Penetrance and Expressivity

Most disease-causing allele combinations are completely penetrant, which means that everyone who inherits it has some symptoms. A genotype is incompletely penetrant if some individuals do not express the phenotype. Polydactyly, having extra fingers or toes, is incompletely penetrant (see fig. 7.48). Some people who inherit the dominant allele have more than five digits on a hand or foot, yet others who are known to have the allele (because they have an affected parent and child) have the normal number of fingers and toes.

The penetrance of a gene is described numerically. If 80 of 100 people who have inherited the dominant polydactyly allele have extra digits, the allele is 80% penetrant. Inherited breast cancer is also incompletely penetrant—that is, not all women who inherit a mutant form of a gene called BRCA1 develop the cancer, but they do have a much greater risk of doing so than a woman who inherits the wild-type allele.

A phenotype is variably expressive if the symptoms vary in intensity in different people. One person with polydactyly might have an extra digit on both hands and a foot; another might have two extra digits on both hands and both feet; a third person might have just one extra fingertip. Penetrance refers to the all-or-none expression of a genotype in an individual; expressivity refers to the severity of a phenotype.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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