how many traits are strictly mendelian?

Answer 1

Good question. Calling something Mendelian implies that it is influenced by ONE AND ONLY ONE GENE. As far as using the word "trait", very few "traits" are influenced by just one gene (i.e. hair color, eye color, skin color, height etc.) These things are influenced by many genes and often environmental factors.

Now, when you talk about expression of genes, we think of Mendelian disorders. There are many many single gene disorders, the most common being CF (cystic fibrosis). Basically if a mutation is inherited dominantly, one copy of a mutation means the phenotype will ALWAYS BE EXPRESSED based on that one mutated gene. Recessive of course (like CF), means both copies of a gene must be mutated to express the phenotype.

Compare this to something like type I diabetes. One mutation in a gene associated with diabetes doesn't guarantee that someone will have this disease. Nor do 2 mutations, or 3, etc. The term we use for this is penetrance by the way. Mendelian inheritance means a penetrance close to 100% (meaning every time the given gene is expressed, a phenotype associated with it will be expressed).