What is the difference between reduced transmission and incomplete penetrance?

While penetrance is a quantitative estimate, reduced expressivity (transmission) is a qualitative estimate.

Explanation:

Penetration

Some individuals who carry the deleterious allele (for example, in an autosomal dominant disease) do not show the expected phenotype: in that case we say that the penetrance is incomplete. This is the case in which the number of individuals presenting an abnormal phenotype is less than the number of individuals carrying the mutation causing this phenotype. Penetrance is a quantitative estimate.

For example, in the case of neurofibromatosis type I, its penetration is estimated to be 80% although it is often difficult to detect mild variants of the disease. Therefore, in these cases it would be useful to be able to detect the different mutations in the affected families and thus establish more precisely the value of their penetrance.

Expressiveness  (transmission)

The observed phenotype can be more or less severe among the different affected individuals. In this case it is said that the deleterious allele has a variable expressiveness. Expressiveness is a qualitative estimate.

In the case of Marfan syndrome, for the same mutation present in a family, some individuals show a severe form of the disease that affects the cardiovascular, ocular and skeletal system while others show only a high height and arachnodactyly, without eye problems or of aortic aneurysm.

Reduced penetrance and incomplete expressivity are mainly described in autosomal dominant diseases.

Do not confuse an isolated case due to poor penetrance with a sporadic case due to the appearance of a mutation.