1. What’s the Difference? Genetic and Common Diseases

2. What’s Inherited? A widespread misconception is that traits due to dominant alleles are the most common in the population. While this is sometimes true, it is not always the case. For example, the allele for Huntington’s Disease is dominant, while the allele for not developing this disorder is recessive. At most, only 1 in 20,000 people will get Huntington’s; most people have two recessive, normal alleles.

3. Gene Interactions While a few traits are due to only one gene (and its alleles), most genetic traits are the product of interactions between several genes. When more than one gene influences a trait, the inheritance pattern is not easily predictable. The predictable patterns referred to as dominant and recessive apply only to single gene traits.

4. Rare Genetic Disorders & Common Disorders The prevalence of rare genetic disorders caused by a single gene such as cystic fibrosis is 1 in 10,000 and the prevalence of more common diseases such as heart disease is 1 in Other genetic disorders include Down Syndrome, Crohn’s Disease, Hemophilia, Muscular Dystrophe. Do common diseases like heart disease, arthritis, diabetes, autism, ADHD or colon cancer have a genetic component?

5. Multifactorial Diseases Most common diseases do have a genetic component and tend to run in families. However, common diseases differ from rare genetic disorders in that they are usually not caused by defects in a single gene. Rather, they result from the combined effects of multiple genes and environmental factors. Thus, they are called multifactorial diseases

6. Common Diseases = Many Genes Because more than one gene is involved in most common diseases, the inheritance of a common disease is not predictable. Information found in a family health history and recorded on a pedigree is used to estimate an individual’s genetic risk (low, medium, or high) of developing a common disease.

7. Common Disease vs. Genetic Disease The chance of a common disease being passed from a grandfather to a grandchild, for example, isn’t high because the chance of all the contributing genes being passed through the generations isn’t great. The chance of a genetic disease (Williams’ Syndrome, for example) being passed along is higher because it’s due to one single gene.