Will every individual who inherits the allele for disease develop disease?

No, inheriting an allele for a disease does not guarantee the development of the disease.

There are several reasons why an individual who inherits a disease allele may not develop the disease:

- Incomplete penetrance: This refers to the phenomenon where not all individuals who inherit a disease-causing allele actually develop the disease. The penetrance of a gene refers to the proportion of individuals with a specific genetic variant who exhibit the associated phenotype (i.e. the disease). Incomplete penetrance occurs when the penetrance is less than 100%. This means that some individuals who inherit the disease allele do not express the disease. For example, some individuals may inherit a mutation in the BRCA1 gene, which significantly increases the risk of breast cancer, but not all individuals with this mutation will develop breast cancer.

- Variable expressivity: Even among individuals who do develop a disease due to a specific genetic variant, the severity and presentation of the disease can vary widely. This phenomenon is called variable expressivity. The expressivity of a gene refers to the range of phenotypic effects associated with a particular genotype. For example, some individuals with sickle cell disease may experience mild symptoms, while others may develop severe and life-threatening complications.

- Genetic heterogeneity: Many diseases have multiple genetic causes, which means that different genetic variations or combinations of variations can lead to the same disease. This is referred to as genetic heterogeneity. For example, cystic fibrosis can be caused by mutations in different genes, including the CFTR gene. Not all individuals with mutations in the CFTR gene will develop cystic fibrosis, and the severity of the disease can also vary widely depending on the specific mutation.

- Environmental factors: In addition to genetic factors, environmental factors can also play a role in the development of disease. For example, exposure to certain toxins or pathogens may increase the risk of developing certain diseases, such as cancer or infectious diseases.