Final answer:
Penetrance is the term for the degree to which genetic traits are expressed in individuals carrying a particular gene, and it is essential in understanding the inheritance of genetic disorders. Geneticists use pedigree analysis to study inheritance patterns and penetrance in families.
Step-by-step explanation:
The phrase you are looking for to describe the parallel levels of symptoms across family members is penetrance. Penetrance refers to the likelihood that a person carrying a particular gene will exhibit the trait or disorder it is associated with. When discussing genetic inheritance, such as the transfer of autosomal dominant disorders across generations, penetrance plays a significant role. In some families, the same disease can be passed down, but with different levels of severity or presence - this variation is often due to incomplete or variable penetrance. For instance, in the context of genetic illnesses like a recessive genetic disorder, a person may carry the gene but not exhibit symptoms if the penetrance is reduced. Heritability also influences this, especially in polygenic and sometimes in oligogenic modes of inheritance.
Geneticists often use pedigree analysis to study the pattern of inheritance for various genetic diseases in lieu of performing unethical test crosses in humans. The analysis of pedigrees helps us understand how traits and disorders are transmitted through the generations and can indicate the degree of penetrance within a family group.