It is the percentage of animals of a specific genotype who express the phenotype associated with that underlying genotype.
Complete and incomplete or reduced penetrance
An allele is said to have complete penetrance if all individuals who have the disease causing mutation have clinical symptoms of the disease. In incomplete or reduced penetrance, some individuals will not express the trait even though they carry the allele.
A specific example of incomplete penetrance is the human bone disease osteogenesis imperfecta (OI). The majority of people with this disease have a dominant mutation in one of the two genes that produce type 1 collagen, COL1A1 or COL1A2. Collagen is a tissue that strengthens bones and muscles and multiple body tissues.
In genetics, expressivity is the degree to which a phenotype is expressed by individuals having a particular genotype. (Alternately, it may refer to the expression of particular gene by individuals having a certain phenotype.)
Three common syndromes that involved phenotypic variability due to expressivity include:
- Marfan Syndrome
- Van der Woude Syndrome
- Neurofibromatosis Syndromes
1. Marfan Syndrome:
The characteristics of Marfan syndrome widely vary among individuals. Marfan syndrome is an inherited disorder that affects connective tissue — the fibers that support and anchor your organs and other structures in your body. Marfan syndrome most commonly affects the heart, eyes, blood vessels and skeleton.
2. Van der Woude Syndrome:
Van der Woude syndrome is a genetic form of cleft lip and palate. Your baby may have: A gap in their lip (cleft lip) or the roof of their mouth (cleft palate) or both. Small mounds of tissue or pits on their lower lip.
3. Neurofibromatosis Syndromes:
It is a genetic disorder of the nervous system. It mainly affects how nerve cells form and grow. It causes tumors to grow on nerves. You can get neurofibromatosis from your parents, or it can happen because of a mutation (change) in your genes.
Once you have it, you can pass it along to your children.