People do not inevitably exhibit the phenotypes that would be predicted from their genotypes. For example, there is a dominant allele that causes polydactyly,16 the presence of extra fingers or toes. We might predict that since it is dominant, anyone who inherited the allele would exhibit this trait. Most do, but others known to have the allele have the normal number of digits. Penetrance is the percentage of a population with a given genotype that actually exhibits the predicted phenotype. If 80% of people with the polydactyly allele actually exhibit extra digits, the allele has 80% penetrance.
Another reason the connection between genotype and phenotype is not inevitable is that environmental factors play an important role in the expression of all genes. At the very least, all gene expression depends on nutrition (fig. 4.19). Children born with the hereditary disease phenylketonuria (FEN-il-KEE-toe-NEW-ree-uh) (PKU), for example, become retarded if they eat a normal diet. However, if PKU is detected early, a child can be placed on a diet low in phenylalanine (an amino acid) and achieve normal mental development.
No gene can produce a phenotypic effect without nutritional and other environmental input, and no nutrients can produce a body or specific phenotype without genetic instructions that tell cells what to do with them. Just as you need both a recipe and ingredients to make a cake, it takes both heredity and environment to make a phenotype.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.