- Basic Concepts
- DNA & RNA
- Simple Inheritance
- Modify Mendelian Ratios
Diagnosis and Screening for Genetic Diseases
Many diseases that affect animals are caused by a single gene mutation that alters the function of an enzyme or protein. These mutations can be passed from parents to offspring, as shown in this animation. In this case, the parents are heterozygous carriers of a recessive genetic disease. These carriers show no symptoms themselves, but when they mate, their offspring have a 1/4 probability of getting the mutated allele (n) from both parents, thus inheriting the genetic disease. Many genetic diseases can be treated effectively if they are diagnosed early. Several methods used to diagnose and screen for genetic diseases and the genes that cause them are discussed below.
1. Blood tests or urinalysis
Blood or urine can be chemically analyzed for abnormal cells or for the buildup of undegraded material due to a defective enzyme. For example, cystinuria is an autosomal recessive disease that can cause fatal kidney failure in Newfoundlands and many other dog breeds. It is caused by an enzyme defect that prevents the proper metabolism of the amino acid cysteine. Urine tests can be used to diagnose this disease because affected dogs have cysteine crystals in their urine.
2. DNA analysis
If the specific gene that produces the defective enzyme is known, it can be studied to determine the exact genetic cause of the disease. Samples of blood or skin can be taken from the patient, and nuclear or mitochondrial DNA can be isolated. Copies of the gene of interest can then be made using PCR and analyzed for mutations either by DNA sequencing or a variety of other techniques.
3. Family studies
Since many harmless gene mutations exist in populations, samples must be collected from related family members as a way to confirm that the mutations detected in the DNA analyses are responsible for the defective enzyme. In short, the inheritance of the mutation and the inheritance of the disease must match. For example, to show that a disease is caused by an autosomal recessive allele, all affected members of a family must be homozygous for that allele, while the affected individuals' parents must each carry at least one copy of the allele. Furthermore, all phenotypically normal individuals must either be heterozygous or homozygous for the allele that is presumed to be normal.
4. DNA markers
If the specific gene that causes a disease is not known, a closely linked gene, called a marker, can be used for diagnosis and screening. In this case, the genetic tests look for the marker allele or phenotype and assume that since it is closely linked to the disease allele, all individuals who inherit one will also inherit the other. Unfortunately, as we know, recombination can separate linked alleles. Thus, tests based on DNA markers are never 100% accurate.