Wednesday, July 6, 2022

Genetic defects afflicting Angus breed explained

The genetic defects AM (curly calf), NHC, and CA are commonly identified as risks in the sale catalogues of Angus studs. 

Each is carried differently and New Zealand Angus says the breed is not the only one trying to remove it from the national herd.

Arthrogryposis Multiplex (AM, or curly calf syndrome) is a lethal genetic defect that is inherited as a simple recessive.

As a simple recessive, the condition is controlled by a single pair of genes. For a calf to be affected, it must inherit the AM gene from both of its parents.

AM calves are dead at birth, with a bent or twisted spine, are small and thin muscled, and their legs are often rigid and hyper-extended.

The chances of having an AM calf are directly related to the probability of herd sires and cows being carriers of the AM gene. Carrier animals possess the AM gene, and although they are not affected themselves, they will pass the effective gene to their progeny 50% of the time.

When a carrier bull is mated to a carrier cow, on average 25% of the calves will have the AM condition and 75% of the calves will be normal.

However, some calves that appear normal will be carriers of the AM gene (one copy of the AM gene, inherited either from their sire or dam) and will have the ability to pass the gene on to their progeny.

Since AM results only when a calf inherits the AM gene from both of its parents, if one parent is free of the AM gene it is not possible to have an affected calf.

Neuropathic Hydrocephaly (NH) is a genetic defect affecting calves, which are dead at birth and have severe hydrocephalus (water on brain), skull malformation, absence of central nervous system tissue (brain and spinal cord), and joint fixation.

It is a lethal defect that is inherited as a simple recessive and is controlled by a single gene. Therefore, for a calf to be affected, it must inherit the NH allele from both of its parents.  

Congenital contractural arachnodactyly (CA), also known as “fawn calf” syndrome (FCS), is a heritable disease of newborn Angus and Angus-derived cattle caused by an error in the DNA genetic code transmitted from parents to their progeny.

Calves with CA are visibly abnormal at birth but are usually born alive. Most survive to adulthood.

CA is a developmental defect involving reduced elasticity of the connective tissue of muscles. 

  • Sources: Department of Primary Industry, New South Wales; Virginia Tec Co-operative Extension, United States

Related story: Breeders ignore defect advice


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