Focus on Genetic Conditions

by Jackie Atkins, Ph.D.

Genetic conditions can be tricky to navigate but please remember, all animals carry genetic defects (including you and me). The difference is that not all genetic conditions have been discovered, documented, and have a developed DNA test. This article centers around the conditions with a DNA test that are tracked in the ASA’s Herdbook system called TraitTrac. 

TraitTrac ASA’s genetic condition and trait tracking tool, called TraitTrac, uses DNA marker tests to provide information concerning genetic traits or conditions for animals in the ASA registry. TraitTrac indicates the status (free, at risk, or carrier) of different traits and genetic conditions based on known information on individual animals. The TraitTrac status can be viewed from the EPD/Pedigree display found in Herdbook Services under Animal Search. An animal’s genetic condition status is determined from information provided to the ASA from recognized laboratories or breed associations.

All the conditions tracked in TraitTrac are recessive, meaning an animal must inherit two copies of the mutation from the sire and the dam (sire and the dam must each be carriers in order to see an affected calf). Dominant mutations always influence an animal’s phenotype, so the dominant mutation can easily be selected for or against. Recessive mutations, however, tend to exist in a population even when harmful to the point of being lethal. This is because animals can carry the recessive gene without showing any signs of it. When carrier animals are mated to other carriers, the resulting offspring have a chance of showing symptoms. Fortunately, technology has evolved for many of these conditions so the carriers of recessive genetic conditions can be identified via DNA testing.

Punnett Square 

A 2 x 2 Punnett Square can be used to illustrate the outcome of various matings. In the examples, one can see the frequency of having free (0 copies of the mutation), carrier (one copy of the mutation) or affected (two copies of the mutation) progeny with various combinations of free, carrier, or affected parents. This process works for any simply inherited traits (not just genetic conditions).

Punnett square

For this example, “A” is the dominant and normal allele and “a” is the recessive mutation. Since the presence of “A” has complete dominance over the expression of “a” (the mutation), an animal with one copy of “A” completely hides the presence of “a.” Only animals with two copies of “a” will show the symptoms of the abnormal gene. Each square has an equal probability (1/4) of occurring. Mating a carrier to another carrier (top right) would result in a free calf 25% of the time, progeny that carry the mutation 50% of the time, and affected calves 25% of the time.  Without a DNA test there is no way of distinguishing an animal with the genotype “AA” from an “Aa,” or a carrier from a free animal for this mutation.

 

Genetic Conditions Tracked in ASA’s TraitTrac

Neuropathic Hydrocephalus (NH): Also called “water head,” affected calves are born dead with an extremely large cranium, with little or no brain material or spinal cord. Recessive, lethal, affecting Angus and Angus-influenced cattle.

Arthrogryposis Multiplex (AM): Known as “Curly Calf Syndrome,” AM results in stillborn calves small in size with diminished muscling, bent limbs, and twisted spines. Recessive, lethal, affecting Angus and Angus-influenced cattle.

Contractural Arachnodactyly (CA): Also known as fawn calf syndrome, the disorder affects the connective tissue of muscles, leading to contraction of the upper limb (most obvious in the hind limbs), and looseness of the joints of the lower limbs. Recessive, not lethal, Angus and Angus-influenced cattle.

Developmental Duplication (DD): Affected calves frequently (but not always) develop extra body parts, often limbs (most commonly, extra front legs), a condition called polymelia. Some animals with two copies of DD may have no outward sign of this trait. Recessive, not lethal, Angus and Angus-influenced cattle.

Tibial Hemimelia (TH): Calves are born with severe deformities including twisted rear legs (possibly missing part or all of bones), with fused joints, large abdominal hernias and/or skull deformities. Recessive, lethal (sometimes live at birth but unable to survive long), Shorthorn-, Maine-Anjou-, and Chianina-derived cattle.

Pulmonary Hypoplasia with Anasarca (PHA): PHA-affected calves are born dead with underdeveloped lungs (pulmonary hypoplasia) and swelling caused by excessive fluid retention (anasarca). Recessive, lethal, Shorthorn-, Maine-Anjou-,Chianina-, and Dexter-derived cattle. 

Osteopetrosis (OS): Known as marble-bone, affected calves are frequently aborted 10 to 30 days early with short lower jaw and missing bone marrow. Recessive, lethal, Red Angus- and some Angus-derived cattle.

 

 

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