These two terms are commonly used in medical science. Expressivity refers to different people who are showing the same traits. Penetrance refers to the number of people who may have a certain gene or trait. There are different conditions that can be under these two categories. Expressivity will show more thorough graduation as compared to penetrance.
Penetrance can be complete for achondroplasia, but most of the time, it is incomplete. Testing should be done to ensure that details about this will be clear. A lot of people say that these two categories exist right now because of modifier genes. The environment where people live may also have an effect on this.
Penetrance and expressivity both have to do with genetics in science. They are used when analyzing genetics. Both the penetrance and the expressivity have to do with gene expression and are modified through varying environments and their backgrounds. They both have to do with the phenotypic expressions. If you need to measure them, they can both be measures with values. However, there are differences between penetrance and expressivity.
If you measure penetrance, you would measure the population, but if you measure the expressivity, then you would measure the single individual. The variability involved in penetrance and expressivity revolves around the statistical variability in penetrance and the individual variability in expressivity. Penetrance is the amount in percent of those who have a certain genotype.