Final answer:
An individual with a risk allele does not automatically develop the genetic disease; it depends on the inheritance pattern and whether the allele is recessive or dominant. For autosomal recessive disorders like cystic fibrosis, two copies of the recessive allele are required for disease manifestation, with carriers remaining asymptomatic.
Step-by-step explanation:
An individual possessing a risk allele for a genetic disease does not necessarily develop the disease. The manifestation of genetic disorders often depends on the pattern of inheritance, such as autosomal dominant, autosomal recessive, or X-linked, and whether the allele is dominant or recessive. Particularly for autosomal recessive disorders like Tay-Sachs disease or cystic fibrosis, an individual needs to inherit two copies of the recessive allele to express the disease phenotype. If they have only one copy, they are considered a carrier without showing symptoms of the disease.
For example, in the case of cystic fibrosis, which is an autosomal recessive disorder, carriers have a functional allele which compensates for the non-functional allele, preventing the disease from manifesting. It's only when an individual inherits two non-functional alleles, one from each parent, that they would develop the disorder. Additionally, genetic testing and the study of family pedigree can help determine an individual's carrier status and the probability of passing on the allele to their offspring.