by a recessive gene. Affected calves are born with deformed legs and backbones and rarely survive birth. Contractural arachnodactyly or fawn calf is rarely fatal but calves are tall and slender and may be weak, with impaired joint motion.
Neuropathic hydrocephalus or water head is a lethal mutation causing excess fluid in the cranium. Idiopathic epilepsy is an inherited condition in which the animal is prone to seizures.
Tibial hemimelia results in abdominal hernia, severe distortion of rear leg structure, exposure of brain and spinal tissue. Protoporphyria results in extreme sensitivity to sunlight and congenital hypotrichosis results in hairlessness.
“Recessive conditions are the result of a single gene on an autosome (any chromosome that is not a sex chromosome). Both parents must be carriers for an animal to be affected,” said Buchanan. “If a certain condition occurs, you can test the sires and remove the carrier and test subsequent bulls to ensure they are not carriers.”
Devani says purebred producers are aware of genetic conditions to avoid, but sometimes commercial producers are not. It is the purebred breeders’ responsibility to ensure that genetics offered are free of unwanted recessive traits.
“Purebred breeders also need to be transparent. If you have a phenomenal bull that carries an unwanted recessive, you can still use him if you do it properly. Many unwanted genetic conditions have been propagated in popular bloodlines because those lines have excellent production traits. This is why they were popular and widely used.”