Diagnosing Carriers of Genetic Diseases
By : James L. Robinson, Professor of Biochemistry, Department of Animal Sciences
Genetic diseases or inherited disorders occur in all animal species. Intense selection for improved animal performance, however, may inadvertently increase the frequency of undesirable traits. Because most genetic diseases in domesticated species are inherited as autosomal recessive traits, and carriers generally give no outward indications, the undesirable trait can be spread widely and covertly.
Electrophoretic patterns diagnostic for normal, carrier, and affected genotypes for bovine citrullinemia. Here, the genes and gene fragments separate in an electric field. Normal animals have two short segments that result from cutting the amplified DNA segment. Affected animals (homozygous recessives) have one long uncut segment because the mutation prevents the cut. Carrier animals (heterozygotes) have three segments (one normal sequence and one mutated sequence).
When carriers of any specific condition become prevalent, matings between carriers occur at a high frequency, resulting in affected offspring (homozygous recessives) that will often die prematurely. Identifying and progressively eliminating carriers from a species or breed constitutes an incremental method for improving animals useful to humans.
Biotechnology offers the promise of rapid, economical detection of carriers of genetic diseases. Similar methods are being developed to diagnose genotypes, such as normal, carrier, or affected, for conditions inherited by humans. It is crucial to identify the nucleotide change in the DNA (genetic material) structure responsible for the condition. The changed nucleotide sequence can often be distinguished by using one of hundreds of restriction enzymes that cut the DNA at specific points. The polymerase chain reaction (PCR) can be used to amplify the sequence that contains the mutation before treatment with the appropriate restriction enzyme.
These methods already permit diagnosis of the genotypes for bovine citrullinemia, a genetic disease of Holstein- Friesian cattle. In Australia, one of every 250 calves born is affected (homozygous recessive) and dies of neurological dysfunction within five days of birth. Among U.S. Holstein cattle, carriers exist; but their frequency, expected to be low, is presently unknown.
The diagnostic protocol consists of
- obtaining a blood, semen, or tissue sample of DNA from the test animal;
- extracting the DNA from the sample;
- amplifying (using PCR) a nucleotide sequence that contains the site of the mutation;
- treating the amplified sequence with a restriction enzyme that cuts only the normal sequence; and
- electrophoresing the resultant DNA to reveal the number and size of DNA segments present.
Electrophoresis is the movement of molecules through a fluid or gel under the action of an electrical current. The figure shows the electrophoretic patterns diagnostic for the normal, carrier, and affected genotypes of bovine citrullinemia.